[[webflux-websocket]]
= WebSockets
[.small]#<<web.adoc#websocket, Same as in the Servlet stack>>#

This part of the reference documentation covers support for reactive-stack WebSocket
messaging.

include::websocket-intro.adoc[leveloffset=+1]




[[webflux-websocket-server]]
== WebSocket API
[.small]#<<web.adoc#websocket-server, Same as in the Servlet stack>>#

The Spring Framework provides a WebSocket API that you can use to write client- and
server-side applications that handle WebSocket messages.



[[webflux-websocket-server-handler]]
=== Server
[.small]#<<web.adoc#websocket-server-handler, Same as in the Servlet stack>>#

To create a WebSocket server, you can first create a `WebSocketHandler`.
The following example shows how to do so:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	import org.springframework.web.reactive.socket.WebSocketHandler;
	import org.springframework.web.reactive.socket.WebSocketSession;

	public class MyWebSocketHandler implements WebSocketHandler {

		@Override
		public Mono<Void> handle(WebSocketSession session) {
			// ...
		}
	}
----

Then you can map it to a URL and add a `WebSocketHandlerAdapter`, as the following example shows:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	static class WebConfig {

		@Bean
		public HandlerMapping handlerMapping() {
			Map<String, WebSocketHandler> map = new HashMap<>();
			map.put("/path", new MyWebSocketHandler());
			int order = -1; // before annotated controllers

			return new SimpleUrlHandlerMapping(map, order);
		}

		@Bean
		public WebSocketHandlerAdapter handlerAdapter() {
			return new WebSocketHandlerAdapter();
		}
	}
----



[[webflux-websockethandler]]
=== `WebSocketHandler`

The `handle` method of `WebSocketHandler` takes `WebSocketSession` and returns `Mono<Void>`
to indicate when application handling of the session is complete. The session is handled
through two streams, one for inbound and one for outbound messages. The following table
describes the two methods that handle the streams:

[options="header"]
|===
| `WebSocketSession` method | Description

| `Flux<WebSocketMessage> receive()`
| Provides access to the inbound message stream and completes when the connection is closed.

| `Mono<Void> send(Publisher<WebSocketMessage>)`
| Takes a source for outgoing messages, writes the messages, and returns a `Mono<Void>` that
  completes when the source completes and writing is done.

|===

A `WebSocketHandler` must compose the inbound and outbound streams into a unified flow and
return a `Mono<Void>` that reflects the completion of that flow. Depending on application
requirements, the unified flow completes when:

* Either the inbound or the outbound message stream completes.
* The inbound stream completes (that is, the connection closed), while the outbound stream is infinite.
* At a chosen point, through the `close` method of `WebSocketSession`.

When inbound and outbound message streams are composed together, there is no need to
check if the connection is open, since Reactive Streams signals terminate activity.
The inbound stream receives a completion or error signal, and the outbound stream
receives a cancellation signal.

The most basic implementation of a handler is one that handles the inbound stream. The
following example shows such an implementation:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
class ExampleHandler implements WebSocketHandler {

	@Override
	public Mono<Void> handle(WebSocketSession session) {
		return session.receive()			<1>
				.doOnNext(message -> {
					// ...					<2>
				})
				.concatMap(message -> {
					// ...					<3>
				})
				.then();					<4>
	}
}
----
<1> Access the stream of inbound messages.
<2> Do something with each message.
<3> Perform nested asynchronous operations that use the message content.
<4> Return a `Mono<Void>` that completes when receiving completes.


TIP: For nested, asynchronous operations, you may need to call `message.retain()` on underlying
servers that use pooled data buffers (for example, Netty). Otherwise, the data buffer may be
released before you have had a chance to read the data. For more background, see
<<core.adoc#databuffers, Data Buffers and Codecs>>.

The following implementation combines the inbound and outbound streams:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
class ExampleHandler implements WebSocketHandler {

	@Override
	public Mono<Void> handle(WebSocketSession session) {

		Flux<WebSocketMessage> output = session.receive()				<1>
				.doOnNext(message -> {
					// ...
				})
				.concatMap(message -> {
					// ...
				})
				.map(value -> session.textMessage("Echo " + value));	<2>

		return session.send(output);									<3>
	}
}
----
<1> Handle the inbound message stream.
<2> Create the outbound message, producing a combined flow.
<3> Return a `Mono<Void>` that does not complete while we continue to receive.


Inbound and outbound streams can be independent and be joined only for completion,
as the following example shows:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
class ExampleHandler implements WebSocketHandler {

	@Override
	public Mono<Void> handle(WebSocketSession session) {

		Mono<Void> input = session.receive()								<1>
				.doOnNext(message -> {
					// ...
				})
				.concatMap(message -> {
					// ...
				})
				.then();

		Flux<String> source = ... ;
		Mono<Void> output = session.send(source.map(session::textMessage));	<2>

		return Mono.zip(input, output).then();								<3>
	}
}
----
<1> Handle inbound message stream.
<2> Send outgoing messages.
<3> Join the streams and return a `Mono<Void>` that completes when either stream ends.



[[webflux-websocket-databuffer]]
=== `DataBuffer`

`DataBuffer` is the representation for a byte buffer in WebFlux. The Spring Core part of
the reference has more on that in the section on
<<core#databuffers, Data Buffers and Codecs>>. The key point to understand is that on some
servers like Netty, byte buffers are pooled and reference counted, and must be released
when consumed to avoid memory leaks.

When running on Netty, applications must use `DataBufferUtils.retain(dataBuffer)` if they
wish to hold on input data buffers in order to ensure they are not released, and
subsequently use `DataBufferUtils.release(dataBuffer)` when the buffers are consumed.




[[webflux-websocket-server-handshake]]
=== Handshake
[.small]#<<web.adoc#websocket-server-handshake, Same as in the Servlet stack>>#

`WebSocketHandlerAdapter` delegates to a `WebSocketService`. By default, that is an instance
of `HandshakeWebSocketService`, which performs basic checks on the WebSocket request and
then uses `RequestUpgradeStrategy` for the server in use. Currently, there is built-in
support for Reactor Netty, Tomcat, Jetty, and Undertow.

`HandshakeWebSocketService` exposes a `sessionAttributePredicate` property that allows
setting a `Predicate<String>` to extract attributes from the `WebSession` and insert them
into the attributes of the `WebSocketSession`.



[[webflux-websocket-server-config]]
=== Server Configation
[.small]#<<web.adoc#websocket-server-runtime-configuration, Same as in the Servlet stack>>#

The `RequestUpgradeStrategy` for each server exposes WebSocket-related configuration
options available for the underlying WebSocket engine. The following example sets
WebSocket options when running on Tomcat:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	static class WebConfig {

		@Bean
		public WebSocketHandlerAdapter handlerAdapter() {
			return new WebSocketHandlerAdapter(webSocketService());
		}

		@Bean
		public WebSocketService webSocketService() {
			TomcatRequestUpgradeStrategy strategy = new TomcatRequestUpgradeStrategy();
			strategy.setMaxSessionIdleTimeout(0L);
			return new HandshakeWebSocketService(strategy);
		}
	}
----

Check the upgrade strategy for your server to see what options are available. Currently,
only Tomcat and Jetty expose such options.



[[webflux-websocket-server-cors]]
=== CORS
[.small]#<<web.adoc#websocket-server-allowed-origins, Same as in the Servlet stack>>#

The easiest way to configure CORS and restrict access to a WebSocket endpoint is to
have your `WebSocketHandler` implement `CorsConfigurationSource` and return a
`CorsConfiguraiton` with allowed origins, headers, and other details. If you cannot do
that, you can also set the `corsConfigurations` property on the `SimpleUrlHandler` to
specify CORS settings by URL pattern. If both are specified, they are combined by using the
`combine` method on `CorsConfiguration`.



[[webflux-websocket-client]]
=== Client

Spring WebFlux provides a `WebSocketClient` abstraction with implementations for
Reactor Netty, Tomcat, Jetty, Undertow, and standard Java (that is, JSR-356).

NOTE: The Tomcat client is effectively an extension of the standard Java one with some extra
functionality in the `WebSocketSession` handling to take advantage of the Tomcat-specific
API to suspend receiving messages for back pressure.

To start a WebSocket session, you can create an instance of the client and use its `execute`
methods:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
WebSocketClient client = new ReactorNettyWebSocketClient();

URI url = new URI("ws://localhost:8080/path");
client.execute(url, session ->
		session.receive()
				.doOnNext(System.out::println)
				.then());
----

Some clients, such as Jetty, implement `Lifecycle` and need to be stopped and started
before you can use them. All clients have constructor options related to configuration
of the underlying WebSocket client.
