I am using NSURLSession for file upload and need to satisfy the following scenarios: Large file uploads Upload tasks should not be interrupted when the app is running in the background After the file upload is completed, the server returns a JSON data to inform the app of some information. When I debug with the my code attached below, I found that the urlSession(_:task:didCompleteWithError:) method is correctly called after the upload is completed, BUT the urlSession(_:dataTask:didReceive:) method is never called. Since I need to read the response data from the server after a successful file upload, if the urlSession(_:dataTask:didReceive:) method is not called, where should I get the server's response data from? PS: When I change URLSessionConfiguration.background to let config = URLSessionConfiguration.default, I can create the upload task with URLSession.uploadTask(with: request, fromFile: fileURL, completionHandler:) and get the server response data in the completionHandler. import Foundation class FileUploader: NSObject, URLSessionDelegate, URLSessionTaskDelegate, URLSessionDataDelegate { public typealias ProgressHandler = (_ bytesSent: Int64, _ totalBytes: Int64) -> Void public typealias CompletionHandler = (Error?, Data?) -> Void private var session: URLSession! private var responedData: Data?; // to hold data responsed from sever private var progressHandler: ProgressHandler? private var completionHandler: CompletionHandler? override init() { super.init() let config = URLSessionConfiguration.background(withIdentifier: "com.example.LABackgroundSession") session = URLSession(configuration: config, delegate: self, delegateQueue: OperationQueue.main) } func upload(fileURL: URL, to url: URL) { var request = URLRequest(url: url) request.httpMethod = "POST" let uploadTask = session.uploadTask(with: request, fromFile: fileURL) uploadTask.resume() } // MARK: - URLSessionDataDelegate methods public func urlSession(_ session: URLSession, task: URLSessionTask, didSendBodyData bytesSent: Int64, totalBytesSent: Int64, totalBytesExpectedToSend: Int64) { self.progressHandler?(totalBytesSent, totalBytesExpectedToSend); } //This method never called, and there is no other way i can get the response data. func urlSession(_ session: URLSession, dataTask: URLSessionDataTask, didReceive data: Data) { self.responedData?.append(data); } func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) { if let error = error { self.completionHandler?(error, self.responedData) } else { self.completionHandler?(nil, self.responedData); } self.responedData = nil; } }