Multipart Upload Overview

Multipart upload allows users of Riak CS to do the following:

  • upload large objects, potentially multiple terabytes, as a set of smaller parts
  • pause and resume the upload of a large object
  • begin an upload without prior knowledge of the total size of the whole object

In general, multipart uploads tend to be more efficient because parts may be uploaded in parallel. In Riak CS they are designed to both behave like Amazon S3 multipart uploads and to utilize the same user-facing API.

Note on file size limit

The size limit on individual parts of a multipart upload is 5 gigabytes.

There are three phases to a multipart upload: initiation, parts upload, and completion. Each phase is described in more detail below.

Multipart Upload Phases


Initiation is done by sending a properly formatted multipart upload initiation request to Riak CS. If the upload initiation is successful, the response from Riak CS includes an upload ID.

This ID is a unique identifier for a particular multipart upload and must be included with all subsequent requests to Riak CS pertaining to this upload. This includes any of the upload operations described in the remainder of this document except for listing all active multipart uploads.

Metadata may be attached to an object uploaded using multipart upload just like any other object stored in Riak CS. To do so, the metadata should be included with the multipart upload initiation request.

Parts Upload

A part upload must include both the upload ID received in response to an initiation request and a part number. Part numbers should be integers between 1 and 10,000. These numbers identify a part within the context of the multipart upload and also specify positioning within the final object. Uploading a part with a part number that has previously been uploaded results in the previous part associated with that part number being overwritten.

For each part that is uploaded, Riak CS returns an ETag header as part of the response. Each ETag value and the part number it corresponds to should be reserved for use in the multipart upload completion request.

The size of each part may be in the range of 5MB to 5GB.


Once a complete multipart upload request is received, Riak CS assembles the object from the uploaded parts. Subsequently, the object is presented as a single entity to the user with no difference from any other object stored in Riak CS. The uploaded parts are no longer individually accessible.

A complete multipart upload request must include the upload ID and a list of all part numbers and their corresponding ETag values. Riak CS returns another ETag that identifies the completed object. It should be noted that this ETag value is not necessarily an MD5 hash of the object data and that this fact may cause warnings to be issued by some client libraries or tools.

Terminating an upload

Abort Multipart Upload

A multipart upload request may be aborted prior to sending an upload completion request. The storage for all parts that have been fully uploaded will be released.

Part uploads that are in-progress may not have their storage released, so the abort request should only be sent after all parts already in progress have uploaded to ensure that all storage is reclaimed.

Once a multipart upload is aborted, the upload ID is no longer valid.

Listing uploads

Active Multipart Uploads

Riak CS can list all of the active multipart uploads for each user account. The number of multipart uploads included in the response is capped at 1000. If there are more than 1000 active multipart uploads for a particular user account, they can be listed by using multiple requests.

Completed Parts From An Active Upload

Riak CS can list the parts that have been successfully uploaded for a specific multipart upload. If a multipart upload is comprised of more than 1000 parts then the parts must be retrieved using multiple parts requests.

The results of this request are not intended to be used when sending a complete multipart upload request. The proper procedure is to record the part numbers and the associated ETag values returned with part upload responses and use that information when completing a multipart upload.

Storage Calculation

As with Amazon S3, once you initiate a multipart upload, Riak CS retains all of the parts of the upload until it is either completed or aborted. If the upload is aborted, Riak CS deletes all upload artifacts and they will no longer be

For example, if a user has uploaded a 10 GB object via multipart upload without completing the request, the object won’t appear in the list objects result but its object size will be included in the user’s usage statistics.