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# Hadoop-AWS module: Integration with Amazon Web Services

<!-- MACRO{toc|fromDepth=0|toDepth=2} -->



## <a name="compatibility"></a> Compatibility


###  <a name="directory-marker-compatibility"></a> Directory Marker Compatibility

1. This release can safely list/index/read S3 buckets where "empty directory"
markers are retained.

1. This release can be configured to retain these directory makers at the
expense of being backwards incompatible.

Consult [Controlling the S3A Directory Marker Behavior](directory_markers.html) for
full details.

## <a name="documents"></a> Documents

* [Encryption](./encryption.html)
* [Performance](./performance.html)
* [S3Guard](./s3guard.html)
* [Troubleshooting](./troubleshooting_s3a.html)
* [Controlling the S3A Directory Marker Behavior](directory_markers.html).
* [Committing work to S3 with the "S3A Committers"](./committers.html)
* [S3A Committers Architecture](./committer_architecture.html)
* [Working with IAM Assumed Roles](./assumed_roles.html)
* [S3A Delegation Token Support](./delegation_tokens.html)
* [S3A Delegation Token Architecture](delegation_token_architecture.html).
* [Auditing](./auditing.html).
* [Auditing Architecture](./auditing_architecture.html).
* [Testing](./testing.html)
* [Prefetching](./prefetching.html)
* [Upcoming upgrade to AWS Java SDK V2](./aws_sdk_upgrade.html)

## <a name="overview"></a> Overview

Apache Hadoop's `hadoop-aws` module provides support for AWS integration.
applications to easily use this support.

To include the S3A client in Apache Hadoop's default classpath:

1. Make sure that`HADOOP_OPTIONAL_TOOLS` in `hadoop-env.sh` includes `hadoop-aws`
in its list of optional modules to add in the classpath.

1. For client side interaction, you can declare that relevant JARs must be loaded
in your `~/.hadooprc` file:

        hadoop_add_to_classpath_tools hadoop-aws

The settings in this file does not propagate to deployed applications, but it will
work for local clients such as the `hadoop fs` command.


## <a name="introduction"></a> Introducing the Hadoop S3A client.

Hadoop's "S3A" client offers high-performance IO against Amazon S3 object store
and compatible implementations.

* Directly reads and writes S3 objects.
* Compatible with standard S3 clients.
* Compatible with files created by the older `s3n://` client and Amazon EMR's `s3://` client.
* Supports partitioned uploads for many-GB objects.
* Offers a high-performance random IO mode for working with columnar data such
as Apache ORC and Apache Parquet files.
* Uses Amazon's Java S3 SDK with support for latest S3 features and authentication
schemes.
* Supports authentication via: environment variables, Hadoop configuration
properties, the Hadoop key management store and IAM roles.
* Supports per-bucket configuration.
* Supports S3 "Server Side Encryption" for both reading and writing:
 SSE-S3, SSE-KMS and SSE-C.
* Instrumented with Hadoop metrics.
* Before S3 was consistent, provided a consistent view of inconsistent storage
  through [S3Guard](./s3guard.html).
* Actively maintained by the open source community.


### Other S3 Connectors

There other Hadoop connectors to S3. Only S3A is actively maintained by
the Hadoop project itself.

1. Apache's Hadoop's original `s3://` client. This is no longer included in Hadoop.
1. Amazon EMR's `s3://` client. This is from the Amazon EMR team, who actively
maintain it.
1. Apache's Hadoop's [`s3n:` filesystem client](./s3n.html).
   This connector is no longer available: users must migrate to the newer `s3a:` client.


## <a name="getting_started"></a> Getting Started

S3A depends upon two JARs, alongside `hadoop-common` and its dependencies.

* `hadoop-aws` JAR.
* `aws-java-sdk-bundle` JAR.

The versions of `hadoop-common` and `hadoop-aws` must be identical.

To import the libraries into a Maven build, add `hadoop-aws` JAR to the
build dependencies; it will pull in a compatible aws-sdk JAR.

The `hadoop-aws` JAR *does not* declare any dependencies other than that
dependencies unique to it, the AWS SDK JAR. This is simplify excluding/tuning
Hadoop dependency JARs in downstream applications. The `hadoop-client` or
`hadoop-common` dependency must be declared


```xml
<properties>
 <!-- Your exact Hadoop version here-->
  <hadoop.version>3.0.0</hadoop.version>
</properties>

<dependencies>
  <dependency>
    <groupId>org.apache.hadoop</groupId>
    <artifactId>hadoop-client</artifactId>
    <version>${hadoop.version}</version>
  </dependency>
  <dependency>
    <groupId>org.apache.hadoop</groupId>
    <artifactId>hadoop-aws</artifactId>
    <version>${hadoop.version}</version>
  </dependency>
</dependencies>
```

## <a name="warning"></a> Warnings

Amazon S3 is an example of "an object store". In order to achieve scalability
and especially high availability, S3 has —as many other cloud object stores have
done— relaxed some of the constraints which classic "POSIX" filesystems promise.

For further discussion on these topics, please consult
[The Hadoop FileSystem API Definition](../../../hadoop-project-dist/hadoop-common/filesystem/index.html).


### Warning #1: Directories are mimicked

The S3A clients mimics directories by:

1. Creating a stub entry after a `mkdirs` call, deleting it when a file
is added anywhere underneath
1. When listing a directory, searching for all objects whose path starts with
the directory path, and returning them as the listing.
1. When renaming a directory, taking such a listing and asking S3 to copying the
individual objects to new objects with the destination filenames.
1. When deleting a directory, taking such a listing and deleting the entries in
batches.
1. When renaming or deleting directories, taking such a listing and working
on the individual files.


Here are some of the consequences:

* Directories may lack modification times.
Parts of Hadoop relying on this can have unexpected behaviour. E.g. the
`AggregatedLogDeletionService` of YARN will not remove the appropriate logfiles.
* Directory listing can be slow. Use `listFiles(path, recursive)` for high
performance recursive listings whenever possible.
* It is possible to create files under files if the caller tries hard.
* The time to rename a directory is proportional to the number of files
underneath it (directory or indirectly) and the size of the files. (The copy is
executed inside the S3 storage, so the time is independent of the bandwidth
from client to S3).
* Directory renames are not atomic: they can fail partway through, and callers
cannot safely rely on atomic renames as part of a commit algorithm.
* Directory deletion is not atomic and can fail partway through.

The final three issues surface when using S3 as the immediate destination
of work, as opposed to HDFS or other "real" filesystem.

The [S3A committers](./committers.html) are the sole mechanism available
to safely save the output of queries directly into S3 object stores
through the S3A filesystem.


### Warning #2: Object stores have different authorization models

The object authorization model of S3 is much different from the file
authorization model of HDFS and traditional file systems.
The S3A client simply reports stub information from APIs that would query this metadata:

* File owner is reported as the current user.
* File group also is reported as the current user. Prior to Apache Hadoop
2.8.0, file group was reported as empty (no group associated), which is a
potential incompatibility problem for scripts that perform positional parsing of
shell output and other clients that expect to find a well-defined group.
* Directory permissions are reported as 777.
* File permissions are reported as 666.

S3A does not really enforce any authorization checks on these stub permissions.
Users authenticate to an S3 bucket using AWS credentials.  It's possible that
object ACLs have been defined to enforce authorization at the S3 side, but this
happens entirely within the S3 service, not within the S3A implementation.

### Warning #4: Your AWS credentials are very, very valuable

Your AWS credentials not only pay for services, they offer read and write
access to the data. Anyone with the credentials can not only read your datasets
—they can delete them.

Do not inadvertently share these credentials through means such as:

1. Checking in to SCM any configuration files containing the secrets.
1. Logging them to a console, as they invariably end up being seen.
1. Including the secrets in bug reports.
1. Logging the `AWS_` environment variables.

If you do any of these: change your credentials immediately!


## <a name="authenticating"></a> Authenticating with S3

Except when interacting with public S3 buckets, the S3A client
needs the credentials needed to interact with buckets.

The client supports multiple authentication mechanisms and can be configured as to
which mechanisms to use, and their order of use. Custom implementations
of `com.amazonaws.auth.AWSCredentialsProvider` may also be used.
However, with the upcoming upgrade to AWS Java SDK V2, these classes will need to be
updated to implement `software.amazon.awssdk.auth.credentials.AwsCredentialsProvider`.
For more information see [Upcoming upgrade to AWS Java SDK V2](./aws_sdk_upgrade.html).

*Important*: The S3A connector no longer supports username and secrets
in URLs of the form `s3a://key:secret@bucket/`.
It is near-impossible to stop those secrets being logged —which is why
a warning has been printed since Hadoop 2.8 whenever such a URL was used.

### Authentication properties

```xml
<property>
  <name>fs.s3a.access.key</name>
  <description>AWS access key ID.
   Omit for IAM role-based or provider-based authentication.</description>
</property>

<property>
  <name>fs.s3a.secret.key</name>
  <description>AWS secret key.
   Omit for IAM role-based or provider-based authentication.</description>
</property>

<property>
  <name>fs.s3a.aws.credentials.provider</name>
  <description>
    Comma-separated class names of credential provider classes which implement
    com.amazonaws.auth.AWSCredentialsProvider.

    These are loaded and queried in sequence for a valid set of credentials.
    Each listed class must implement one of the following means of
    construction, which are attempted in order:
    1. a public constructor accepting java.net.URI and
        org.apache.hadoop.conf.Configuration,
    2. a public static method named getInstance that accepts no
       arguments and returns an instance of
       com.amazonaws.auth.AWSCredentialsProvider, or
    3. a public default constructor.

    Specifying org.apache.hadoop.fs.s3a.AnonymousAWSCredentialsProvider allows
    anonymous access to a publicly accessible S3 bucket without any credentials.
    Please note that allowing anonymous access to an S3 bucket compromises
    security and therefore is unsuitable for most use cases. It can be useful
    for accessing public data sets without requiring AWS credentials.

    If unspecified, then the default list of credential provider classes,
    queried in sequence, is:
    1. org.apache.hadoop.fs.s3a.SimpleAWSCredentialsProvider:
       Uses the values of fs.s3a.access.key and fs.s3a.secret.key.
    2. com.amazonaws.auth.EnvironmentVariableCredentialsProvider: supports
        configuration of AWS access key ID and secret access key in
        environment variables named AWS_ACCESS_KEY_ID and
        AWS_SECRET_ACCESS_KEY, as documented in the AWS SDK.
    3. com.amazonaws.auth.InstanceProfileCredentialsProvider: supports use
        of instance profile credentials if running in an EC2 VM.
  </description>
</property>

<property>
  <name>fs.s3a.session.token</name>
  <description>
    Session token, when using org.apache.hadoop.fs.s3a.TemporaryAWSCredentialsProvider
    as one of the providers.
  </description>
</property>
```

### <a name="auth_env_vars"></a> Authenticating via the AWS Environment Variables

S3A supports configuration via [the standard AWS environment variables](http://docs.aws.amazon.com/cli/latest/userguide/cli-chap-getting-started.html#cli-environment).

The core environment variables are for the access key and associated secret:

```bash
export AWS_ACCESS_KEY_ID=my.aws.key
export AWS_SECRET_ACCESS_KEY=my.secret.key
```

If the environment variable `AWS_SESSION_TOKEN` is set, session authentication
using "Temporary Security Credentials" is enabled; the Key ID and secret key
must be set to the credentials for that specific session.

```bash
export AWS_SESSION_TOKEN=SECRET-SESSION-TOKEN
export AWS_ACCESS_KEY_ID=SESSION-ACCESS-KEY
export AWS_SECRET_ACCESS_KEY=SESSION-SECRET-KEY
```

These environment variables can be used to set the authentication credentials
instead of properties in the Hadoop configuration.

*Important:*
These environment variables are generally not propagated from client to server when
YARN applications are launched. That is: having the AWS environment variables
set when an application is launched will not permit the launched application
to access S3 resources. The environment variables must (somehow) be set
on the hosts/processes where the work is executed.

### <a name="auth_providers"></a> Changing Authentication Providers

The standard way to authenticate is with an access key and secret key set in
the Hadoop configuration files.

By default, the S3A client follows the following authentication chain:

1. The options `fs.s3a.access.key`, `fs.s3a.secret.key` and `fs.s3a.sesson.key`
are looked for in the Hadoop XML configuration/Hadoop credential providers,
returning a set of session credentials if all three are defined.
1. The `fs.s3a.access.key` and `fs.s3a.secret.key` are looked for in the Hadoop
XML configuration//Hadoop credential providers, returning a set of long-lived
credentials if they are defined.
1. The [AWS environment variables](http://docs.aws.amazon.com/cli/latest/userguide/cli-chap-getting-started.html#cli-environment),
are then looked for: these will return session or full credentials depending
on which values are set.
1. An attempt is made to query the Amazon EC2 Instance Metadata Service to
 retrieve credentials published to EC2 VMs.

S3A can be configured to obtain client authentication providers from classes
which integrate with the AWS SDK by implementing the `com.amazonaws.auth.AWSCredentialsProvider`
Interface. This is done by listing the implementation classes, in order of
preference, in the configuration option `fs.s3a.aws.credentials.provider`.

*Important*: AWS Credential Providers are distinct from _Hadoop Credential Providers_.
As will be covered later, Hadoop Credential Providers allow passwords and other secrets
to be stored and transferred more securely than in XML configuration files.
AWS Credential Providers are classes which can be used by the Amazon AWS SDK to
obtain an AWS login from a different source in the system, including environment
variables, JVM properties and configuration files.

All Hadoop `fs.s3a.` options used to store login details can all be secured
in [Hadoop credential providers](../../../hadoop-project-dist/hadoop-common/CredentialProviderAPI.html);
this is advised as a more secure way to store valuable secrets.

There are a number of AWS Credential Providers inside the `hadoop-aws` JAR:

| classname | description |
|-----------|-------------|
| `org.apache.hadoop.fs.s3a.TemporaryAWSCredentialsProvider`| Session Credentials |
| `org.apache.hadoop.fs.s3a.SimpleAWSCredentialsProvider`| Simple name/secret credentials |
| `org.apache.hadoop.fs.s3a.AnonymousAWSCredentialsProvider`| Anonymous Login |
| `org.apache.hadoop.fs.s3a.auth.AssumedRoleCredentialProvider<`| [Assumed Role credentials](assumed_roles.html) |


There are also many in the Amazon SDKs, in particular two which are automatically
set up in the authentication chain:

| classname | description |
|-----------|-------------|
| `com.amazonaws.auth.InstanceProfileCredentialsProvider`| EC2 Metadata Credentials |
| `com.amazonaws.auth.EnvironmentVariableCredentialsProvider`| AWS Environment Variables |


### <a name="auth_iam"></a> EC2 IAM Metadata Authentication with `InstanceProfileCredentialsProvider`

Applications running in EC2 may associate an IAM role with the VM and query the
[EC2 Instance Metadata Service](http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-instance-metadata.html)
for credentials to access S3.  Within the AWS SDK, this functionality is
provided by `InstanceProfileCredentialsProvider`, which internally enforces a
singleton instance in order to prevent throttling problem.

### <a name="auth_named_profile"></a> Using Named Profile Credentials with `ProfileCredentialsProvider`

You can configure Hadoop to authenticate to AWS using a [named profile](https://docs.aws.amazon.com/cli/latest/userguide/cli-configure-profiles.html).

To authenticate with a named profile:

1. Declare `com.amazonaws.auth.profile.ProfileCredentialsProvider` as the provider.
1. Set your profile via the `AWS_PROFILE` environment variable.
1. Due to a [bug in version 1 of the AWS Java SDK](https://github.com/aws/aws-sdk-java/issues/803),
you'll need to remove the `profile` prefix from the AWS configuration section heading.

    Here's an example of what your AWS configuration files should look like:

    ```
    $ cat ~/.aws/config
    [user1]
    region = us-east-1
    $ cat ~/.aws/credentials
    [user1]
    aws_access_key_id = ...
    aws_secret_access_key = ...
    aws_session_token = ...
    aws_security_token = ...
    ```
Note:

1. The `region` setting is only used if `fs.s3a.endpoint.region` is set to the empty string.
1. For the credentials to be available to applications running in a Hadoop cluster, the
   configuration files MUST be in the `~/.aws/` directory on the local filesystem in
   all hosts in the cluster.

### <a name="auth_session"></a> Using Session Credentials with `TemporaryAWSCredentialsProvider`

[Temporary Security Credentials](http://docs.aws.amazon.com/IAM/latest/UserGuide/id_credentials_temp.html)
can be obtained from the Amazon Security Token Service; these
consist of an access key, a secret key, and a session token.

To authenticate with these:

1. Declare `org.apache.hadoop.fs.s3a.TemporaryAWSCredentialsProvider` as the
provider.
1. Set the session key in the property `fs.s3a.session.token`,
and the access and secret key properties to those of this temporary session.

Example:

```xml
<property>
  <name>fs.s3a.aws.credentials.provider</name>
  <value>org.apache.hadoop.fs.s3a.TemporaryAWSCredentialsProvider</value>
</property>

<property>
  <name>fs.s3a.access.key</name>
  <value>SESSION-ACCESS-KEY</value>
</property>

<property>
  <name>fs.s3a.secret.key</name>
  <value>SESSION-SECRET-KEY</value>
</property>

<property>
  <name>fs.s3a.session.token</name>
  <value>SECRET-SESSION-TOKEN</value>
</property>
```

The lifetime of session credentials are fixed when the credentials
are issued; once they expire the application will no longer be able to
authenticate to AWS.

### <a name="auth_anon"></a> Anonymous Login with `AnonymousAWSCredentialsProvider`

Specifying `org.apache.hadoop.fs.s3a.AnonymousAWSCredentialsProvider` allows
anonymous access to a publicly accessible S3 bucket without any credentials.
It can be useful for accessing public data sets without requiring AWS credentials.

```xml
<property>
  <name>fs.s3a.aws.credentials.provider</name>
  <value>org.apache.hadoop.fs.s3a.AnonymousAWSCredentialsProvider</value>
</property>
```

Once this is done, there's no need to supply any credentials
in the Hadoop configuration or via environment variables.

This option can be used to verify that an object store does
not permit unauthenticated access: that is, if an attempt to list
a bucket is made using the anonymous credentials, it should fail —unless
explicitly opened up for broader access.

```bash
hadoop fs -ls \
 -D fs.s3a.aws.credentials.provider=org.apache.hadoop.fs.s3a.AnonymousAWSCredentialsProvider \
 s3a://landsat-pds/
```

1. Allowing anonymous access to an S3 bucket compromises
security and therefore is unsuitable for most use cases.

1. If a list of credential providers is given in `fs.s3a.aws.credentials.provider`,
then the Anonymous Credential provider *must* come last. If not, credential
providers listed after it will be ignored.

### <a name="auth_simple"></a> Simple name/secret credentials with `SimpleAWSCredentialsProvider`*

This is is the standard credential provider, which supports the secret
key in `fs.s3a.access.key` and token in `fs.s3a.secret.key`
values.

```xml
<property>
  <name>fs.s3a.aws.credentials.provider</name>
  <value>org.apache.hadoop.fs.s3a.SimpleAWSCredentialsProvider</value>
</property>
```

This is the basic authenticator used in the default authentication chain.

This means that the default S3A authentication chain can be defined as

```xml
<property>
  <name>fs.s3a.aws.credentials.provider</name>
  <value>
    org.apache.hadoop.fs.s3a.TemporaryAWSCredentialsProvider,
    org.apache.hadoop.fs.s3a.SimpleAWSCredentialsProvider,
    com.amazonaws.auth.EnvironmentVariableCredentialsProvider,
    org.apache.hadoop.fs.s3a.auth.IAMInstanceCredentialsProvider
  </value>
  <description>
    Comma-separated class names of credential provider classes which implement
    com.amazonaws.auth.AWSCredentialsProvider.

    When S3A delegation tokens are not enabled, this list will be used
    to directly authenticate with S3 and other AWS services.
    When S3A Delegation tokens are enabled, depending upon the delegation
    token binding it may be used
    to communicate with the STS endpoint to request session/role
    credentials.

    These are loaded and queried in sequence for a valid set of credentials.
    Each listed class must implement one of the following means of
    construction, which are attempted in order:
    * a public constructor accepting java.net.URI and
        org.apache.hadoop.conf.Configuration,
    * a public constructor accepting org.apache.hadoop.conf.Configuration,
    * a public static method named getInstance that accepts no
       arguments and returns an instance of
       com.amazonaws.auth.AWSCredentialsProvider, or
    * a public default constructor.

    Specifying org.apache.hadoop.fs.s3a.AnonymousAWSCredentialsProvider allows
    anonymous access to a publicly accessible S3 bucket without any credentials.
    Please note that allowing anonymous access to an S3 bucket compromises
    security and therefore is unsuitable for most use cases. It can be useful
    for accessing public data sets without requiring AWS credentials.

    If unspecified, then the default list of credential provider classes,
    queried in sequence, is:
    * org.apache.hadoop.fs.s3a.TemporaryAWSCredentialsProvider: looks
       for session login secrets in the Hadoop configuration.
    * org.apache.hadoop.fs.s3a.SimpleAWSCredentialsProvider:
       Uses the values of fs.s3a.access.key and fs.s3a.secret.key.
    * com.amazonaws.auth.EnvironmentVariableCredentialsProvider: supports
        configuration of AWS access key ID and secret access key in
        environment variables named AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY,
        and AWS_SESSION_TOKEN as documented in the AWS SDK.
    * org.apache.hadoop.fs.s3a.auth.IAMInstanceCredentialsProvider: picks up
       IAM credentials of any EC2 VM or AWS container in which the process is running.
  </description>
</property>
```

## <a name="auth_security"></a> Protecting the AWS Credentials

It is critical that you never share or leak your AWS credentials.
Loss of credentials can leak/lose all your data, run up large bills,
and significantly damage your organisation.

1. Never share your secrets.

1. Never commit your secrets into an SCM repository.
The [git secrets](https://github.com/awslabs/git-secrets) can help here.

1. Never include AWS credentials in bug reports, files attached to them,
or similar.

1. If you use the `AWS_` environment variables,  your list of environment variables
is equally sensitive.

1. Never use root credentials.
Use IAM user accounts, with each user/application having its own set of credentials.

1. Use IAM permissions to restrict the permissions individual users and applications
have. This is best done through roles, rather than configuring individual users.

1. Avoid passing in secrets to Hadoop applications/commands on the command line.
The command line of any launched program is visible to all users on a Unix system
(via `ps`), and preserved in command histories.

1. Explore using [IAM Assumed Roles](assumed_roles.html) for role-based permissions
management: a specific S3A connection can be made with a different assumed role
and permissions from the primary user account.

1. Consider a workflow in which users and applications are issued with short-lived
session credentials, configuring S3A to use these through
the `TemporaryAWSCredentialsProvider`.

1. Have a secure process in place for cancelling and re-issuing credentials for
users and applications. Test it regularly by using it to refresh credentials.

1. In installations where Kerberos is enabled, [S3A Delegation Tokens](delegation_tokens.html)
can be used to acquire short-lived session/role credentials and then pass them
into the shared application. This can ensure that the long-lived secrets stay
on the local system.

When running in EC2, the IAM EC2 instance credential provider will automatically
obtain the credentials needed to access AWS services in the role the EC2 VM
was deployed as.
This AWS credential provider is enabled in S3A by default.


## <a name="hadoop_credential_providers"></a>Storing secrets with Hadoop Credential Providers

The Hadoop Credential Provider Framework allows secure "Credential Providers"
to keep secrets outside Hadoop configuration files, storing them in encrypted
files in local or Hadoop filesystems, and including them in requests.

The S3A configuration options with sensitive data
(`fs.s3a.secret.key`, `fs.s3a.access.key`,  `fs.s3a.session.token`
and `fs.s3a.encryption.key`) can
have their data saved to a binary file stored, with the values being read in
when the S3A filesystem URL is used for data access. The reference to this
credential provider then declared in the Hadoop configuration.

For additional reading on the Hadoop Credential Provider API see:
[Credential Provider API](../../../hadoop-project-dist/hadoop-common/CredentialProviderAPI.html).


The following configuration options can be stored in Hadoop Credential Provider
stores.

```
fs.s3a.access.key
fs.s3a.secret.key
fs.s3a.session.token
fs.s3a.encryption.key
fs.s3a.encryption.algorithm
```

The first three are for authentication; the final two for
[encryption](./encryption.html). Of the latter, only the encryption key can
be considered "sensitive". However, being able to include the algorithm in
the credentials allows for a JCECKS file to contain all the options needed
to encrypt new data written to S3.

### Step 1: Create a credential file

A credential file can be created on any Hadoop filesystem; when creating one on HDFS or
a Unix filesystem the permissions are automatically set to keep the file
private to the reader —though as directory permissions are not touched,
users should verify that the directory containing the file is readable only by
the current user.

```bash
hadoop credential create fs.s3a.access.key -value 123 \
    -provider jceks://hdfs@nn1.example.com:9001/user/backup/s3.jceks

hadoop credential create fs.s3a.secret.key -value 456 \
    -provider jceks://hdfs@nn1.example.com:9001/user/backup/s3.jceks
```

A credential file can be listed, to see what entries are kept inside it

```bash
hadoop credential list -provider jceks://hdfs@nn1.example.com:9001/user/backup/s3.jceks

Listing aliases for CredentialProvider: jceks://hdfs@nn1.example.com:9001/user/backup/s3.jceks
fs.s3a.secret.key
fs.s3a.access.key
```
At this point, the credentials are ready for use.

### Step 2: Configure the `hadoop.security.credential.provider.path` property

The URL to the provider must be set in the configuration property
`hadoop.security.credential.provider.path`, either on the command line or
in XML configuration files.

```xml
<property>
  <name>hadoop.security.credential.provider.path</name>
  <value>jceks://hdfs@nn1.example.com:9001/user/backup/s3.jceks</value>
  <description>Path to interrogate for protected credentials.</description>
</property>
```

Because this property only supplies the path to the secrets file, the configuration
option itself is no longer a sensitive item.

The property `hadoop.security.credential.provider.path` is global to all
filesystems and secrets.
There is another property, `fs.s3a.security.credential.provider.path`
which only lists credential providers for S3A filesystems.
The two properties are combined into one, with the list of providers in the
`fs.s3a.` property taking precedence
over that of the `hadoop.security` list (i.e. they are prepended to the common list).

```xml
<property>
  <name>fs.s3a.security.credential.provider.path</name>
  <value />
  <description>
    Optional comma separated list of credential providers, a list
    which is prepended to that set in hadoop.security.credential.provider.path
  </description>
</property>
```

This was added to support binding different credential providers on a per
bucket basis, without adding alternative secrets in the credential list.
However, some applications (e.g Hive) prevent the list of credential providers
from being dynamically updated by users. As per-bucket secrets are now supported,
it is better to include per-bucket keys in JCEKS files and other sources
of credentials.

### Using secrets from credential providers

Once the provider is set in the Hadoop configuration, Hadoop commands
work exactly as if the secrets were in an XML file.

```bash
hadoop distcp \
    hdfs://nn1.example.com:9001/user/backup/007020615 s3a://glacier1/

hadoop fs -ls s3a://glacier1/
```

The path to the provider can also be set on the command line:

```bash
hadoop distcp \
    -D hadoop.security.credential.provider.path=jceks://hdfs@nn1.example.com:9001/user/backup/s3.jceks \
    hdfs://nn1.example.com:9001/user/backup/007020615 s3a://glacier1/

hadoop fs \
  -D fs.s3a.security.credential.provider.path=jceks://hdfs@nn1.example.com:9001/user/backup/s3.jceks \
  -ls s3a://glacier1/
```

Because the provider path is not itself a sensitive secret, there is no risk
from placing its declaration on the command line.


## <a name="general_configuration"></a>General S3A Client configuration

All S3A client options are configured with options with the prefix `fs.s3a.`.

The client supports <a href="per_bucket_configuration">Per-bucket configuration</a>
to allow different buckets to override the shared settings. This is commonly
used to change the endpoint, encryption and authentication mechanisms of buckets.
and various minor options.

Here are the S3A properties for use in production; some testing-related
options are covered in [Testing](./testing.md).

```xml
<property>
  <name>fs.s3a.connection.maximum</name>
  <value>15</value>
  <description>Controls the maximum number of simultaneous connections to S3.</description>
</property>

<property>
  <name>fs.s3a.connection.ssl.enabled</name>
  <value>true</value>
  <description>Enables or disables SSL connections to S3.</description>
</property>

<property>
  <name>fs.s3a.endpoint</name>
  <description>AWS S3 endpoint to connect to. An up-to-date list is
    provided in the AWS Documentation: regions and endpoints. Without this
    property, the standard region (s3.amazonaws.com) is assumed.
  </description>
</property>

<property>
  <name>fs.s3a.endpoint.region</name>
  <description>AWS S3 region for a bucket, which bypasses the parsing of
    fs.s3a.endpoint to know the region. Would be helpful in avoiding errors
    while using privateLink URL and explicitly set the bucket region.
    If set to a blank string (or 1+ space), falls back to the
    (potentially brittle) SDK region resolution process.
  </description>
</property>

<property>
  <name>fs.s3a.path.style.access</name>
  <value>false</value>
  <description>Enable S3 path style access ie disabling the default virtual hosting behaviour.
    Useful for S3A-compliant storage providers as it removes the need to set up DNS for virtual hosting.
  </description>
</property>

<property>
  <name>fs.s3a.proxy.host</name>
  <description>Hostname of the (optional) proxy server for S3 connections.</description>
</property>

<property>
  <name>fs.s3a.proxy.port</name>
  <description>Proxy server port. If this property is not set
    but fs.s3a.proxy.host is, port 80 or 443 is assumed (consistent with
    the value of fs.s3a.connection.ssl.enabled).</description>
</property>

<property>
  <name>fs.s3a.proxy.username</name>
  <description>Username for authenticating with proxy server.</description>
</property>

<property>
  <name>fs.s3a.proxy.password</name>
  <description>Password for authenticating with proxy server.</description>
</property>

<property>
  <name>fs.s3a.proxy.domain</name>
  <description>Domain for authenticating with proxy server.</description>
</property>

<property>
  <name>fs.s3a.proxy.workstation</name>
  <description>Workstation for authenticating with proxy server.</description>
</property>

<property>
  <name>fs.s3a.attempts.maximum</name>
  <value>20</value>
  <description>How many times we should retry commands on transient errors.</description>
</property>

<property>
  <name>fs.s3a.connection.establish.timeout</name>
  <value>5000</value>
  <description>Socket connection setup timeout in milliseconds.</description>
</property>

<property>
  <name>fs.s3a.connection.timeout</name>
  <value>200000</value>
  <description>Socket connection timeout in milliseconds.</description>
</property>

<property>
  <name>fs.s3a.paging.maximum</name>
  <value>5000</value>
  <description>How many keys to request from S3 when doing
     directory listings at a time.</description>
</property>

<property>
  <name>fs.s3a.threads.max</name>
  <value>10</value>
  <description> Maximum number of concurrent active (part)uploads,
  which each use a thread from the threadpool.</description>
</property>

<property>
  <name>fs.s3a.socket.send.buffer</name>
  <value>8192</value>
  <description>Socket send buffer hint to amazon connector. Represented in bytes.</description>
</property>

<property>
  <name>fs.s3a.socket.recv.buffer</name>
  <value>8192</value>
  <description>Socket receive buffer hint to amazon connector. Represented in bytes.</description>
</property>

<property>
  <name>fs.s3a.threads.keepalivetime</name>
  <value>60</value>
  <description>Number of seconds a thread can be idle before being
    terminated.</description>
</property>

<property>
  <name>fs.s3a.max.total.tasks</name>
  <value>5</value>
  <description>Number of (part)uploads allowed to the queue before
  blocking additional uploads.</description>
</property>

<property>
  <name>fs.s3a.multipart.size</name>
  <value>64M</value>
  <description>How big (in bytes) to split upload or copy operations up into.
    A suffix from the set {K,M,G,T,P} may be used to scale the numeric value.
  </description>
</property>

<property>
  <name>fs.s3a.multipart.threshold</name>
  <value>128MB</value>
  <description>How big (in bytes) to split upload or copy operations up into.
    This also controls the partition size in renamed files, as rename() involves
    copying the source file(s).
    A suffix from the set {K,M,G,T,P} may be used to scale the numeric value.
  </description>
</property>

<property>
  <name>fs.s3a.multiobjectdelete.enable</name>
  <value>true</value>
  <description>When enabled, multiple single-object delete requests are replaced by
    a single 'delete multiple objects'-request, reducing the number of requests.
    Beware: legacy S3-compatible object stores might not support this request.
  </description>
</property>

<property>
  <name>fs.s3a.acl.default</name>
  <description>Set a canned ACL for newly created and copied objects. Value may be Private,
    PublicRead, PublicReadWrite, AuthenticatedRead, LogDeliveryWrite, BucketOwnerRead,
    or BucketOwnerFullControl.
    If set, caller IAM role must have "s3:PutObjectAcl" permission on the bucket.
    </description>
</property>

<property>
  <name>fs.s3a.multipart.purge</name>
  <value>false</value>
  <description>True if you want to purge existing multipart uploads that may not have been
     completed/aborted correctly</description>
</property>

<property>
  <name>fs.s3a.multipart.purge.age</name>
  <value>86400</value>
  <description>Minimum age in seconds of multipart uploads to purge</description>
</property>

<property>
  <name>fs.s3a.signing-algorithm</name>
  <description>Override the default signing algorithm so legacy
    implementations can still be used</description>
</property>

<property>
  <name>fs.s3a.encryption.algorithm</name>
  <description>Specify a server-side encryption or client-side
     encryption algorithm for s3a: file system. Unset by default. It supports the
     following values: 'AES256' (for SSE-S3), 'SSE-KMS', 'SSE-C', and 'CSE-KMS'
  </description>
</property>

<property>
    <name>fs.s3a.encryption.key</name>
    <description>Specific encryption key to use if fs.s3a.encryption.algorithm
        has been set to 'SSE-KMS', 'SSE-C' or 'CSE-KMS'. In the case of SSE-C
    , the value of this property should be the Base64 encoded key. If you are
     using SSE-KMS and leave this property empty, you'll be using your default's
     S3 KMS key, otherwise you should set this property to the specific KMS key
     id. In case of 'CSE-KMS' this value needs to be the AWS-KMS Key ID
     generated from AWS console.
    </description>
</property>

<property>
  <name>fs.s3a.buffer.dir</name>
  <value>${env.LOCAL_DIRS:-${hadoop.tmp.dir}}/s3a</value>
  <description>Comma separated list of directories that will be used to buffer file
    uploads to.
    Yarn container path will be used as default value on yarn applications,
    otherwise fall back to hadoop.tmp.dir
  </description>
</property>

<property>
  <name>fs.s3a.block.size</name>
  <value>32M</value>
  <description>Block size to use when reading files using s3a: file system.
  </description>
</property>

<property>
  <name>fs.s3a.user.agent.prefix</name>
  <value></value>
  <description>
    Sets a custom value that will be prepended to the User-Agent header sent in
    HTTP requests to the S3 back-end by S3AFileSystem.  The User-Agent header
    always includes the Hadoop version number followed by a string generated by
    the AWS SDK.  An example is "User-Agent: Hadoop 2.8.0, aws-sdk-java/1.10.6".
    If this optional property is set, then its value is prepended to create a
    customized User-Agent.  For example, if this configuration property was set
    to "MyApp", then an example of the resulting User-Agent would be
    "User-Agent: MyApp, Hadoop 2.8.0, aws-sdk-java/1.10.6".
  </description>
</property>

<property>
  <name>fs.s3a.impl</name>
  <value>org.apache.hadoop.fs.s3a.S3AFileSystem</value>
  <description>The implementation class of the S3A Filesystem</description>
</property>

<property>
  <name>fs.AbstractFileSystem.s3a.impl</name>
  <value>org.apache.hadoop.fs.s3a.S3A</value>
  <description>The implementation class of the S3A AbstractFileSystem.</description>
</property>

<property>
  <name>fs.s3a.readahead.range</name>
  <value>64K</value>
  <description>Bytes to read ahead during a seek() before closing and
  re-opening the S3 HTTP connection. This option will be overridden if
  any call to setReadahead() is made to an open stream.</description>
</property>

<property>
  <name>fs.s3a.input.async.drain.threshold</name>
  <value>64K</value>
  <description>Bytes to read ahead during a seek() before closing and
  re-opening the S3 HTTP connection. This option will be overridden if
  any call to setReadahead() is made to an open stream.</description>
</property>

<property>
  <name>fs.s3a.list.version</name>
  <value>2</value>
  <description>Select which version of the S3 SDK's List Objects API to use.
  Currently support 2 (default) and 1 (older API).</description>
</property>

<property>
  <name>fs.s3a.connection.request.timeout</name>
  <value>0</value>
  <description>
  Time out on HTTP requests to the AWS service; 0 means no timeout.
  Measured in seconds; the usual time suffixes are all supported

  Important: this is the maximum duration of any AWS service call,
  including upload and copy operations. If non-zero, it must be larger
  than the time to upload multi-megabyte blocks to S3 from the client,
  and to rename many-GB files. Use with care.

  Values that are larger than Integer.MAX_VALUE milliseconds are
  converged to Integer.MAX_VALUE milliseconds
  </description>
</property>

<property>
  <name>fs.s3a.bucket.probe</name>
  <value>0</value>
  <description>
     The value can be 0 (default), 1 or 2.
     When set to 0, bucket existence checks won't be done
     during initialization thus making it faster.
     Though it should be noted that when the bucket is not available in S3,
     or if fs.s3a.endpoint points to the wrong instance of a private S3 store
     consecutive calls like listing, read, write etc. will fail with
     an UnknownStoreException.
     When set to 1, the bucket existence check will be done using the
     V1 API of the S3 protocol which doesn't verify the client's permissions
     to list or read data in the bucket.
     When set to 2, the bucket existence check will be done using the
     V2 API of the S3 protocol which does verify that the
     client has permission to read the bucket.
  </description>
</property>

<property>
  <name>fs.s3a.object.content.encoding</name>
  <value></value>
  <description>
    Content encoding: gzip, deflate, compress, br, etc.
    This will be set in the "Content-Encoding" header of the object,
    and returned in HTTP HEAD/GET requests.
  </description>
</property>

<property>
  <name>fs.s3a.create.storage.class</name>
  <value></value>
  <description>
      Storage class: standard, reduced_redundancy, intelligent_tiering, etc.
      Specify the storage class for S3A PUT object requests.
      If not set the storage class will be null
      and mapped to default standard class on S3.
  </description>
</property>

<property>
  <name>fs.s3a.prefetch.enabled</name>
  <value>false</value>
  <description>
    Enables prefetching and caching when reading from input stream.
  </description>
</property>

<property>
  <name>fs.s3a.prefetch.block.size</name>
  <value>8MB</value>
  <description>
      The size of a single prefetched block of data.
  </description>
</property>

<property>
  <name>fs.s3a.prefetch.block.count</name>
  <value>8</value>
  <description>
      Maximum number of blocks prefetched concurrently at any given time.
  </description>
</property>
```

## <a name="retry_and_recovery"></a>Retry and Recovery

The S3A client makes a best-effort attempt at recovering from network failures;
this section covers the details of what it does.

The S3A divides exceptions returned by the AWS SDK into different categories,
and chooses a different retry policy based on their type and whether or
not the failing operation is idempotent.


### Unrecoverable Problems: Fail Fast

* No object/bucket store: `FileNotFoundException`
* No access permissions: `AccessDeniedException`
* Network errors considered unrecoverable (`UnknownHostException`,
 `NoRouteToHostException`, `AWSRedirectException`).
* Interruptions: `InterruptedIOException`, `InterruptedException`.
* Rejected HTTP requests: `InvalidRequestException`

These are all considered unrecoverable: S3A will make no attempt to recover
from them.

### Possibly Recoverable Problems: Retry

* Connection timeout: `ConnectTimeoutException`. Timeout before
setting up a connection to the S3 endpoint (or proxy).
* HTTP response status code 400, "Bad Request"

The status code 400, Bad Request usually means that the request
is unrecoverable; it's the generic "No" response. Very rarely it
does recover, which is why it is in this category, rather than that
of unrecoverable failures.

These failures will be retried with an exponential sleep interval set in
`fs.s3a.retry.interval`, up to the limit set in `fs.s3a.retry.limit`.


### Only retriable on idempotent operations

Some network failures are considered to be retriable if they occur on
idempotent operations; there's no way to know if they happened
after the request was processed by S3.

* `SocketTimeoutException`: general network failure.
* `EOFException` : the connection was broken while reading data
* "No response from Server" (443, 444) HTTP responses.
* Any other AWS client, service or S3 exception.

These failures will be retried with an exponential sleep interval set in
`fs.s3a.retry.interval`, up to the limit set in `fs.s3a.retry.limit`.

*Important*: DELETE is considered idempotent, hence: `FileSystem.delete()`
and `FileSystem.rename()` will retry their delete requests on any
of these failures.

The issue of whether delete should be idempotent has been a source
of historical controversy in Hadoop.

1. In the absence of any other changes to the object store, a repeated
DELETE request will eventually result in the named object being deleted;
it's a no-op if reprocessed. As indeed, is `Filesystem.delete()`.
1. If another client creates a file under the path, it will be deleted.
1. Any filesystem supporting an atomic `FileSystem.create(path, overwrite=false)`
operation to reject file creation if the path exists MUST NOT consider
delete to be idempotent, because a `create(path, false)` operation will
only succeed if the first `delete()` call has already succeeded.
1. And a second, retried `delete()` call could delete the new data.

Because S3 is eventually consistent *and* doesn't support an
atomic create-no-overwrite operation, the choice is more ambiguous.

Currently S3A considers delete to be
idempotent because it is convenient for many workflows, including the
commit protocols. Just be aware that in the presence of transient failures,
more things may be deleted than expected. (For anyone who considers this to
be the wrong decision: rebuild the `hadoop-aws` module with the constant
`S3AFileSystem.DELETE_CONSIDERED_IDEMPOTENT` set to `false`).






### Throttled requests from S3


When AWS S3 returns a response indicating that requests
from the caller are being throttled, an exponential back-off with
an initial interval and a maximum number of requests.

```xml
<property>
  <name>fs.s3a.retry.throttle.limit</name>
  <value>${fs.s3a.attempts.maximum}</value>
  <description>
    Number of times to retry any throttled request.
  </description>
</property>

<property>
  <name>fs.s3a.retry.throttle.interval</name>
  <value>1000ms</value>
  <description>
    Interval between retry attempts on throttled requests.
  </description>
</property>
```

Notes

1. There is also throttling taking place inside the AWS SDK; this is managed
by the value `fs.s3a.attempts.maximum`.
1. Throttling events are tracked in the S3A filesystem metrics and statistics.
1. Amazon KMS may throttle a customer based on the total rate of uses of
KMS *across all user accounts and applications*.

Throttling of S3 requests is all too common; it is caused by too many clients
trying to access the same shard of S3 Storage. This generally
happen if there are too many reads, those being the most common in Hadoop
applications. This problem is exacerbated by Hive's partitioning
strategy used when storing data, such as partitioning by year and then month.
This results in paths with little or no variation at their start, which ends
up in all the data being stored in the same shard(s).

Here are some expensive operations; the more of these taking place
against part of an S3 bucket, the more load it experiences.
* Many clients trying to list directories or calling `getFileStatus` on
paths (LIST and HEAD requests respectively)
* The GET requests issued when reading data.
* Random IO used when reading columnar data (ORC, Parquet) means that many
more GET requests than a simple one-per-file read.
* The number of active writes to that part of the S3 bucket.

A special case is when enough data has been written into part of an S3 bucket
that S3 decides to split the data across more than one shard: this
is believed to be one by some copy operation which can take some time.
While this is under way, S3 clients access data under these paths will
be throttled more than usual.


Mitigation strategies

1. Use separate buckets for intermediate data/different applications/roles.
1. Use significantly different paths for different datasets in the same bucket.
1. Increase the value of `fs.s3a.retry.throttle.interval` to provide
longer delays between attempts.
1. Reduce the parallelism of the queries. The more tasks trying to access
data in parallel, the more load.
1. Reduce `fs.s3a.threads.max` to reduce the amount of parallel operations
performed by clients.
!. Maybe: increase `fs.s3a.readahead.range` to increase the minimum amount
of data asked for in every GET request, as well as how much data is
skipped in the existing stream before aborting it and creating a new stream.
1. KMS: "consult AWS about increasing your capacity".



## Handling Read-During-Overwrite

Read-during-overwrite is the condition where a writer overwrites a file while
a reader has an open input stream on the file.  Depending on configuration,
the S3AFileSystem may detect this and throw a `RemoteFileChangedException` in
conditions where the reader's input stream might otherwise silently switch over
from reading bytes from the original version of the file to reading bytes from
the new version.

The configurations items controlling this behavior are:

```xml
<property>
  <name>fs.s3a.change.detection.source</name>
  <value>etag</value>
  <description>
    Select which S3 object attribute to use for change detection.
    Currently support 'etag' for S3 object eTags and 'versionid' for
    S3 object version IDs.  Use of version IDs requires object versioning to be
    enabled for each S3 bucket utilized.  Object versioning is disabled on
    buckets by default. When version ID is used, the buckets utilized should
    have versioning enabled before any data is written.
  </description>
</property>

<property>
  <name>fs.s3a.change.detection.mode</name>
  <value>server</value>
  <description>
    Determines how change detection is applied to alert to S3 objects
    rewritten while being read. Value 'server' indicates to apply the attribute
    constraint directly on GetObject requests to S3. Value 'client' means to do a
    client-side comparison of the attribute value returned in the response.  Value
    'server' would not work with third-party S3 implementations that do not
    support these constraints on GetObject. Values 'server' and 'client' generate
    RemoteObjectChangedException when a mismatch is detected.  Value 'warn' works
    like 'client' but generates only a warning.  Value 'none' will ignore change
    detection completely.
  </description>
</property>

<property>
  <name>fs.s3a.change.detection.version.required</name>
  <value>true</value>
  <description>
    Determines if S3 object version attribute defined by
    fs.s3.change.detection.source should be treated as required.  If true and the
    referred attribute is unavailable in an S3 GetObject response,
    NoVersionAttributeException is thrown.  Setting to 'true' is encouraged to
    avoid potential for inconsistent reads with third-party S3 implementations or
    against S3 buckets that have object versioning disabled.
  </description>
</property>
```

In the default configuration, S3 object eTags are used to detect changes.  When
the filesystem retrieves a file from S3 using
[Get Object](https://docs.aws.amazon.com/AmazonS3/latest/API/RESTObjectGET.html),
it captures the eTag and uses that eTag in an `If-Match` condition on each
subsequent request.  If a concurrent writer has overwritten the file, the
'If-Match' condition will fail and a `RemoteFileChangedException` will be thrown.

Even in this default configuration, a new write may not trigger this exception
on an open reader.  For example, if the reader only reads forward in the file
then only a single S3 'Get Object' request is made and the full contents of the
file are streamed from a single response.  An overwrite of the file after the
'Get Object' request would not be seen at all by a reader with an input stream
that had already read the first byte.  Seeks backward on the other hand can
result in new 'Get Object' requests that can trigger the
`RemoteFileChangedException`.

### Change detection with S3 Versions.

It is possible to switch to using the
[S3 object version id](https://docs.aws.amazon.com/AmazonS3/latest/dev/ObjectVersioning.html)
instead of eTag as the change detection mechanism.  Use of this option requires
object versioning to be enabled on any S3 buckets used by the filesystem.  The
benefit of using version id instead of eTag is potentially reduced frequency
of `RemoteFileChangedException`. With object versioning enabled, old versions
of objects remain available after they have been overwritten.
This means an open input stream will still be able to seek backwards after a
concurrent writer has overwritten the file.
The reader will retain their consistent view of the version of the file from
which they read the first byte.
Because the version ID is null for objects written prior to enablement of
object versioning, **this option should only be used when the S3 buckets
have object versioning enabled from the beginning.**

Note: when you rename files the copied files may have a different version number.

### Change Detection Modes.

Configurable change detection mode is the next option.  Different modes are
available primarily for compatibility with third-party S3 implementations which
may not support all change detection mechanisms.

* `server`: the version/etag check is performed on the server by adding
extra headers to the `GET` request. This is the default.
* `client` : check on the client by comparing the eTag/version ID of a
reopened file with the previous version.
This is useful when the implementation doesn't support the `If-Match` header.
* `warn`: check on the client, but only warn on a mismatch, rather than fail.
* `none` do not check. Useful if the implementation doesn't provide eTag
or version ID support at all or you would like to retain previous behavior
where the reader's input stream silently switches over to the new object version
(not recommended).

The final option (`fs.s3a.change.detection.version.required`) is present
primarily to ensure the filesystem doesn't silently ignore the condition
where it is configured to use version ID on a bucket that doesn't have
object versioning enabled or alternatively it is configured to use eTag on
an S3 implementation that doesn't return eTags.

When `true` (default) and 'Get Object' doesn't return eTag or
version ID (depending on configured 'source'), a `NoVersionAttributeException`
will be thrown.  When `false` and and eTag or version ID is not returned,
the stream can be read, but without any version checking.


## <a name="per_bucket_configuration"></a>Configuring different S3 buckets with Per-Bucket Configuration

Different S3 buckets can be accessed with different S3A client configurations.
This allows for different endpoints, data read and write strategies, as well
as login details.

1. All `fs.s3a` options other than a small set of unmodifiable values
 (currently `fs.s3a.impl`) can be set on a per bucket basis.
1. The bucket specific option is set by replacing the `fs.s3a.` prefix on an option
with `fs.s3a.bucket.BUCKETNAME.`, where `BUCKETNAME` is the name of the bucket.
1. When connecting to a bucket, all options explicitly set will override
the base `fs.s3a.` values.

As an example, a configuration could have a base configuration to use the IAM
role information available when deployed in Amazon EC2.

```xml
<property>
  <name>fs.s3a.aws.credentials.provider</name>
  <value>com.amazonaws.auth.InstanceProfileCredentialsProvider</value>
</property>
```

This will become the default authentication mechanism for S3A buckets.

A bucket `s3a://nightly/` used for nightly data can then be given
a session key:

```xml
<property>
  <name>fs.s3a.bucket.nightly.access.key</name>
  <value>AKAACCESSKEY-2</value>
</property>

<property>
  <name>fs.s3a.bucket.nightly.secret.key</name>
  <value>SESSIONSECRETKEY</value>
</property>

<property>
  <name>fs.s3a.bucket.nightly.session.token</name>
  <value>Short-lived-session-token</value>
</property>

<property>
  <name>fs.s3a.bucket.nightly.aws.credentials.provider</name>
  <value>org.apache.hadoop.fs.s3a.TemporaryAWSCredentialsProvider</value>
</property>
```

Finally, the public `s3a://landsat-pds/` bucket can be accessed anonymously:

```xml
<property>
  <name>fs.s3a.bucket.landsat-pds.aws.credentials.provider</name>
  <value>org.apache.hadoop.fs.s3a.AnonymousAWSCredentialsProvider</value>
</property>
```

#### per-bucket configuration and deprecated configuration options

Per-bucket declaration of the deprecated encryption options
will take priority over a global option -even when the
global option uses the newer configuration keys.

This means that when setting encryption options in XML files,
the option, `fs.bucket.BUCKET.fs.s3a.server-side-encryption-algorithm`
will take priority over the global value of `fs.bucket.s3a.encryption.algorithm`.
The same holds for the encryption key option `fs.s3a.encryption.key`
and its predecessor `fs.s3a.server-side-encryption.key`.


For a site configuration of:

```xml
<property>
  <name>fs.s3a.bucket.nightly.server-side-encryption-algorithm</name>
  <value>SSE-KMS</value>
</property>

<property>
  <name>fs.s3a.bucket.nightly.server-side-encryption.key</name>
  <value>arn:aws:kms:eu-west-2:1528130000000:key/753778e4-2d0f-42e6-b894-6a3ae4ea4e5f</value>
</property>

<property>
  <name>fs.s3a.encryption.algorithm</name>
  <value>AES256</value>
</property>

<property>
  <name>fs.s3a.encryption.key</name>
  <value>unset</value>
</property>


```

The bucket "nightly" will be encrypted with SSE-KMS using the KMS key
`arn:aws:kms:eu-west-2:1528130000000:key/753778e4-2d0f-42e6-b894-6a3ae4ea4e5f`

###  <a name="per_bucket_endpoints"></a>Using Per-Bucket Configuration to access data round the world

S3 Buckets are hosted in different "regions", the default being "US-East".
The S3A client talks to this region by default, issuing HTTP requests
to the server `s3.amazonaws.com`.

S3A can work with buckets from any region. Each region has its own
S3 endpoint, documented [by Amazon](http://docs.aws.amazon.com/general/latest/gr/rande.html#s3_region).

1. Applications running in EC2 infrastructure do not pay for IO to/from
*local S3 buckets*. They will be billed for access to remote buckets. Always
use local buckets and local copies of data, wherever possible.
1. The default S3 endpoint can support data IO with any bucket when the V1 request
signing protocol is used.
1. When the V4 signing protocol is used, AWS requires the explicit region endpoint
to be used —hence S3A must be configured to use the specific endpoint. This
is done in the configuration option `fs.s3a.endpoint`.
1. All endpoints other than the default endpoint only support interaction
with buckets local to that S3 instance.

While it is generally simpler to use the default endpoint, working with
V4-signing-only regions (Frankfurt, Seoul) requires the endpoint to be identified.
Expect better performance from direct connections —traceroute will give you some insight.

If the wrong endpoint is used, the request may fail. This may be reported as a 301/redirect error,
or as a 400 Bad Request: take these as cues to check the endpoint setting of
a bucket.

Here is a list of properties defining all AWS S3 regions, current as of June 2017:

```xml
<!--
 This is the default endpoint, which can be used to interact
 with any v2 region.
 -->
<property>
  <name>central.endpoint</name>
  <value>s3.amazonaws.com</value>
</property>

<property>
  <name>canada.endpoint</name>
  <value>s3.ca-central-1.amazonaws.com</value>
</property>

<property>
  <name>frankfurt.endpoint</name>
  <value>s3.eu-central-1.amazonaws.com</value>
</property>

<property>
  <name>ireland.endpoint</name>
  <value>s3-eu-west-1.amazonaws.com</value>
</property>

<property>
  <name>london.endpoint</name>
  <value>s3.eu-west-2.amazonaws.com</value>
</property>

<property>
  <name>mumbai.endpoint</name>
  <value>s3.ap-south-1.amazonaws.com</value>
</property>

<property>
  <name>ohio.endpoint</name>
  <value>s3.us-east-2.amazonaws.com</value>
</property>

<property>
  <name>oregon.endpoint</name>
  <value>s3-us-west-2.amazonaws.com</value>
</property>

<property>
  <name>sao-paolo.endpoint</name>
  <value>s3-sa-east-1.amazonaws.com</value>
</property>

<property>
  <name>seoul.endpoint</name>
  <value>s3.ap-northeast-2.amazonaws.com</value>
</property>

<property>
  <name>singapore.endpoint</name>
  <value>s3-ap-southeast-1.amazonaws.com</value>
</property>

<property>
  <name>sydney.endpoint</name>
  <value>s3-ap-southeast-2.amazonaws.com</value>
</property>

<property>
  <name>tokyo.endpoint</name>
  <value>s3-ap-northeast-1.amazonaws.com</value>
</property>

<property>
  <name>virginia.endpoint</name>
  <value>${central.endpoint}</value>
</property>
```

This list can be used to specify the endpoint of individual buckets, for example
for buckets in the central and EU/Ireland endpoints.

```xml
<property>
  <name>fs.s3a.bucket.landsat-pds.endpoint</name>
  <value>${central.endpoint}</value>
  <description>The endpoint for s3a://landsat-pds URLs</description>
</property>

<property>
  <name>fs.s3a.bucket.eu-dataset.endpoint</name>
  <value>${ireland.endpoint}</value>
  <description>The endpoint for s3a://eu-dataset URLs</description>
</property>
```

Why explicitly declare a bucket bound to the central endpoint? It ensures
that if the default endpoint is changed to a new region, data store in
US-east is still reachable.

## <a name="accesspoints"></a>Configuring S3 AccessPoints usage with S3A
S3a now supports [S3 Access Point](https://aws.amazon.com/s3/features/access-points/) usage which
improves VPC integration with S3 and simplifies your data's permission model because different
policies can be applied now on the Access Point level. For more information about why to use and
how to create them make sure to read the official documentation.

Accessing data through an access point, is done by using its ARN, as opposed to just the bucket name.
You can set the Access Point ARN property using the following per bucket configuration property:
```xml
<property>
    <name>fs.s3a.bucket.sample-bucket.accesspoint.arn</name>
    <value> {ACCESSPOINT_ARN_HERE} </value>
    <description>Configure S3a traffic to use this AccessPoint</description>
</property>
```

This configures access to the `sample-bucket` bucket for S3A, to go through the
new Access Point ARN. So, for example `s3a://sample-bucket/key` will now use your
configured ARN when getting data from S3 instead of your bucket.

The `fs.s3a.accesspoint.required` property can also require all access to S3 to go through Access
Points. This has the advantage of increasing security inside a VPN / VPC as you only allow access
to known sources of data defined through Access Points. In case there is a need to access a bucket
directly (without Access Points) then you can use per bucket overrides to disable this setting on a
bucket by bucket basis i.e. `fs.s3a.bucket.{YOUR-BUCKET}.accesspoint.required`.

```xml
<!-- Require access point only access -->
<property>
    <name>fs.s3a.accesspoint.required</name>
    <value>true</value>
</property>
<!-- Disable it on a per-bucket basis if needed -->
<property>
    <name>fs.s3a.bucket.example-bucket.accesspoint.required</name>
    <value>false</value>
</property>
```

Before using Access Points make sure you're not impacted by the following:
- `ListObjectsV1` is not supported, this is also deprecated on AWS S3 for performance reasons;
- The endpoint for S3 requests will automatically change from `s3.amazonaws.com` to use
`s3-accesspoint.REGION.amazonaws.{com | com.cn}` depending on the Access Point ARN. While
considering endpoints, if you have any custom signers that use the host endpoint property make
sure to update them if needed;

## <a name="requester_pays"></a>Requester Pays buckets

S3A supports buckets with
[Requester Pays](https://docs.aws.amazon.com/AmazonS3/latest/userguide/RequesterPaysBuckets.html)
enabled. When a bucket is configured with requester pays, the requester must cover
the per-request cost.

For requests to be successful, the S3 client must acknowledge that they will pay
for these requests by setting a request flag, usually a header, on each request.

To enable this feature within S3A, configure the `fs.s3a.requester.pays.enabled` property.

```xml
<property>
    <name>fs.s3a.requester.pays.enabled</name>
    <value>true</value>
</property>
```

## <a name="storage_classes"></a>Storage Classes

Amazon S3 offers a range of [Storage Classes](https://aws.amazon.com/s3/storage-classes/)
that you can choose from based on behavior of your applications. By using the right
storage class, you can reduce the cost of your bucket.

S3A uses Standard storage class for PUT object requests by default, which is suitable for
general use cases. To use a specific storage class, set the value in `fs.s3a.create.storage.class` property to
the storage class you want.

```xml
<property>
    <name>fs.s3a.create.storage.class</name>
    <value>intelligent_tiering</value>
</property>
```

Please note that S3A does not support reading from archive storage classes at the moment.
`AccessDeniedException` with InvalidObjectState will be thrown if you're trying to do so.

## <a name="upload"></a>How S3A writes data to S3

The original S3A client implemented file writes by
buffering all data to disk as it was written to the `OutputStream`.
Only when the stream's `close()` method was called would the upload start.

This made output slow, especially on large uploads, and could even
fill up the disk space of small (virtual) disks.

Hadoop 2.7 added the `S3AFastOutputStream` alternative, which Hadoop 2.8 expanded.
It is now considered stable and has replaced the original `S3AOutputStream`,
which is no longer shipped in hadoop.

The "fast" output stream

1.  Uploads large files as blocks with the size set by
    `fs.s3a.multipart.size`. That is: the threshold at which multipart uploads
    begin and the size of each upload are identical. This behavior can be enabled
    or disabled by using the flag `fs.s3a.multipart.uploads.enabled` which by
    default is set to true.
1.  Buffers blocks to disk (default) or in on-heap or off-heap memory.
1.  Uploads blocks in parallel in background threads.
1.  Begins uploading blocks as soon as the buffered data exceeds this partition
    size.
1.  When buffering data to disk, uses the directory/directories listed in
    `fs.s3a.buffer.dir`. The size of data which can be buffered is limited
    to the available disk space.
1.  Generates output statistics as metrics on the filesystem, including
    statistics of active and pending block uploads.
1.  Has the time to `close()` set by the amount of remaining data to upload, rather
    than the total size of the file.

Because it starts uploading while data is still being written, it offers
significant benefits when very large amounts of data are generated.
The in memory buffering mechanisms may also offer speedup when running adjacent to
S3 endpoints, as disks are not used for intermediate data storage.


```xml
<property>
  <name>fs.s3a.fast.upload.buffer</name>
  <value>disk</value>
  <description>
    The buffering mechanism to use.
    Values: disk, array, bytebuffer.

    "disk" will use the directories listed in fs.s3a.buffer.dir as
    the location(s) to save data prior to being uploaded.

    "array" uses arrays in the JVM heap

    "bytebuffer" uses off-heap memory within the JVM.

    Both "array" and "bytebuffer" will consume memory in a single stream up to the number
    of blocks set by:

        fs.s3a.multipart.size * fs.s3a.fast.upload.active.blocks.

    If using either of these mechanisms, keep this value low

    The total number of threads performing work across all threads is set by
    fs.s3a.threads.max, with fs.s3a.max.total.tasks values setting the number of queued
    work items.
  </description>
</property>

<property>
  <name>fs.s3a.multipart.size</name>
  <value>100M</value>
  <description>How big (in bytes) to split upload or copy operations up into.
    A suffix from the set {K,M,G,T,P} may be used to scale the numeric value.
  </description>
</property>

<property>
  <name>fs.s3a.fast.upload.active.blocks</name>
  <value>8</value>
  <description>
    Maximum Number of blocks a single output stream can have
    active (uploading, or queued to the central FileSystem
    instance's pool of queued operations.

    This stops a single stream overloading the shared thread pool.
  </description>
</property>
```

**Notes**

* If the amount of data written to a stream is below that set in `fs.s3a.multipart.size`,
the upload is performed in the `OutputStream.close()` operation —as with
the original output stream.

* The published Hadoop metrics monitor include live queue length and
upload operation counts, so identifying when there is a backlog of work/
a mismatch between data generation rates and network bandwidth. Per-stream
statistics can also be logged by calling `toString()` on the current stream.

* Files being written are still invisible until the write
completes in the `close()` call, which will block until the upload is completed.


### <a name="upload_disk"></a>Buffering upload data on disk `fs.s3a.fast.upload.buffer=disk`

When `fs.s3a.fast.upload.buffer` is set to `disk`, all data is buffered
to local hard disks prior to upload. This minimizes the amount of memory
consumed, and so eliminates heap size as the limiting factor in queued uploads
—exactly as the original "direct to disk" buffering.


```xml
<property>
  <name>fs.s3a.fast.upload.buffer</name>
  <value>disk</value>
</property>

<property>
  <name>fs.s3a.buffer.dir</name>
  <value>${env.LOCAL_DIRS:-${hadoop.tmp.dir}}/s3a</value>
  <description>Comma separated list of directories that will be used to buffer file
    uploads to.
    Yarn container path will be used as default value on yarn applications,
    otherwise fall back to hadoop.tmp.dir
  </description>
</property>
```

This is the default buffer mechanism. The amount of data which can
be buffered is limited by the amount of available disk space.

### <a name="upload_bytebuffer"></a>Buffering upload data in ByteBuffers: `fs.s3a.fast.upload.buffer=bytebuffer`

When `fs.s3a.fast.upload.buffer` is set to `bytebuffer`, all data is buffered
in "Direct" ByteBuffers prior to upload. This *may* be faster than buffering to disk,
and, if disk space is small (for example, tiny EC2 VMs), there may not
be much disk space to buffer with.

The ByteBuffers are created in the memory of the JVM, but not in the Java Heap itself.
The amount of data which can be buffered is
limited by the Java runtime, the operating system, and, for YARN applications,
the amount of memory requested for each container.

The slower the upload bandwidth to S3, the greater the risk of running out
of memory —and so the more care is needed in
[tuning the upload settings](#upload_thread_tuning).


```xml
<property>
  <name>fs.s3a.fast.upload.buffer</name>
  <value>bytebuffer</value>
</property>
```

### <a name="upload_array"></a>Buffering upload data in byte arrays: `fs.s3a.fast.upload.buffer=array`

When `fs.s3a.fast.upload.buffer` is set to `array`, all data is buffered
in byte arrays in the JVM's heap prior to upload.
This *may* be faster than buffering to disk.

The amount of data which can be buffered is limited by the available
size of the JVM heap heap. The slower the write bandwidth to S3, the greater
the risk of heap overflows. This risk can be mitigated by
[tuning the upload settings](#upload_thread_tuning).

```xml
<property>
  <name>fs.s3a.fast.upload.buffer</name>
  <value>array</value>
</property>
```

### <a name="upload_thread_tuning"></a>Upload Thread Tuning

Both the [Array](#upload_array) and [Byte buffer](#upload_bytebuffer)
buffer mechanisms can consume very large amounts of memory, on-heap or
off-heap respectively. The [disk buffer](#upload_disk) mechanism
does not use much memory up, but will consume hard disk capacity.

If there are many output streams being written to in a single process, the
amount of memory or disk used is the multiple of all stream's active memory/disk use.

Careful tuning may be needed to reduce the risk of running out memory, especially
if the data is buffered in memory.

There are a number parameters which can be tuned:

1. The total number of threads available in the filesystem for data
uploads *or any other queued filesystem operation*. This is set in
`fs.s3a.threads.max`

1. The number of operations which can be queued for execution:, *awaiting
a thread*: `fs.s3a.max.total.tasks`

1. The number of blocks which a single output stream can have active,
that is: being uploaded by a thread, or queued in the filesystem thread queue:
`fs.s3a.fast.upload.active.blocks`

1. How long an idle thread can stay in the thread pool before it is retired: `fs.s3a.threads.keepalivetime`


When the maximum allowed number of active blocks of a single stream is reached,
no more blocks can be uploaded from that stream until one or more of those active
blocks' uploads completes. That is: a `write()` call which would trigger an upload
of a now full datablock, will instead block until there is capacity in the queue.

How does that come together?

* As the pool of threads set in `fs.s3a.threads.max` is shared (and intended
to be used across all threads), a larger number here can allow for more
parallel operations. However, as uploads require network bandwidth, adding more
threads does not guarantee speedup.

* The extra queue of tasks for the thread pool (`fs.s3a.max.total.tasks`)
covers all ongoing background S3A operations (future plans include: parallelized
rename operations, asynchronous directory operations).

* When using memory buffering, a small value of `fs.s3a.fast.upload.active.blocks`
limits the amount of memory which can be consumed per stream.

* When using disk buffering a larger value of `fs.s3a.fast.upload.active.blocks`
does not consume much memory. But it may result in a large number of blocks to
compete with other filesystem operations.


We recommend a low value of `fs.s3a.fast.upload.active.blocks`; enough
to start background upload without overloading other parts of the system,
then experiment to see if higher values deliver more throughput —especially
from VMs running on EC2.

```xml

<property>
  <name>fs.s3a.fast.upload.active.blocks</name>
  <value>4</value>
  <description>
    Maximum Number of blocks a single output stream can have
    active (uploading, or queued to the central FileSystem
    instance's pool of queued operations.

    This stops a single stream overloading the shared thread pool.
  </description>
</property>

<property>
  <name>fs.s3a.threads.max</name>
  <value>10</value>
  <description>The total number of threads available in the filesystem for data
    uploads *or any other queued filesystem operation*.</description>
</property>

<property>
  <name>fs.s3a.max.total.tasks</name>
  <value>5</value>
  <description>The number of operations which can be queued for execution</description>
</property>

<property>
  <name>fs.s3a.threads.keepalivetime</name>
  <value>60</value>
  <description>Number of seconds a thread can be idle before being
    terminated.</description>
</property>
```

### <a name="multipart_purge"></a>Cleaning up after partial Upload Failures

There are two mechanisms for cleaning up after leftover multipart
uploads:
- Hadoop s3guard CLI commands for listing and deleting uploads by their
age. Documented in the [S3Guard](./s3guard.html) section.
- The configuration parameter `fs.s3a.multipart.purge`, covered below.

If a large stream write operation is interrupted, there may be
intermediate partitions uploaded to S3 —data which will be billed for.

These charges can be reduced by enabling `fs.s3a.multipart.purge`,
and setting a purge time in seconds, such as 86400 seconds —24 hours.
When an S3A FileSystem instance is instantiated with the purge time greater
than zero, it will, on startup, delete all outstanding partition requests
older than this time.

```xml
<property>
  <name>fs.s3a.multipart.purge</name>
  <value>true</value>
  <description>True if you want to purge existing multipart uploads that may not have been
     completed/aborted correctly</description>
</property>

<property>
  <name>fs.s3a.multipart.purge.age</name>
  <value>86400</value>
  <description>Minimum age in seconds of multipart uploads to purge</description>
</property>
```

If an S3A client is instantiated with `fs.s3a.multipart.purge=true`,
it will delete all out of date uploads *in the entire bucket*. That is: it will affect all
multipart uploads to that bucket, from all applications.

Leaving `fs.s3a.multipart.purge` to its default, `false`,
means that the client will not make any attempt to reset or change the partition
rate.

The best practise for using this option is to disable multipart purges in
normal use of S3A, enabling only in manual/scheduled housekeeping operations.

### S3A "fadvise" input policy support

The S3A Filesystem client supports the notion of input policies, similar
to that of the Posix `fadvise()` API call. This tunes the behavior of the S3A
client to optimise HTTP GET requests for the different use cases.

See [Improving data input performance through fadvise](./performance.html#fadvise)
for the details.

##<a name="metrics"></a>Metrics

S3A metrics can be monitored through Hadoop's metrics2 framework. S3A creates
its own metrics system called s3a-file-system, and each instance of the client
will create its own metrics source, named with a JVM-unique numerical ID.

As a simple example, the following can be added to `hadoop-metrics2.properties`
to write all S3A metrics to a log file every 10 seconds:

    s3a-file-system.sink.my-metrics-config.class=org.apache.hadoop.metrics2.sink.FileSink
    s3a-file-system.sink.my-metrics-config.filename=/var/log/hadoop-yarn/s3a-metrics.out
    *.period=10

Lines in that file will be structured like the following:

    1511208770680 s3aFileSystem.s3aFileSystem: Context=s3aFileSystem, s3aFileSystemId=892b02bb-7b30-4ffe-80ca-3a9935e1d96e, bucket=bucket,
    Hostname=hostname-1.hadoop.apache.com, files_created=1, files_copied=2, files_copied_bytes=10000, files_deleted=5, fake_directories_deleted=3,
    directories_created=3, directories_deleted=0, ignored_errors=0, op_copy_from_local_file=0, op_exists=0, op_get_file_status=15, op_glob_status=0,
    op_is_directory=0, op_is_file=0, op_list_files=0, op_list_located_status=0, op_list_status=3, op_mkdirs=1, op_rename=2, object_copy_requests=0,
    object_delete_requests=6, object_list_requests=23, object_continue_list_requests=0, object_metadata_requests=46, object_multipart_aborted=0,
    object_put_bytes=0, object_put_requests=4, object_put_requests_completed=4, stream_write_failures=0, stream_write_block_uploads=0,
    stream_write_block_uploads_committed=0, stream_write_block_uploads_aborted=0, stream_write_total_time=0, stream_write_total_data=0,
    s3guard_metadatastore_put_path_request=10, s3guard_metadatastore_initialization=0, object_put_requests_active=0, object_put_bytes_pending=0,
    stream_write_block_uploads_active=0, stream_write_block_uploads_pending=0, stream_write_block_uploads_data_pending=0,
    S3guard_metadatastore_put_path_latencyNumOps=0, S3guard_metadatastore_put_path_latency50thPercentileLatency=0,
    S3guard_metadatastore_put_path_latency75thPercentileLatency=0, S3guard_metadatastore_put_path_latency90thPercentileLatency=0,
    S3guard_metadatastore_put_path_latency95thPercentileLatency=0, S3guard_metadatastore_put_path_latency99thPercentileLatency=0

Depending on other configuration, metrics from other systems, contexts, etc. may
also get recorded, for example the following:

    1511208770680 metricssystem.MetricsSystem: Context=metricssystem, Hostname=s3a-metrics-4.gce.cloudera.com, NumActiveSources=1, NumAllSources=1,
    NumActiveSinks=1, NumAllSinks=0, Sink_fileNumOps=2, Sink_fileAvgTime=1.0, Sink_fileDropped=0, Sink_fileQsize=0, SnapshotNumOps=5,
    SnapshotAvgTime=0.0, PublishNumOps=2, PublishAvgTime=0.0, DroppedPubAll=0

Note that low-level metrics from the AWS SDK itself are not currently included
in these metrics.

##<a name="further_reading"></a> Other Topics

### <a name="distcp"></a> Copying Data with distcp

Hadoop's `distcp` tool is often used to copy data between a Hadoop
cluster and Amazon S3.
See [Copying Data Between a Cluster and Amazon S3](https://hortonworks.github.io/hdp-aws/s3-copy-data/index.html)
for details on S3 copying specifically.

The `distcp update` command tries to do incremental updates of data.
It is straightforward to verify when files do not match when they are of
different length, but not when they are the same size.

Distcp addresses this by comparing file checksums on the source and destination
filesystems, which it tries to do *even if the filesystems have incompatible
checksum algorithms*.

The S3A connector can provide the HTTP etag header to the caller as the
checksum of the uploaded file. Doing so will break distcp operations
between hdfs and s3a.

For this reason, the etag-as-checksum feature is disabled by default.

```xml
<property>
  <name>fs.s3a.etag.checksum.enabled</name>
  <value>false</value>
  <description>
    Should calls to getFileChecksum() return the etag value of the remote
    object.
    WARNING: if enabled, distcp operations between HDFS and S3 will fail unless
    -skipcrccheck is set.
  </description>
</property>
```

If enabled, `distcp` between two S3 buckets can use the checksum to compare
objects. Their checksums should be identical if they were either each uploaded
as a single file PUT, or, if in a multipart PUT, in blocks of the same size,
as configured by the value `fs.s3a.multipart.size`.

To disable checksum verification in `distcp`, use the `-skipcrccheck` option:

```bash
hadoop distcp -update -skipcrccheck -numListstatusThreads 40 /user/alice/datasets s3a://alice-backup/datasets
```

### <a name="customsigners"></a> Advanced - Custom Signers

AWS uees request signing to authenticate requests. In general, there should
be no need to override the signers, and the defaults work out of the box.
If, however, this is required - this section talks about how to configure
custom signers. There’s 2 broad config categories to be set - one for
registering a custom signer and another to specify usage.

#### Registering Custom Signers
```xml
<property>
  <name>fs.s3a.custom.signers</name>
  <value>comma separated list of signers</value>
  <!-- Example
  <value>AWS4SignerType,CS1:CS1ClassName,CS2:CS2ClassName:CS2InitClass</value>
  -->
</property>
```
Acceptable value for each custom signer

`SignerName`- this is used in case one of the default signers is being used.
(E.g `AWS4SignerType`, `QueryStringSignerType`, `AWSS3V4SignerType`).
If no custom signers are being used - this value does not need to be set.

`SignerName:SignerClassName` - register a new signer with the specified name,
and the class for this signer.
The Signer Class must implement `com.amazonaws.auth.Signer`.

`SignerName:SignerClassName:SignerInitializerClassName` - similar time above
except also allows for a custom SignerInitializer
(`org.apache.hadoop.fs.s3a.AwsSignerInitializer`) class to be specified.

#### Usage of the Signers
Signers can be set at a per-service level (S3, etc) or a common
signer for all services.

```xml
<property>
  <name>fs.s3a.s3.signing-algorithm</name>
  <value>${S3SignerName}</value>
  <description>Specify the signer for S3</description>
</property>

<property>
  <name>fs.s3a.signing-algorithm</name>
  <value>${SignerName}</value>
</property>
```

For a specific service, the service specific signer is looked up first.
If that is not specified, the common signer is looked up. If this is
not specified as well, SDK settings are used.
