I have a scenario where i need to find a method that return a string response from external API. I have two possibilities for this response give me a valid response (with parameter 1 or parameter 2) or if both responses are not valid, return a final empty publisher to chain.
Mono<String> checkResponse(String parameter)
Check if call checkResponse(parameter1) is acceptable, ignore second call (switchIfEmpty) and continue chain, or
Check if call checkResponse(parameter2) is acceptable and continue chain, or
return Mono.Empty() and discard chain
Actually i have
checkResponse(stringArg1)
.switchIfEmpty(checkResponse(stringArg2))
.flatMapMany ...
.flatMap ...
method
public Mono<String> checkResponse(String s)
return webClient.post()
.uri(URI)
.body(BodyInserters.fromValue(s))
.retrieve()
.bodyToMono(String.class)
But switchIfEmpty is always executing.
Regards,
Are you sure that it's actually emitting twice?
There are two aspects of Project Reactor that is important to understand:
On Assembly
On Subscription
This code:
checkResponse(stringArg1)
.switchIfEmpty(checkResponse(stringArg2));
will assemble the Monos for both checkResponse calls.
In essence, the checkResponse-method is called twice - however only the Mono returned from the first checkResponse-call will be subscribed to as long as it emits an item.
You can verify this behaviour with this:
checkResponse(stringArg1)
.doOnSubscribe(s -> System.out.println("First checkResponse subscription"))
.switchIfEmpty(checkResponse(stringArg2)
.doOnSubscribe(s -> System.out.println("Second checkResponse subscription"))
);
Something that's very typical of reactive code is that top-level code within a method that returns a Mono/Flux usually executes at assembly time while all the lambdas passed to their various operators such as map/flatMap/concatMap/etc execute at subscription time.
To illustrate:
public Mono<String> getName(int id) {
// Assembly time
System.out.println("This executes at assembly time");
return userRepo.get(id)
.map(user -> {
// Subscription time
System.out.println("This executes at subscription time");
return user.name;
});
}
If assembling the Mono might be expensive while it may never be subscribed to like in your case here, you can defer assembly until subscription-time using Mono.defer:
checkResponse(stringArg1)
.switchIfEmpty(Mono.defer(() -> checkResponse(stringArg2)));
Actually there's difference between assembly time and subscription time.
Assembly time is when you create your pipeline by building the reactive chain.
Subscription time is when the execution triggered and the data starts to flow. You should consider using callbacks and lambdas since they are lazily evaluated.
So your checkResponse() method called "twice" on assembly time, because it is not a lambda, but just a regular method. And it returns Mono
You can use Mono.defer(() -> checkResponse()) and delay the execution and assembling inner mono until you subscribed.
Related
The view model is given below
class ClickRowViewModel #Inject constructor(
private val clickRowRepository: ClickRowRepository
): ViewModel() {
private val _clickRowsFlow = MutableStateFlow<List<ClickRow>>(mutableListOf())
val clickRowsFlow = _clickRowsFlow.asStateFlow()
fun fetchAndInitialiseClickRows() {
viewModelScope.launch {
_clickRowsFlow.update {
clickRowRepository.fetchClickRows()
}
}
}
}
My test is as follows:
I am using InstantTaskExecutorRule as follows
#get:Rule
val instantTaskExecutorRule = InstantTaskExecutorRule()
The actual value never resolves to the expected value even though $result seems to have two elements but the actualValue is an empty list. I don't know what I am doing wrong.
Update
I tried to use the first terminal operator as well but the returned output returns an empty list.
Update # 2
I tried async but I got the following error
kotlinx.coroutines.test.UncompletedCoroutinesError: After waiting for 60000 ms, the test coroutine is not completing, there were active child jobs: [DeferredCoroutine{Active}#a4a38f0]
at kotlinx.coroutines.test.TestBuildersKt__TestBuildersKt$runTestCoroutine$3$3.invokeSuspend(TestBuilders.kt:342)
Update # 3
This test passes in Android Studio, but fails using CLI
Test failing in CLI
You can't call toList on a SharedFlow like that:
Shared flow never completes. A call to Flow.collect on a shared flow never completes normally, and neither does a coroutine started by the Flow.launchIn function.
So calling toList will hang forever, because the flow never hits an end point where it says "ok that's all the elements", and toList needs to return a final value. Since StateFlow only contains one element at a time anyway, and you're not collecting over a period of time, you probably just want take(1).toList().
Or use first() if you don't want the wrapping list, which it seems you don't - each element in the StateFlow is a List<ClickRow>, which is what clickRowRepository.fetchClickRows() returns too. So expectedValue is a List<ClickRow>, whereas actualValue is a List<List<ClickRow>> - so they wouldn't match anyway!
edit your update (using first()) has a couple of issues.
First of all, the clickRowsFlow StateFlow in your ViewModel only updates when you call fetchAndInitialiseClickRows(), because that's what fetches a value and sets it on the StateFlow. You're not calling that in your second example, so it won't update.
Second, that StateFlow is going to go through two state values, right? The first is the initial empty list, the second is the row contents you get back from the repo. So when you access that StateFlow, it either needs to be after the update has happened, or (better) you need to ignore the first state and only return the second one:
val actualValue = clickRowViewModel.clickRowsFlow
.drop(1) // ignore the initial state
.first() // then take the first result after that
// start the update -after- setting up the flow collection,
// so there's no race condition to worry about
clickRowsViewModel.fetchAndInitialiseClickRows()
This way, you subscribe to the StateFlow and immediately get (and drop) the initial state. Then when the update happens, it should push another value to the subscriber, which takes that first new value as its final result.
But there's another complication - because fetchAndInitialiseClickRows() kicks off its own coroutine and returns immediately, that means the fetch-and-update task is running asynchronously. You need to give it time to finish, before you start asserting any results from it.
One option is to start the coroutine and then block waiting for the result to show up:
// start the update
clickRowsViewModel.fetchAndInitialiseClickRows()
// run the collection as a blocking operation, which completes when you get
// that second result
val actualValue = clickRowViewModel.clickRowsFlow
.drop(1)
.first()
This works so long as fetchAndInitialiseClickRows doesn't complete immediately. That consumer chain up there requires at least two items to be produced while it's subscribed - if it never gets to see the initial state, it'll hang waiting for that second (really a third) value that's never coming. This introduces a race condition and even if it's "probably fine in practice" it still makes the test brittle.
Your other option is to subscribe first, using a coroutine so that execution can continue, and then start the update - that way the subscriber can see the initial state, and then the update that arrives later:
// async is like launch, but it returns a `Deferred` that produces a result later
val actualValue = async {
clickRowViewModel.clickRowsFlow
.drop(1)
.first()
}
// now you can start the update
clickRowsViewModel.fetchAndInitialiseClickRows()
// then use `await` to block until the result is available
assertEquals(expected, actualValue.await())
You always need to make sure you handle waiting on your coroutines, otherwise the test could finish early (i.e. you do your asserting before the results are in). Like in your first example, you're launching a coroutine to populate your list, but not ensuring that has time to complete before you check the list's contents.
In that case you'd have to do something like advanceUntilIdle() - have a look at this section on testing coroutines, it shows you some ways to wait on them. This might also work for the one you're launching with fetchAndInitialiseClickRows (since it says it waits for other coroutines on the scheduler, not the same scope) but I'm not really familiar with it, you could look into it if you like!
The Kotlin flow states the following:
A suspending function asynchronously returns a single value, but how
can we return multiple asynchronously computed values? This is where
Kotlin Flows come in.
However, if the source of my flow is such that when it completes but returns no data, is there a way to determine that from the flow? For example, if the source of the flow calls a backend API but the API returns no data, is there a way to determine when the flow completes and has no data?
There is an onEmpty method that will invoke an action if the flow completes without emitting any items.
It can also be used to emit a special value to the flow to indicate that it was empty. For example, if you have a flow of events you can emit an EmptyDataEvent.
Or you can just do whatever it is that you want to do inside this onEmpty lambda.
Highly related is also the onCompletion method
If the API returns a Flow, it is usually expected to return 0 or more elements. That flow will typically be collected by some piece of code in order to process the values. The collect() call is a suspending function, and will return when the flow completes:
val flow = yourApiCallReturningFlow()
flow.collect { element ->
// process element here
}
// if we're here, the flow has completed
Any other terminal operator like first(), toList(), etc. will handle the flow's completion in some way (and may even cancel the flow early).
I'm not sure about what you're looking for here, but for example there is a terminal operator count:
val flow = yourApiCallReturningFlow()
val hasAnyElement = flow.count() == 0
There is also onEmpty that allows to emit some specific values instead of the empty flow.
I guess it depends on what you want to do when there are items in the flow.
You can just do toList() on the flow and check if it's empty
When I create a client with the JacksonSerializer() feature and make some API calls, then run that script on my local machine, I get no error and the script runs successfully. However, when I upload this script as an AWS Lambda, I get the following error:
com.fasterxml.jackson.databind.JsonMappingException: Can not construct instance of kotlin.coroutines.Continuation, problem: abstract types either need to be mapped to concrete types, have custom deserializer, or be instantiated with additional type information
At first, I thought the error originated from me constructing the client outside of the Handler class, but when I made the client a private value inside the Handler class, I still get the error. I've included println() statements in my function, but they don't even run. That tells me that my handleRequest() funciton isn't getting run. Is there some AWS/Lambda'ism that prevents me from using the JacksonSerializer() feature as? If so, are there any alternatives on how to parse JSON responses with the Ktor client?
My client construction:
private val client = HttpClient(Apache) {
install(JsonFeature) {
serializer = JacksonSerializer()
}
}
An example call using the client:
val response = client.post<JsonNode> {
url(URL(GITHUB_GRAPHQL_ENDPOINT))
body = reqBody
headers {
append("Authorization", "bearer $token")
}
}
I'm guessing that you made your handler function be a kotlin suspend function? If so, that's your problem.
When you mark a function suspend, the compiler applies a bunch of magic. Most of the time, you don't need to know anything about this, other than the fact that any suspend function gets an extra parameter of type kotlin.coroutines.Continuation added to its signature. You usually don't notice this, since the compiler also makes calls to the function pass along their own hidden Continuation parameter.
Continuation, by design, can't be created by a tool like Jackson - it's an internal thing. What you probably need to do (assuming that you did make your handler function suspend) is to wrap your method in runBlocking {} and make it not be a suspend function. It's probably easiest to create a new handler, like so:
fun fixedHandler(input: MyInput, context: Context) = runBlocking {
originalHandler(input, context)
}
suspend fun originalHandler(input: MyInput, context: Context): MyOutput {
TODO("This is your original code")
}
PS - I've usually found it best to leverage the pre-defined Lambda interfaces to write my Lambda functions - it prevents you from encountering issues like this. See https://docs.aws.amazon.com/lambda/latest/dg/java-handler-using-predefined-interfaces.html for how to do it.
Have you checked out your dependencies running locally versus your dependencies in AWS? I've had issues where locally I'm running with a version, but the version in AWS was different. That could especially explain the error about continuations... Perhaps the method signature is different in whatever version you're using?
Look especially for provided scopes in your gradle/maven pom. Those are easy places for version to be out of sync.
I have a unit test that started to fail on Circle CI only. It fails on the last line in this (Kotlin) example:
generator.generateNames(50) // returns List<String>
.parallelStream()
.map { name ->
val playerId = "${name.firstName.toLowerCase()}"
Player(playerId = playerId)
}.collect(Collectors.toList()).last()
throwing: Caused by: java.util.NoSuchElementException.
It works always on my local machine or on Circle CI if I do not use a parallel stream. My theory is that the collect call returns a List snapshot (it actually doesn't block until the List is completely filled) and that CI doesn't have enough CPU to collect a single element in other threads?
However, my stream is ordered and so is the Collector right? Is this even collecting in parallel?
The exception you are getting probably has a message in it, and not just the name of the exception. That message is likely telling you the error. For example, the last part of your code calls the Kotlin extension function last() which in the implementation:
public fun <T> List<T>.last(): T {
if (isEmpty())
throw NoSuchElementException("List is empty.")
return this[lastIndex]
}
So if you are seeing "List is empty" message in the stack trace for java.util.NoSuchElementException then that is the cause.
Also, if you share the stack trace you can actually see what is throwing the exception. But looking at your code this is the only likely candidate.
The question then is, "why is the final list empty?!" ... is generateNames(50) working differently in this environment? The problem is not with collect(Collectors.toList()) which provides a synchronous result.
What are the differences in the following cases:
fun a(params: String) = Completable.fromAction {
if (params.isEmpty()) {
throw EmptyRequiredFieldException()
}
}
VS
fun b(params: String) = if(params.isEmpty())
Completable.error(EmptyRequiredFieldException())
else
Completable.complete()
Specifically in the context of android, if it matters (even though I don't think it does)
Thanks!
According to documentation,
If the Action throws an exception, the respective Throwable is delivered to the downstream via CompletableObserver.onError(Throwable), except when the downstream has disposed this Completable source. In this latter case, the Throwable is delivered to the global error handler via RxJavaPlugins.onError(Throwable) as an UndeliverableException.
So both of two ways you described are similar (except when the downstream has disposed). Note, that first approach (with manually throwing exception) allow to modify behavior of Completable at runtime. And second one - statically defined as you return particular type of Completable and can't modify it.
What to choose depends on your needs.