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I'm uploading a lot of files to a server using URLSession, because the uploaded files can be big I'm using URLSessionConfiguration.background so that my uploads can continue in the background.

My url session is declared like this:

urlSessionConfiguration = URLSessionConfiguration.background(withIdentifier: UploadQueue.uploadQueueIdentifier)
urlSessionConfiguration.sessionSendsLaunchEvents = true
urlSessionConfiguration.shouldUseExtendedBackgroundIdleMode = true
urlSessionConfiguration.allowsCellularAccess = true
urlSessionConfiguration.sharedContainerIdentifier = appGroup
backgroundUploadSession = URLSession(configuration: urlSessionConfiguration, delegate: self, delegateQueue: nil)

URLSessionUploadTask are always created in the foreground (we use background session only to ensure that tasks can finish).

var request = try! URLRequest(url: url, method: .put)
backgroundUploadSession.uploadTask(with: request, fromFile: fileUrl).resume()

All the necessary delegates are implemented and called normally when uploading ~ 100 files and staying in the foreground. However when uploading a lot of files (~1000), the first files upload correctly but after some time the session seems "stuck" and no callback are delivered. (Still with the app in the foreground) I noticed that if I just wait the upload restarts after ~5 minutes.

I tried replacing URLSessionConfiguration.background(withIdentifier: UploadQueue.uploadQueueIdentifier) with URLSessionConfiguration.default and it's working perfectly in the foreground. Is it a bug with with URLSessionConfiguration.background or am I doing something wrong ?

Philou
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  • Are all of the tasks in a single session? Does the problem go away if you split it across multiple sessions? (I ask because I vaguely recall reading somewhere that NSURLSession starts to misbehave when you get above a couple of hundred tasks.) – dgatwood Mar 03 '21 at 23:38
  • After investigation there were multiple problems one of which is that all the tasks were in the same session (I will answer this post for documentation, thank you for taking the time to answer) – Philou Mar 04 '21 at 16:21

1 Answers1

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After investigating there were multiple problems:

  1. We were expecting that URLSessionConfiguration.httpMaximumConnectionsPerHost would limit the number of open connections. However this limit doesn't seem to be respected when using http/2 or when the server is behind a proxy.

Solution: Do your own connection limiting (eg. with DispatchGroup)

  1. Even when limiting the number of concurrent uploads, the background sessions seem stuck after some time even when the app is in the foreground. There is a rate limiter for background session but the documentation says it is not endorsed while the app is in the background.

Solution: This was discussed on Apple Developer Forum and this is a bug in the CF Network Framework. A bug report has been made. For the time being, the solution is to simply not create too much background session with small files.

Philou
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