Edit3: Okay, it seems like it's an issue with Firebase, someone else tweeted about having the same issue. I also contacted support.
A piece of Swift code that handles creating documents suddenly stopped working. No errors are thrown, Firebase doesn't complain in the log and I can verify from the console that the document is not created, I can verify that the device has a healthy internet connection. I also disabled offline persistence for Firebase just to be sure.
When I try debugging it, the debugger jumps straight over the block that handles errors or successes, never running it (i.e. never finishing the Firestore request?).
Here is the code
func createConversation(){
let conversation : [String : Any] = ["owners" : [
UserProfile().getProfile().uid!],
"seeking" : true,
"timestamp" : Timestamp(date: Date())
]
var ref: DocumentReference? = nil
ref = DB().firestore().collection("Conversations").addDocument(data: conversation){ err in
if let err = err {
print("Error creating a convo: \(err)")
} else {
print("Conversation created with ID: \(ref!.documentID)")
StateMachine().action(a: .seekingStarted(ref!.documentID))
}
print("Conversation Creation finished")
}
let documentID = ref?.documentID
print(ref.debugDescription)
}
I'm not sure how to approach this issue, any ideas?
Edit: Okay, the issue is not limited to this block of code, it looks like Firebase is not communicating with the servers. I've waited for more than 5min for the addDocument to return(with error or success) but that never happened.
I noticed that at the initiation of the App BoringSSL complains a bit but this is not new and I don't have problems with the other Firebase services, they work just fine - reading and creating data with no problems.
Edit2: Apparently I can fetch collections and documents from Firestore, the issue seems to be limited to document/collection creation.
The document creation operation takes a little time, if you place the breakpoint inside the asynchronus completion block you will surely get an error or success.
I am developing a app which has the connection with Firebase DB. This is one part of my code,
let ref1 = Database.database().reference()
ref1.child("UserId").child("\(user_id)").setValue(["name": "\(name)"])
And this one is working fine in simulator(updating value in firebase DB), but not in real device. I have tried some solutions from this site, still I can't get the output.
Is anyone there to help me?
This could be because you are not authenticated on your device or any other reasons. In order to debug it, I suggest you to add a completion block for your setValue which is going to be called whenever the write has been committed to the database. If the call was unsuccessful for any reasons, you can get the error object and take a look at the error.
yourRef.setValue(["name": "\(name)"]) { (error, dbRef) in
if (error != nil) {
//catch the error here
print("Data could not be saved.")
} else {
print("Data saved successfully!")
}
}
At last, the issue was the real device's date and time. The date and time was incorrect in my device. So, the connection with firebase DB failed. Now, it's fixed. Thanks for you suggestions!!
In short
We have a mobile app that streams fairly high volumes of data to and from a server through various bidirectional streams. The streams need to be closed on occasion (for example when the app is backgrounded). They are then reopened as needed. Sometimes when this happens, something goes wrong:
From what I can tell, the stream is up and running on the device's side (the status of both the GRPCProtocall and the GRXWriter involved is either started or paused)
The device sends data on the stream fine (the server receives the data)
The server seems to send data back to the device fine (the server's Stream.Send calls return as successful)
On the device, the result handler for data received on the stream is never called
More detail
Our code is heavily simplified below, but this should hopefully provide enough detail to indicate what we're doing. A bidirection stream is managed by a Switch class:
class Switch {
/** The protocall over which we send and receive data */
var protocall: GRPCProtoCall?
/** The writer object that writes data to the protocall. */
var writer: GRXBufferedPipe?
/** A static GRPCProtoService as per the .proto */
static let service = APPDataService(host: Settings.grpcHost)
/** A response handler. APPData is the datatype defined by the .proto. */
func rpcResponse(done: Bool, response: APPData?, error: Error?) {
NSLog("Response received")
// Handle response...
}
func start() {
// Create a (new) instance of the writer
// (A writer cannot be used on multiple protocalls)
self.writer = GRXBufferedPipe()
// Setup the protocall
self.protocall = Switch.service.rpcToStream(withRequestWriter: self.writer!, eventHandler: self.rpcRespose(done:response:error:))
// Start the stream
self.protocall.start()
}
func stop() {
// Stop the writer if it is started.
if self.writer.state == .started || self.writer.state == .paused {
self.writer.finishWithError(nil)
}
// Stop the proto call if it is started
if self.protocall?.state == .started || self.protocall?.state == .paused {
protocall?.cancel()
}
self.protocall = nil
}
private var needsRestart: Bool {
if let protocall = self.protocall {
if protocall.state == .notStarted || protocall.state == .finished {
// protocall exists, but isn't running.
return true
} else if writer.state == .notStarted || writer.state == .finished {
// writer isn't running
return true
} else {
// protocall and writer are running
return false
}
} else {
// protocall doesn't exist.
return true
}
}
func restartIfNeeded() {
guard self.needsRestart else { return }
self.stop()
self.start()
}
func write(data: APPData) {
self.writer.writeValue(data)
}
}
Like I said, heavily simplified, but it shows how we start, stop, and restart streams, and how we check whether a stream is healthy.
When the app is backgrounded, we call stop(). When it is foregrounded and we need the stream again, we call start(). And we periodically call restartIfNeeded(), eg. when screens that use the stream come into view.
As I mentioned above, what happens occasionally is that our response handler (rpcResponse) stops getting called when server writes data to the stream. The stream appears to be healthy (server receives the data we write to it, and protocall.state is neither .notStarted nor .finished). But not even the log on the first line of the response handler is executed.
First question: Are we managing the streams correctly, or is our way of stopping and restarting streams prone to errors? If so, what is the correct way of doing something like this?
Second question: How do we debug this? Everything we could think of that we can query for a status tells us that the stream is up and running, but it feels like the objc gRPC library keeps a lot of its mechanics hidden from us. Is there a way to see whether responses from server may do reach us, but fail to trigger our response handler?
Third question: As per the code above, we use the GRXBufferedPipe provided by the library. Its documentation advises against using it in production because it doesn't have a push-back mechanism. To our understanding, the writer is only used to feed data to the gRPC core in a synchronised, one-at-a-time fashion, and since server receives data from us fine, we don't think this is an issue. Are we wrong though? Is the writer also involved in feeding data received from server to our response handler? I.e. if the writer broke due to overload, could that manifest as a problem reading data from the stream, rather than writing to it?
UPDATE: Over a year after asking this, we have finally found a deadlock bug in our server-side code that was causing this behaviour on client-side. The streams appeared to hang because no communication sent by the client was handled by server, and vice-versa, but the streams were actually alive and well. The accepted answer provides good advice for how to manage these bi-directional streams, which I believe is still valuable (it helped us a lot!). But the issue was actually due to a programming error.
Also, for anyone running into this type of issue, it might be worth investigating whether you're experiencing this known issue where a channel gets silently dropped when iOS changes its network. This readme provides instructions for using Apple's CFStream API rather than TCP sockets as a possible fix for that issue.
First question: Are we managing the streams correctly, or is our way of stopping and restarting streams prone to errors? If so, what is the correct way of doing something like this?
From what I can tell by looking at your code, the start() function seems to be right. In the stop() function, you do not need to call cancel() of self.protocall; the call will be finished with the previous self.writer.finishWithError(nil).
needsrestart() is where it gets a bit messy. First, you are not supposed to poll/set the state of protocall yourself. That state is altered by itself. Second, setting those state does not close your stream. It only pause a writer, and if app is in background, pausing a writer is like a no-op. If you want to close a stream, you should use finishWithError to terminate this call, and maybe start a new call later when needed.
Second question: How do we debug this?
One way is to turn on gRPC log (GRPC_TRACE and GRPC_VERBOSITY). Another way is to set breakpoint at here where gRPC objc library receives a gRPC message from the server.
Third question: Is the writer also involved in feeding data received from server to our response handler?
No. If you create a buffered pipe and feed that as request of your call, it only feed data to be sent to server. The receiving path is handled by another writer (which is in fact your protocall object).
I don't see where the usage of GRXBufferedPipe in production is discouraged. The known drawback about this utility is that if you pause the writer but keep writing data to it with writeWithValue, you end up buffering a lot of data without being able to flush them, which may cause memory issue.
I'm trying to figure out how I can retry an API call at the ViewController layer when dealing with an RxSwift stream. I understand that I can either wrap my errors in an Element or I can return a stream Error. In either case, it's not at all clear how I can "retry" the operation the discretion of the user (e.g. after putting up an alert).
If I wrap the error, how can/should I communicate back "down" to the API layer to attempt a retry? If I don't wrap the error, I have two questions:
1) How can I make the retry conditional on the user's response to the alert? and
2) How can/should I "re-initialize" the stream?
The only thing I've been able to come up with is passing back a "retry subject" with the error and having the view controller indicate the retry request by emitting something on the associated observable (i.e. assign a value to the subject) which would in term be associated with the API stream. That seems awfully convoluted, however.
More generally, I'd appreciate any references to helpful literature on the subject of application error handling with streams/observables. I feel like I have a pretty good grasp of the RxSwift objects and operators and how to handle the "happy path" situation, but it's just not clear to me how to robustly handle errors in the context of continuously running application.
My solution for this is to wrap them to this type Result<AnyObject,ErrorType>
For example, user login scenario
// where API.login() takes the response data and try to map to JSON
// then try to map into Result<JSON,ErrorType>
let loginStream: Result<JSON,ErrorType> = loginTaps.flatMap { API.login() }
.shareReplay()
let loginResponse = loginStream.filter { $0.value != nil }.map { $0.value }
let loginError = loginStream.filter { $0.error != nil }.map { $0.error }
The stream will never be terminated, so no need to re-subscribe them.
In my application I need to download a JSON file from the web. I have made a ResourceService class that have a download method as seen below. I use this service in "higher level" services of my app. You can see there are multiple of things that may go wrong during the download. The server could be on fire and not be able to successfully respond at the moment, there could be go something wrong during the moving of the temporary file etc.
Now, there is probably not much a user can do with this other than trying later. However, he/she probably want to know that there was something wrong and that the download or the behaviour of the "higher level" methods could not succeed.
Me as a developer is confused as this point because I don't understand how to deal with errors in Swift. I have a completionHandler that takes an error if there was one, but I don't know what kind of error I should pass back to the caller.
Thoughts:
1) If I pass the error objects I get from the NSFileManager API or the NSURLSession API, I would think that I am "leaking" some of the implementation of download method to the callers. And how would the caller know what kind of errors to expect based on the error? It could be both.
2) If I am supposed to catch and wrap those errors that could happen inside the download method, how would that look like?
3) How do I deal with multiple error sources inside a method, and how would the code that calls the method that may throw/return NSError objects look like?
Should you as a caller start intercepting the errors you get back and then write a lot of code that differentiates the messages/action taken based on the error code? I don't get this error handling stuff at all and how it would look like when there are many things that could go wrong in a single method.
func download(destinationUrl: NSURL, completionHandler: ((error: NSError?) -> Void)) {
let request = NSURLRequest(URL: resourceUrl!)
let task = downloadSession.downloadTaskWithRequest(request) {
(url: NSURL?, response: NSURLResponse?, error: NSError?) in
if error == nil {
do {
try self.fileManager.moveItemAtURL(url!, toURL: destinationUrl)
} catch let e {
print(e)
}
} else {
}
}.resume()
}
First of all this is a great question. Error handling is a specific task that applies to a incredible array of situations with who know's what repercussions with your App's state. The key issue is what is meaningful to your user, app and you the developer.
I like to see this conceptually as how the Responder chain is used to handle events. Like an event traversing the responder chain an error has the possibility of bubbling up your App's levels of abstraction. Depending on the error you might want to do a number of things related to the type of the error. Different components of your app may need to know about error, it maybe an error that depending on the state of the app requires no action.
You as the developer ultimately know where errors effect your app and how. So given that how do we choose to implement a technical solution.
I would suggest using Enumerations and Closures as to build my error handling solution.
Here's a contrived example of an ENUM. As you can see it is represents the core of the error handling solution.
public enum MyAppErrorCode {
case NotStartedCode(Int, String)
case ResponseOkCode
case ServiceInProgressCode(Int, String)
case ServiceCancelledCode(Int, String, NSError)
func handleCode(errorCode: MyAppErrorCode) {
switch(errorCode) {
case NotStartedCode(let code, let message):
print("code: \(code)")
print("message: \(message)")
case ResponseOkCode:
break
case ServiceInProgressCode(let code, let message):
print("code: \(code)")
print("message: \(message)")
case ServiceCancelledCode(let code, let message, let error):
print("code: \(code)")
print("message: \(message)")
print("error: \(error.localizedDescription)")
}
}
}
Next we want to define our completionHandler which will replace ((error: NSError?) -> Void) the closure you have in your download method.
((errorCode: MyAppErrorCode) -> Void)
New Download Function
func download(destinationUrl: NSURL, completionHandler: ((errorCode: MyAppErrorCode) -> Void)) {
let request = NSURLRequest(URL: resourceUrl!)
let task = downloadSession.downloadTaskWithRequest(request) {
(url: NSURL?, response: NSURLResponse?, error: NSError?) in
if error == nil {
do {
try self.fileManager.moveItemAtURL(url!, toURL: destinationUrl)
completionHandler(errorCode: MyAppErrorCode.ResponseOkCode)
} catch let e {
print(e)
completionHandler(errorCode: MyAppErrorCode.MoveItemFailedCode(170, "Text you would like to display to the user..", e))
}
} else {
completionHandler(errorCode: MyAppErrorCode.DownloadFailedCode(404, "Text you would like to display to the user.."))
}
}.resume()
}
In the closure you pass in you could call handleCode(errorCode: MyAppErrorCode) or any other function you have defined on the ENUM.
You have now the components to define your own error handling solution that is easy to tailor to your app and which you can use to map http codes and any other third party error/response codes to something meaningful in your app. You can also choose if it is useful to let the NSError bubble up.
EDIT
Back to our contrivances.
How do we deal with interacting with our view controllers? We can choose to have a centralized mechanism as we have now or we could handle it in the view controller and keep the scope local. For that we would move the logic from the ENUM to the view controller and target the very specific requirements of our view controller's task (downloading in this case), you could also move the ENUM to the view controller's scope. We achieve encapsulation, but will most lightly end up repeating our code elsewhere in the project. Either way your view controller is going to have to do something with the error/result code
An approach I prefer would be to give the view controller a chance to handle specific behavior in the completion handler, or/then pass it to our ENUM for more general behavior such as sending out a notification that the download had finished, updating app state or just throwing up a AlertViewController with a single action for 'OK'.
We do this by adding methods to our view controller that can be passed the MyAppErrorCode ENUM and any related variables (URL, Request...) and add any instance variables to keep track of our task, i.e. a different URL, or the number of attempts before we give up on trying to do the download.
Here is a possible method for handling the download at the view controller:
func didCompleteDownloadWithResult(resultCode: MyAppErrorCode, request: NSURLRequest, url: NSURL) {
switch(resultCode) {
case .ResponseOkCode:
// Made up method as an example
resultCode.postSuccessfulDownloadNotification(url, dictionary: ["request" : request])
case .FailedDownloadCode(let code, let message, let error):
if numberOfAttempts = maximumAttempts {
// Made up method as an example
finishedAttemptingDownload()
} else {
// Made up method as an example
AttemptDownload(numberOfAttempts)
}
default:
break
}
}
Long story short: yes
... and then write a lot of code that differentiates the
messages/action taken based on the error code?
Most code examples leave the programmer alone about how to do any error handling at all, but in order to do it right, your error handling code might be more than the code for successful responses. Especially when it comes to networking and json parsing.
In one of my last projects (a lot of stateful json server communication) I have implemented the following approach: I have asked myself: How should the app possibly react to the user in case of an error (and translate it to be more user friendly)?
ignore it
show a message/ an alert (possibly only one)
retry by itself (how often?)
force the user to start over
assume (i.e. a previously cached response)
To achieve this, I have create a central ErrorHandler class, which does have several enums for the different types of errors (i.e. enum NetworkResponseCode, ServerReturnCode, LocationStatusCode) and one enum for the different ErrorDomains:
enum MyErrorDomain : String {
// if request data has errors (i.e. json not valid)
case NetworkRequestDomain = "NetworkRequest"
// if network response has error (i.e. offline or http status code != 200)
case NetworkResponseDomain = "NetworkResponse"
// server return code in json: value of JSONxxx_JSON_PARAM_xxx_RETURN_CODE
case ServerReturnDomain = "ServerReturnCode"
// server return code in json: value of JSONxxxStatus_xxx_JSON_PARAM_xxx_STATUS_CODE
case ServerStatusDomain = "ServerStatus"
// if CLAuthorizationStatus
case LocationStatusDomain = "LocationStatus"
....
}
Furthermore there exists some helper functions named createError. These methods do some checking of the error condition (i.e. network errors are different if you are offline or if the server response !=200). They are shorter than you would expect.
And to put it all together there is a function which handles the error.
func handleError(error: NSError, msgType: String, shouldSuppressAlert: Bool = false){
...
}
This method started with on switch statement (and needs some refactoring now, so I won't show it as it still is one). In this statement all possible reactions are implemented. You might need a different return type to keep your state correctly in the app.
Lessons learned:
Although I thought that I have started big (different enums, central user alerting), the architecture could have been better (i.e. multiple classes, inheritance, ...).
I needed to keep track of previous errors (as some are follow ups) in order to only show one error message to the user -> state.
There are good reasons to hide errors.
Within the errorObj.userInfo map, it exits a user friendly error message and a technicalErrorMessage (which is send to a tracking provider).
We have introduced numeric error codes (the error domain is prefixed with a letter) which are consistent between client and server. They are also shown to the user. This has really helped to track bugs.
I have implemented a handleSoftwareBug function (which is almost the same as the handleError but much less cases). It is used in a lot of else-blocks which you normally do not bother to write (as you think that this state can never be reached). Surprisingly it can.
ErrorHandler.sharedInstance.handleSoftwareBug("SW bug? Unknown received error code string was code: \(code)")
How does it look like in code: There are a lot of similar backend network requests where a lot of code looks something like the following:
func postAllXXX(completionHandler:(JSON!, NSError!) -> Void) -> RegisteringSessionTask {
log.function()
return postRegistered(jsonDict: self.jsonFactory.allXXX(),
outgoingMsgType: JSONClientMessageToServerAllXXX,
expectedIncomingUserDataType: JSONServerResponseAllXXX,
completionHandler: {(json, error) in
if error != nil {
log.error("error: \(error.localizedDescription)")
ErrorHandler.sharedInstance.handleError(error,
msgType: JSONServerResponseAllXXX, shouldSuppressAlert: true)
dispatch_async(dispatch_get_main_queue(), {
completionHandler(json, error)
})
return
}
// handle request payload
var returnList:[XXX] = []
let xxxList = json[JSONServerResponse_PARAM_XXX][JSONServerResponse_PARAM_YYY].arrayValue
.....
dispatch_async(dispatch_get_main_queue(), {
completionHandler(json, error)
})
})
}
Within the above code you see that I call a completionHandler and give this caller the chance to customize error handling, too. Most of the time, this caller only handles success.
Whenever I have had the need for retries and other and not so common handling, I have also done it on the caller side, i.e.
private func postXXXMessageInternal(completionHandler:(JSON!, NSError!) -> Void) -> NSURLSessionDataTask {
log.function()
return self.networkquery.postServerJsonEphemeral(url, jsonDict: self.jsonFactory.xxxMessage(),
outgoingMsgType: JSONClientMessageToServerXXXMessage,
expectedIncomingUserDataType: JSONServerResponseXXXMessage,
completionHandler: {(json, error) in
if error != nil {
self.xxxMessageErrorWaitingCounter++
log.error("error(\(self.xxxMessageErrorWaitingCounter)): \(error.localizedDescription)")
if (something || somethingelse) &&
self.xxxMessageErrorWaitingCounter >= MAX_ERROR_XXX_MESSAGE_WAITING {
// reset app because of too many errors
xxx.currentState = AppState.yyy
ErrorHandler.sharedInstance.genericError(MAX_ERROR_XXX_MESSAGE_WAITING, shouldSuppressAlert: false)
dispatch_async(dispatch_get_main_queue(), {
completionHandler(json, nil)
})
self.xxxMessageErrorWaitingCounter = 0
return
}
// handle request payload
if let msg = json[JSONServerResponse_PARAM_XXX][JSONServerResponse_PARAM_ZZZ].stringValue {
.....
}
.....
dispatch_async(dispatch_get_main_queue(), {
completionHandler(json, error)
})
})
}
Here is another example where the user is forced to retry
// user did not see a price. should have been fetched earlier (something is wrong), cancel any ongoing requests
ErrorHandler.sharedInstance.handleSoftwareBug("potentially sw bug (or network to slow?): no payment there? user must retry")
if let st = self.sessionTask {
st.cancel()
self.sessionTask = nil
}
// tell user
ErrorHandler.sharedInstance.genericInfo(MESSAGE_XXX_PRICE_REQUIRED)
// send him back
xxx.currentState = AppState.zzz
return
For any request, you get either an error or an http status code. Error means: Your application never managed to talk properly to the server. http status code means: Your application talked to a server. Be aware that if you take your iPhone into the nearest Starbucks, "your application talked to a server" doesn't mean "your application talked to the server it wanted to talk to". It might mean "your application managed to talk to the Starbucks server which asks you to log in and you have no idea how to do that".
I divide the possible errors into categories: "It's a bug in my code". That's where you need to fix your code. "Something went wrong, and the user can do something about it". For example when WiFi is turned off. "Something went wrong, maybe it works later". You can tell the user to try later. "Something went wrong, and the user can't do anything about it". Tough. "I got a reply from the server that I expected. Maybe an error, maybe not, but something that I know how to handle". You handle it.
I also divide calls into categories: Those that should run invisibly in the background, and those that run as a result of a direct user action. Things running invisibly in the background shouldn't give error messages. (Bloody iTunes telling me it cannot connect to the iTunes Store when I had no interest in connecting to the iTunes Store in the first place is an awful example of getting that wrong).
When you show things to the user, remember that the user doesn't care. To the user: Either it worked, or it didn't work. If it didn't work, the user can fix the problem if it is a problem they can fix, they can try again later, or it's just tough luck. In an enterprise app, you might have a message "call your help desk at xxxxxx and tell them yyyyyy".
And when things don't work, don't annoy the user by showing error after error after error. If you send then requests, don't tell the user ten times that the server is on fire.
There are things that you just don't expect to go wrong. If you download a file, and you can't put it where it belongs, well, that's tough. It shouldn't happen. The user can't do anything about it. (Well, maybe they can. If the storage of the device is full then you can tell the user). Apart from that, it's the same category as "Something went wrong, and the user can't do anything about it". You may find out as a developer what the cause is and fix it, but if it happens with an application out in the user's hands, there's nothing reasonable you can do.
Since all such requests should be asynchronous, you will always pass either one or two callback blocks to the call, one for success and one for failure. I have most of the error handling in the download code, so things like asking the user to turn WiFi on happen only once, and calls may even be repeated automatically if such an error condition is fixed by the user. The error callback is mostly used to inform the application that it won't get the data that it wanted; sometimes the fact that there is an error is useful information in itself.
For consistent error handling, I create my own errors representing either errors returned by the session, or html status codes interpreted as errors. Plus two additional errors "user cancelled" and "no user interaction allowed" if either there was a UI involved and the user cancelled the operation, or I wanted to use some user interaction but wasn't allowed to. The last two errors are different - these errors will never be reported to the user.
I would wrap the errors in your own, but pass the underlying error as a property on your error class (ala C#'s InnerException). That way you are giving consumers a consistent interface, but also providing lower level error detail if required. However, the main reason I would do this is for unit testing. It makes it much easier to mock your ResourceService class and test the code paths for the various errors that could occur.
I don't like the thought of passing back an array of errors, as it adds complexity for the consumer. Instead I would provide an array of InnerException instances. If they are instances of your own error class, they would potentially have their own InnerException's with underlying errors. However, this would probably only make sense if you were doing your own validations where multiple errors might make sense. Your download method will probably have to bail out after the first error encountered.