Request timeout when app is in background

Hi,

We have an enterprise application and we are trying to download couple of files when application is in background.

We have used URLSession to execute the downloadTask. We use timeout value of 15 mins for each file. BackgroundFetch is enabled for the app.

Now we are facing problem sometimes when application tries to download file in background which receives request timeout.

In the ipa logs we can see that download has been started (that means code to download file has been executed) However we do not receive any chunks of data back. and after waiting for timeout value 15 mins, we get request timeout error in delegate method -

didCompleteWithError:

This do not happen every time. We also checked there is no latency in our network or any network delays.

We came across an observation that as soon as we connect device to the charger (device battery should be > 10%), download starts immediately in background reliably.

So we thought that iOS might have some threshold battery value above which it gives priority to background tasks. But that was not true!

We had faced this issue even when our client's iPad battery was > 90% (charger not connected)

The files we are downloading are with size varying from 100kbs - 50mb

We have received request timeout for small files also of 250kbs

It only works perfectly in background when device is connected to charger OR application is in foreground mode.

We are aware that background tasks should not be taken granted and should be performed for time insensitive tasks but wanted to know more about this behaviour of iOS where it performs downloading instantly as soon as charger is connected.

While researching more we came across a link which explains similar scenario throughly.

https://topologyeyewear.github.io/engineering-blog/2017/11/20/background_transfer/

We are using below API calls.

func backgroundUrlSessionWithIdentifier(_ identifier: String) -> URLSession {
        let config = URLSessionConfiguration.background(withIdentifier: "\(identifier)")
        config.timeoutIntervalForResource = 900.00
        config.isDiscretionary = false
        config.sessionSendsLaunchEvents = true
        print("Downloading start progress NetworkManager ::  \(#function)")
        return URLSession(configuration: config, delegate: self, delegateQueue: OperationQueue())
    }


        let session = backgroundUrlSessionWithIdentifier(identifier)
        let request = URLRequest(url: url)
        let donwloadTask = session.downloadTask(with: request)
        donwloadTask?.resume()