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S3 Multipart Upload

S3 has a series of multipart upload operations. These can be used to upload an object to S3 in multiple parts. If your object is larger than 5GB you are required to use the multipart operations for uploading, but multipart also has the advantage that if one part fails to upload you don't need to re-upload the whole object, just the parts that failed. Finally you can upload multiple parts at the same time and thus improve your upload speed. The multiple part uploading isn't support by Soto at the moment, but it is on the list of improvements to make. You can read Amazon's documentation on multipart upload here.

Multipart upload has three stages. First you initiate the upload with an S3.CreateMultipartUpload()). Next, you upload each part using S3.UploadPart() and then you complete the upload by calling S3.CompleteMultipartUpload(). If there is an error and you don't want to finish the upload you need to call S3.AbortMultipartUpload(). The code to implement this can get quite complex so Soto provides you with a function that implements all of this for you.

let request = S3.CreateMultipartUploadRequest(bucket: "MyBucket", key: "MyFile.txt")
let responseFuture = s3.multipartUpload(
    partSize: 5*1024*1024,
    filename: "/Users/home/myfile.txt"
    abortOnFail: true,
    on: eventLoop,
    threadPoolProvider: .createNew
) { progress in

The function parameters for multipartUpload are as follows

  • request is the request object you would create to call S3.CreateMultipartUpload.
  • partSize is the size of each part you upload. The minimum size for a part is 5MB.
  • filename is the full path to the file you want to upload.
  • abortOnFail is a flag indicating whether you want S3.AbortMultipartUpload to be called when a part upload fails. If you set this flag to false the abort function will not be called and the error S3ErrorType.multipart.abortedUpload(resumeRequest:error:) will be thrown. This holds an S3.ResumeMultipartUploadRequest object which can be used in the function S3.resumeMultipartUpload to resume the multipart upload. In this situation if you do not call the resume function you should call S3.AbortMultipartUpload with the uploadId contained in the S3.ResumeMultipartUploadRequest to delete the parts you have already uploaded.
  • on indicates the EventLoop the upload should run on.
  • threadPoolProvider: The file loading requires a ThreadPool to run. You can either provide your own, or have the function create its own, which it will destroy once the function is complete.
  • progress is a closure that gets called after every part is loaded. It is called with a value between 0 and 1 indicating how far we are through the multipart upload. You can also use the progress closure as a way to cancel the upload, by throwing an error.

Resuming an upload

As mentioned above if you call s3.multipartUpload(_:filename:abortOnFail:) with abortOnFail set to false, you can resume the upload if it fails. You can use the function resumeMultipartUpload(_:filename:) in the following manner. While not implemented here you can also set the abortOnFail to false again, and resume the upload again if the first resumeMultipartUpload(_:filename:) fails.

let request = S3.CreateMultipartUploadRequest(bucket: name, key: name)
let responseFuture = s3.multipartUpload(
    filename: filename, 
    abortOnFail: false
).flatMapError { error -> EventLoopFuture<S3.CompleteMultipartUploadOutput> in
    switch error {
    case S3ErrorType.multipart.abortedUpload(let resumeRequest, let error):
        return s3.resumeMultipartUpload(resumeRequest, filename: filename) { 
            print("Progress \($0 * 100)")