S3 API

Alluxio supports a RESTful API that is compatible with the basic operations of the Amazon S3 API.

The Alluxio S3 API should be used by applications designed to communicate with an S3-like storage and would benefit from the other features provided by Alluxio, such as data caching, data sharing with file system based applications, and storage system abstraction (e.g., using Ceph instead of S3 as the backing store). For example, a simple application that downloads reports generated by analytic tasks can use the S3 API instead of the more complex file system API.

Prerequisites

To use the S3 API provided in the worker process, you need to modify conf/alluxio-site.properties to include:

alluxio.worker.s3.api.enabled=true

It is recommended to set up a load balancer to distribute the API calls among all the worker nodes. You can consider using different load balancing solutions such as DNS, Nginx, or LVS.

Limitations and Disclaimers

Alluxio Filesystem Limitations

Only top-level Alluxio directories are treated as buckets by the S3 API. Hence the root directory of the Alluxio filesystem is not treated as an S3 bucket. Any root-level objects (eg: alluxio://file) will be inaccessible through the Alluxio S3 API.

Alluxio uses / as a reserved separator. Therefore, any S3 paths with objects or folders named /; s3://example-bucket// will cause undefined behavior.

Also note that the Alluxio filesystem does not handle the following special characters and patterns:

Question mark ('?')
Patterns with period (./ and ../)
Backslash ('\')

No Bucket Virtual Hosting

Virtual hosting of buckets is not supported. S3 clients must utilize path-style requests (i.e: http://s3.amazonaws.com/{bucket}/{object}) rather than http://{bucket}.s3.amazonaws.com/{object}.

S3 Writes Implicitly Overwrite

As described in the AWS S3 docs for PutObject:

Amazon S3 is a distributed system. If it receives multiple write requests for the same object simultaneously, it overwrites all but the last object written. Amazon S3 does not provide object locking; if you need this, make sure to build it into your application layer or use versioning instead.

Note that at the moment the Alluxio S3 API does not support object versioning

Alluxio S3 will overwrite the existing key and the temporary directory for multipart upload.

Folders in ListObjects(V2)

All sub-directories in Alluxio are returned as 0-byte folders when using ListObjects(V2). This behavior is in accordance with if you used the AWS S3 console to create all parent folders for each object.

Tagging & Metadata Limits

To support the Tagging function in S3 API, you need to modify conf/alluxio-site.properties to include:

alluxio.underfs.xattr.change.enabled=true

User-defined tags on buckets & objects are limited to 10 and obey the S3 tag restrictions.

Set the property key alluxio.proxy.s3.tagging.restrictions.enabled=false to disable this behavior.

The maximum size for user-defined metadata in PUT-requests is 2KB by default in accordance with S3 object metadata restrictions.

Set the property key alluxio.proxy.s3.header.metadata.max.size to change this behavior.

Performance

Since the S3 API implementation adopts a redirection mechanism and data zero-copy, it requires the client that supports HTTP redirection to access it.

Global request headers

Header

Content

Description

Authorization

AWS4-HMAC-SHA256 Credential={user}/..., SignedHeaders=..., Signature=...

There is currently no support for access & secret keys in the Alluxio S3 API. The only supported authentication scheme is the SIMPLE authentication type. By default, the user that is used to perform any operations is the user that was used to launch the Alluxio process. Therefore this header is used exclusively to specify an Alluxio ACL username to perform an operation with. In order to remain compatible with other S3 clients, the header is still expected to follow the AWS Signature Version 4 format. When supplying an access key to an S3 client, put the intended Alluxio ACL username. The secret key is unused, so you may use any dummy value.