Expose Tomcat response output stream buffer size as a configuration property

[dsyer]

After some recent changes in Spring MVC (7.0.6) Tomcat will write the content-length header for us if we write a response body that fits in its output stream buffer. The default size is 8192 bytes, which is completely arbitrary, and some applications (where they often return JSON results slightly larger than that, for instance) would benefit from being able to change the value.

Jetty behaves the same way, and it probably has the same opportunity to configure the buffer size.

[wilkinsona]

For Tomcat, I guess that's the NIO-specific socket.appWriteBufSize? It can be configured programatically, although it's stringly typed:

@Bean
TomcatConnectorCustomizer increaseWriteBufferSize() {
	return (connector) -> ((AbstractProtocol<?>) connector.getProtocolHandler())
		.setProperty("socket.appWriteBufSize", "16384");
}

[dsyer]

That's the one for Tomcat. Good work tracking that down. (I tested it on a JSON sample.) Looks like it works for all AbstractProtocol implementations (including Nio which is the default in Spring Boot).

[wilkinsona]

Looks like it works for all AbstractProtocol implementations

While SocketProperties is managed in AbstractEndpoint, as far as I can tell it's only NioEndpoint and Nio2Endpoint that call SocketProperties.getAppWriteBufSize() so it does seem to be specific to NIO.

[dsyer]

Actually I was wrong. That property doesn't seem to change the content-length writing behaviour in Tomcat. We might need to ask the Tomcat team for clarification (there is another buffer size in the ServletResponse that is never changed from its default). Moreover, there are 2 constants that are set to 8192 by default, and this "socket.appWriteBufSize" is the only one you can change. The other is the one that seems important - when the org.apachje.catalina.connector.Response is created it is always initialized with a default buffer size because the ProtocolHandler doesn't express a preference.

UPDATE: I tried it with a custom Http11NioProtocol overriding getDesiredBufferSize() and that does work, but only if the desired size is greater than the default 8192, it seems. There's some switcheroo going on somewhere where Tomcat refuses to go lower than that. I didn't figure out exactly what's going on but I think the ask might be for Tomcat to support setting this property natively (e.g. adding a default value and providing a setter), and then we can do it in a protocol customizer.