7.1 Using the Low-Level HTTP Client - 7.1.4 Streaming JSON over HTTP - 《Micronaut v2.2.3 Documentation》

7.1.4 Streaming JSON over HTTP

7.1.4 Streaming JSON over HTTP

Micronaut’s HTTP client includes support for streaming data over HTTP via the RxStreamingHttpClient interface which includes methods specific to HTTP streaming including:

Table 1. HTTP Streaming Methods
Method	Description
`dataStream(HttpRequest<I> request)`	Returns a stream of data as a Flowable of ByteBuffer
`exchangeStream(HttpRequest<I> request)`	Returns the HttpResponse wrapping a Flowable of ByteBuffer
`jsonStream(HttpRequest<I> request)`	Returns a non-blocking stream of JSON objects

In order to do JSON streaming you should on the server side declare a controller method that returns a application/x-json-stream of JSON objects. For example:

Streaming JSON on the Server

import io.micronaut.http.MediaType;
import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Get;
import io.reactivex.Flowable;
import java.time.ZonedDateTime;
import java.util.concurrent.TimeUnit;
    @Get(value = "/headlines", processes = MediaType.APPLICATION_JSON_STREAM) (1)
    Flowable<Headline> streamHeadlines() {
        return Flowable.fromCallable(() -> {  (2)
            Headline headline = new Headline();
            headline.setText("Latest Headline at " + ZonedDateTime.now());
            return headline;
        }).repeat(100) (3)
          .delay(1, TimeUnit.SECONDS); (4)
    }

Streaming JSON on the Server

import io.micronaut.http.MediaType
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get
import io.reactivex.Flowable
import java.time.ZonedDateTime
import java.util.concurrent.TimeUnit
    @Get(value = "/headlines", processes = MediaType.APPLICATION_JSON_STREAM) (1)
    Flowable<Headline> streamHeadlines() {
        Flowable.fromCallable({ (2)
            Headline headline = new Headline()
            headline.setText("Latest Headline at " + ZonedDateTime.now())
            return headline
        }).repeat(100) (3)
          .delay(1, TimeUnit.SECONDS) (4)
    }

Streaming JSON on the Server

import io.micronaut.http.MediaType
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get
import io.reactivex.Flowable
import java.time.ZonedDateTime
import java.util.concurrent.TimeUnit
    @Get(value = "/headlines", processes = [MediaType.APPLICATION_JSON_STREAM]) (1)
    internal fun streamHeadlines(): Flowable<Headline> {
        return Flowable.fromCallable {
            (2)
            val headline = Headline()
            headline.text = "Latest Headline at " + ZonedDateTime.now()
            headline
        }.repeat(100) (3)
                .delay(1, TimeUnit.SECONDS) (4)
    }

1	A method `streamHeadlines` is defined that produces `application/x-json-stream`
2	A Flowable is created from a `Callable` function (note no blocking occurs within the function so this is ok, otherwise you would want to `subscribeOn` an I/O thread pool).
3	The Flowable is set to repeat 100 times
4	The Flowable will emit items with a delay of 1 second between each item

The server does not have to be written in Micronaut, any server that supports JSON streaming will do.

Then on the client simply subscribe to the stream using jsonStream and every time the server emits a JSON object the client will decode and consume it:

Streaming JSON on the Client

        Flowable<Headline> headlineStream = client.jsonStream(GET("/streaming/headlines"), Headline.class); (1)
        CompletableFuture<Headline> future = new CompletableFuture<>(); (2)
        headlineStream.subscribe(new Subscriber<Headline>() {
            @Override
            public void onSubscribe(Subscription s) {
                s.request(1); (3)
            }
            @Override
            public void onNext(Headline headline) {
                System.out.println("Received Headline = " + headline.getText());
                future.complete(headline); (4)
            }
            @Override
            public void onError(Throwable t) {
                future.completeExceptionally(t); (5)
            }
            @Override
            public void onComplete() {
                // no-op (6)
            }
        });

Streaming JSON on the Client

        Flowable<Headline> headlineStream = client.jsonStream(GET("/streaming/headlines"), Headline.class) (1)
        CompletableFuture<Headline> future = new CompletableFuture<>() (2)
        headlineStream.subscribe(new Subscriber<Headline>() {
            @Override
            void onSubscribe(Subscription s) {
                s.request(1) (3)
            }
            @Override
            void onNext(Headline headline) {
                System.out.println("Received Headline = " + headline.getText())
                future.complete(headline) (4)
            }
            @Override
            void onError(Throwable t) {
                future.completeExceptionally(t) (5)
            }
            @Override
            void onComplete() {
                // no-op (6)
            }
        })

Streaming JSON on the Client

            val headlineStream = client.jsonStream(GET<Any>("/streaming/headlines"), Headline::class.java) (1)
            val future = CompletableFuture<Headline>() (2)
            headlineStream.subscribe(object : Subscriber<Headline> {
                override fun onSubscribe(s: Subscription) {
                    s.request(1) (3)
                }
                override fun onNext(headline: Headline) {
                    println("Received Headline = " + headline.text!!)
                    future.complete(headline) (4)
                }
                override fun onError(t: Throwable) {
                    future.completeExceptionally(t) (5)
                }
                override fun onComplete() {
                    // no-op (6)
                }
            })

1	The `jsonStream` method is used return a Flowable
2	A `CompletableFuture` is used in the example to receive a value, but what you do with each emitted item is application specific
3	The Subscription is used to request a single item. You can use the Subscription to regulate back pressure and demand.
4	The `onNext` method is called when an item is emitted
5	The `onError` method is called when an error occurs
6	The `onComplete` method is called when all `Headline` instances have been emitted

Note neither the server or the client in the example above perform blocking I/O at any point.