Downloading multiple HLS audio files simultaneously fails.

I have been trying to multiple HLS audio files together. The files are encrypted with a key. I| need to fetch the key and store it locally for offline use. When I download a small number (2 or 3 files) of files simultaneously it works fine, but if I start downloading 10-15 files simultaneously most of them fails with the error message-

Error Domain=AVFoundationErrorDomain Code=-11800 "The operation could not be completed" UserInfo={NSLocalizedFailureReason=An unknown error occurred (-17377), NSLocalizedDescription=The operation could not be completed}

I am getting error message from NSURLErrorDomain as well but they are rare.

Below is my download function -

class AudioDownloader {

var productKey: String

var downloadUrl: URL

var downloadSession: AVAssetDownloadURLSession?

var fakeDownloadUrl: URL?

var downloadTask: AVAssetDownloadTask?

func downloadAudio() {

if downloadSession == nil {

let configuration = URLSessionConfiguration.background(withIdentifier: self.productKey)

downloadSession = AVAssetDownloadURLSession(configuration: configuration, assetDownloadDelegate: self, delegateQueue: OperationQueue.main)

configuration.shouldUseExtendedBackgroundIdleMode = true

configuration.httpShouldSetCookies = true

configuration.httpShouldUsePipelining = false

configuration.allowsCellularAccess = true

configuration.isDiscretionary = true

}

self.fakeDownloadUrl = self.convertToScheme(url: self.downloadUrl, scheme: "fakehttp")

let asset = AVURLAsset(url: self.fakeDownloadUrl!)

let loader = asset.resourceLoader

loader.setDelegate(self, queue: DispatchQueue(label: "dispatch2"))

self.downloadTask = downloadSession?.makeAssetDownloadTask(asset: asset, assetTitle: "assetTitle \(self.productKey)", assetArtworkData: nil, options: nil)!

self.downloadTask?.taskDescription = self.productKey

self.downloadTask?.resume()

}

}

The link of the approach which I am using to fetch the key for offline use is below -

https://stackoverflow.com/questions/45670774/playing-offline-hls-with-aes-128-encryption-ios

Any help would be appreciated.