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Performance

There is no silver bullet when it comes to performance. Many factors affect it, including the size and volume of messages, whether application methods perform work that requires blocking, and external factors (such as network speed and other issues). The goal of this section is to provide an overview of the available configuration options along with some thoughts on how to reason about scaling.

In a messaging application, messages are passed through channels for asynchronous executions that are backed by thread pools. Configuring such an application requires good knowledge of the channels and the flow of messages. Therefore, it is recommended to review Flow of Messages.

The obvious place to start is to configure the thread pools that back the clientInboundChannel and the clientOutboundChannel. By default, both are configured at twice the number of available processors.

If the handling of messages in annotated methods is mainly CPU-bound, the number of threads for the clientInboundChannel should remain close to the number of processors. If the work they do is more IO-bound and requires blocking or waiting on a database or other external system, the thread pool size probably needs to be increased.

ThreadPoolExecutor has three important properties: the core thread pool size, the max thread pool size, and the capacity for the queue to store tasks for which there are no available threads.

A common point of confusion is that configuring the core pool size (for example, 10) and max pool size (for example, 20) results in a thread pool with 10 to 20 threads. In fact, if the capacity is left at its default value of Integer.MAX_VALUE, the thread pool never increases beyond the core pool size, since all additional tasks are queued.

See the javadoc of ThreadPoolExecutor to learn how these properties work and understand the various queuing strategies.

On the clientOutboundChannel side, it is all about sending messages to WebSocket clients. If clients are on a fast network, the number of threads should remain close to the number of available processors. If they are slow or on low bandwidth, they take longer to consume messages and put a burden on the thread pool. Therefore, increasing the thread pool size becomes necessary.

While the workload for the clientInboundChannel is possible to predict — after all, it is based on what the application does — how to configure the "clientOutboundChannel" is harder, as it is based on factors beyond the control of the application. For this reason, two additional properties relate to the sending of messages: sendTimeLimit and sendBufferSizeLimit. You can use those methods to configure how long a send is allowed to take and how much data can be buffered when sending messages to a client.

The general idea is that, at any given time, only a single thread can be used to send to a client. All additional messages, meanwhile, get buffered, and you can use these properties to decide how long sending a message is allowed to take and how much data can be buffered in the meantime. See the javadoc and documentation of the XML schema for important additional details.

The following example shows a possible configuration:

@Configuration
@EnableWebSocketMessageBroker
public class WebSocketConfig implements WebSocketMessageBrokerConfigurer {

	@Override
	public void configureWebSocketTransport(WebSocketTransportRegistration registration) {
		registration.setSendTimeLimit(15 * 1000).setSendBufferSizeLimit(512 * 1024);
	}

	// ...

}

The following example shows the XML configuration equivalent of the preceding example:

<beans xmlns="http://www.springframework.org/schema/beans"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xmlns:websocket="http://www.springframework.org/schema/websocket"
	xsi:schemaLocation="
		http://www.springframework.org/schema/beans
		https://www.springframework.org/schema/beans/spring-beans.xsd
		http://www.springframework.org/schema/websocket
		https://www.springframework.org/schema/websocket/spring-websocket.xsd">

	<websocket:message-broker>
		<websocket:transport send-timeout="15000" send-buffer-size="524288" />
		<!-- ... -->
	</websocket:message-broker>

</beans>

You can also use the WebSocket transport configuration shown earlier to configure the maximum allowed size for incoming STOMP messages. In theory, a WebSocket message can be almost unlimited in size. In practice, WebSocket servers impose limits — for example, 8K on Tomcat and 64K on Jetty. For this reason, STOMP clients such as stomp-js/stompjs and others split larger STOMP messages at 16K boundaries and send them as multiple WebSocket messages, which requires the server to buffer and re-assemble.

Spring’s STOMP-over-WebSocket support does this ,so applications can configure the maximum size for STOMP messages irrespective of WebSocket server-specific message sizes. Keep in mind that the WebSocket message size is automatically adjusted, if necessary, to ensure they can carry 16K WebSocket messages at a minimum.

The following example shows one possible configuration:

@Configuration
@EnableWebSocketMessageBroker
public class WebSocketConfig implements WebSocketMessageBrokerConfigurer {

	@Override
	public void configureWebSocketTransport(WebSocketTransportRegistration registration) {
		registration.setMessageSizeLimit(128 * 1024);
	}

	// ...

}

The following example shows the XML configuration equivalent of the preceding example:

<beans xmlns="http://www.springframework.org/schema/beans"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xmlns:websocket="http://www.springframework.org/schema/websocket"
	xsi:schemaLocation="
		http://www.springframework.org/schema/beans
		https://www.springframework.org/schema/beans/spring-beans.xsd
		http://www.springframework.org/schema/websocket
		https://www.springframework.org/schema/websocket/spring-websocket.xsd">

	<websocket:message-broker>
		<websocket:transport message-size="131072" />
		<!-- ... -->
	</websocket:message-broker>

</beans>

An important point about scaling involves using multiple application instances. Currently, you cannot do that with the simple broker. However, when you use a full-featured broker (such as RabbitMQ), each application instance connects to the broker, and messages broadcast from one application instance can be broadcast through the broker to WebSocket clients connected through any other application instances.