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Retrofit Kotlin - make an API request followed by two more concurrent ones

I want to make an api request, then I need to make two more requests after I receive the data. I found a great SO answer that uses rxjava2 to make two concurrent requests here:
How to make multiple request and wait until data is come from all the requests in retrofit 2.0 - android
I suppose I could just chain the logic for this after the first request, but my intuition tells me thats a bad idea because I'd be duplicating some code (I'd have separate logic for the first request in a function, then some similar logic for the second two requests in a function)
Is there a better way to accomplish this? I'd prefer Kotlin, but Java is ok.
Here is the code for concurrent requests from the SO answer.
val retrofit = Retrofit.Builder()
.baseUrl("https://api.example.com/")
.build()
val backendApi = retrofit.create(MyBackendAPI::class.java)
val requests = ArrayList<Observable<*>>()
requests.add(backendApi.getUser())
requests.add(backendApi.listPhotos())
requests.add(backendApi.listFriends())
Observable
.zip(requests) {
// do something with those results and emit new event
Any() // <-- Here we emit just new empty Object(), but you can emit anything
}
// Will be triggered if all requests will end successfully (4xx and 5xx also are successful requests too)
.subscribe({
//Do something on successful completion of all requests
}) {
//Do something on error completion of requests
}
Thanks

Spring Webflux endpoint working as a topic

I have an Flux endpoint that I provide to clients (subscribers) to receive updated prices. I'm testing it accessing the URL (http://localhost:8080/prices) though the browser and it works fine. The problem I'm facing (I'm maybe missing some concepts here) is when I open this URL in many browsers and I expect to receive the notification in all of them, but just one receives. It is working as a queue instead of a topic (like in message Brokers). Is that correct behavior?
#GetMapping(value = "prices", produces = MediaType.TEXT_EVENT_STREAM_VALUE)
public Flux<ServerSentEvent<Collection<Price>>> prices() {
return Flux.interval(Duration.ofSeconds(5))
.map(sec -> pricesQueue.get())
.filter(prices -> !prices.isEmpty())
.map(prices -> ServerSentEvent.<Collection<Price>> builder()
.event("status-changed")
.data(prices)
.build());
}

get isn't a standard queue operation, but this is almost certainly because your pricesQueue.get() method isn't idempotent. With every request (with every browser window you open in this case), you'll get a new flux that calls pricesQueue.get() every 5 seconds. Now if pricesQueue.get() just retrieves the latest item in the queue and does nothing with it, all is good - all your subscribers receive the same item, and the same item is displayed. But if it acts more like a poll() where it removes the item in the queue after it's retrieved it, then only the first flux will get that value - the rest won't, as by that point it will have been removed.
You've really two main options here:
Change your get() implementation (or implement a new method) so that it doesn't mutate the queue, only retrieves a value.
Turn the flux into a hot flux. Store Flux.interval(Duration.ofSeconds(5)).map(sec -> pricesQueue.get()).publish().autoConnect() somewhere as a field (let's say as queueFlux), then just return queueFlux.filter(prices -> !prices.isEmpty()).map(...) in your controller method.

Android call Rest API in a loop

I'm new to android, so have some questions regarding api calls.
Currently i use Retrofit to accomplish my api calls.
Here is example of my api call with retrofit
#POST("posts/new")
fun createPost(#Body post: Post, #Header("Authorization") token: String): Single<PostResult>
So, assume i have 10 posts and i need to call createPost 10 times (Yes, i know i can have list input on BE side, but ... ). Best way is to iterate over posts (for/map) and send them to the server.
But here is problem:
- How do i can track that all calls are done?
In JS i can have something like Promise.all - could i do something
similar in android?
I thought about counting the finished vs started requests, but i think it's bit ugly isn't?

in your response success you need to call again get/post method.
after success you know your api call is done or else it throw error.

Thanks to #shkschneider
Your question is broad. You could use RxJava to zip, or coroutines to async/await or other methods.
Ended up with usage of .zip
Single.zip(observables) { args -> args }
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe({ ...Success }, { ...Failure })

Vertx.io GET silently fails

I'm writing a POC using vertx, looking for alternatives when we have to migrate Spring Web from 4.x to 5 to be java 9 compliant.
I've written a simple client, just a GET towards a publicly available server just to get something working but it silently fails me.
public List<String> pull() {
Vertx vertx = Vertx.vertx();
HttpClientOptions options = new HttpClientOptions().setLogActivity(true);
HttpClient hc = vertx.createHttpClient(options);
hc.getNow(80, "http://sunet.se", "/",r -> {
System.out.println("\n****** Handler called! ***\n");
});
return new ArrayList<>();
}
This will silently fail and I cannot understand why.
As far as I can tell, I do exactly as in the examples given in the docs.
In desperation I fired up wire shark and according to WS, there is no actual call (when I use the browser WS captures that). So, it seems my call is never actually done. I don't get any exceptions or anything. Setting the log level to debug gives nothing noteworthy other than
Failed to get SOMAXCONN from sysctl and file /proc/sys/net/core/somaxconn. Default: 128
And that should not fail the call.
I've also tried using vertx.io WebClient but that fails also, in the same manner.
UPDATE
I've managed to get it to work but with a caveat.
As #tsegismont states in his answer, the protocol part of the URI shouldn't be there, that was not in the examples, I just missed it myself.
I ran my example as a stand-alone and then it worked.
My original example was run as a junit test (it's an easy way to test code and I usually try to write the test code first) and when it's run as a junit test it still doesn't work. Why that is, I have no idea. I would greatly appreciate if someone could tell me how to get that to work.

The getNow variant you use expects the server host, not a URL. It should be:
hc.getNow(80, "sunet.se", "/",r -> {
System.out.println("\n****** Handler called! ***\n");
}
If you found a snippet like this in the Vert.x docs it's a bug. Would you mind to report it?
Now a few comments.
1/ The HttpClient is a low-level client.
Most users should prefer the Vert.x Web Client
Here's an example for your use case:
WebClient client = WebClient.create(vertx);
client
.get(80, "sunet.se", "/")
.send(ar -> {
if (ar.succeeded()) {
// Obtain response
HttpResponse<Buffer> response = ar.result();
System.out.println("Received response with status code" + response.statusCode());
} else {
System.out.println("Something went wrong " + ar.cause().getMessage());
}
});
2/ Create a single Vert.x and WebClient instance
Do not create a Vert.x and WebClient instance on every method call.
It wastes resources and is inefficient.

How do I keep track of requests issued/completed without the use of additional state variables? (Java/Grpc)

I am using the StreamObserver class found in the grpc-java project to set up some bidirectional streaming.
When I run my program, I make an undetermined number of requests to the server, and I only want to call onCompleted() on the requestObserver once I have finished making all of the requests.
Currently, to solve this, I am using a variable "inFlight" to keep track of the requests that have been issued, and when a response comes back, I decrement "inFlight". So, something like this.
// issuing requests
while (haveRequests) {
MessageRequest request = mkRequest();
this.requestObserver.onNext(request);
this.inFlight++;
}
// response observer
StreamObserver<Message> responseObserver = new StreamObserver<Message> {
#Override
public void onNext(Message response) {
if (--this.onFlight == 0) {
this.requestObserver.onCompleted();
}
// work on message
}
// other methods
}
A bit pseudo-codey, but this logic works. However, I would like to get rid of the "inFlight" variable if possible. Is there anything within the StreamObserver class that allows this sort of functionality, without the need of an additional variable to track state? Something that would tell the number of requests issued and when they completed.
I've tried inspecting the object within the intellij IDE debugger, but nothing is popping out to me.

To answer your direct question, you can simply call onComplete from the while loop. All the messages passed to onNext. Under the hood, gRPC will send what is called a "half close", indicating that it won't send any more messages, but it is willing to receive them. Specifically:
// issuing requests
while (haveRequests) {
MessageRequest request = mkRequest();
this.requestObserver.onNext(request);
this.inFlight++;
}
requestObserver.onCompleted();
This ensures that all responses are sent, and in the order that you sent them. On the server side, when it sees the corresponding onCompleted callback, it can half-close its side of the connection by calling onComplete on its observer. (There are two observers on the server side one for receiving info from the client, one for sending info).
Back on the client side, you just need to wait for the server to half close to know that all messages were received and processed. Note that if there were any errors, you would get an onError callback instead.
If you don't know how many requests you are going to make on the client side, you might consider using an AtomicInteger, and call decrementAndGet when you get back a response. If the return value is 0, you'll know all the requests have completed.