Multipart Upload

Uploading objects using multipart upload

What is Multipart Upload?

Multipart upload allows you to upload a single object as a set of parts. Each part is a contiguous portion of the object's data. You can upload these object parts independently and in any order. If transmission of any part fails, you can retransmit that part without affecting other parts. After all parts of your object are uploaded, OORT Storage assembles these parts and creates the object. In general, when your object size reaches 100 MB, you should consider using multipart uploads instead of uploading the object in a single operation.

Using multipart upload provides the following advantages:

  • Improved throughput – You can upload parts in parallel to improve throughput.

  • Quick recovery from any network issues – Smaller part size minimizes the impact of restarting a failed upload due to a network error.

  • Pause and resume object uploads – You can upload object parts over time. After you initiate a multipart upload, there is no expiry; you must explicitly complete or stop the multipart upload.

  • Begin an upload before you know the final object size – You can upload an object as you are creating it.

We recommend that you use multipart upload in the following ways:

  • If you're uploading large objects over a stable high-bandwidth network, use multipart upload to maximize the use of your available bandwidth by uploading object parts in parallel for multi-threaded performance.

  • If you're uploading over a spotty network, use multipart upload to increase resiliency to network errors by avoiding upload restarts. When using multipart upload, you need to retry uploading only the parts that are interrupted during the upload. You don't need to restart uploading your object from the beginning.

Multipart upload process

Multipart upload is a three-step process: You initiate the upload, you upload the object parts, and after you have uploaded all the parts, you complete the multipart upload. Upon receiving the complete multipart upload request, OORT Storage constructs the object from the uploaded parts, and you can then access the object just as you would any other object in your bucket.

You can list all of your in-progress multipart uploads or get a list of the parts that you have uploaded for a specific multipart upload. Each of these operations is explained in this section.

Multipart upload initiation

When you send a request to initiate a multipart upload, OORT Storage returns a response with an upload ID, which is a unique identifier for your multipart upload. You must include this upload ID whenever you upload parts, list the parts, complete an upload, or stop an upload. If you want to provide any metadata describing the object being uploaded, you must provide it in the request to initiate multipart upload.

Parts upload

When uploading a part, in addition to the upload ID, you must specify a part number. You can choose any part number between 1 and 10,000. A part number uniquely identifies a part and its position in the object you are uploading. The part number that you choose doesn’t need to be in a consecutive sequence (for example, it can be 1, 5, and 14). If you upload a new part using the same part number as a previously uploaded part, the previously uploaded part is overwritten.

Whenever you upload a part, OORT Storage returns an entity tag (ETag) header in its response. For each part upload, you must record the part number and the ETag value. You must include these values in the subsequent request to complete the multipart upload.


After you initiate a multipart upload and upload one or more parts, you must either complete or stop the multipart upload to stop getting charged for storage of the uploaded parts.

After stopping a multipart upload, you cannot upload any part using that upload ID again. If any part uploads were in-progress, they can still succeed or fail even after you stop the upload. To make sure you free all storage consumed by all parts, you must stop a multipart upload only after all part uploads have been completed.

Multipart upload completion

When you complete a multipart upload, OORT Storage creates an object by concatenating the parts in ascending order based on the part number. If any object metadata was provided in the initiate multipart upload request, Amazon S3 associates that metadata with the object. After a successful complete request, the parts no longer exist.

Your complete multipart upload request must include the upload ID and a list of both part numbers and corresponding ETag values. The OORT Storage response includes an ETag that uniquely identifies the combined object data. This ETag is not necessarily an MD5 hash of the object data.

Sample multipart upload calls

For this example, assume that you are generating a multipart upload for a 100 GB file. In this case, you would have the following API calls for the entire process. There would be a total of 1002 API calls.

  • A CreateMultipartUpload call to start the process.

  • 1000 individual UploadPart calls, each uploading a part of 100 MB, for a total size of 100 GB.

  • A CompleteMultipartUpload call to finish the process.

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