Facing io.netty.handler.timeout.ReadTimeoutException: null while consuming server sent events - spring

Question

I am new to spring web flux, I have a client application that consumes server-sent events, The events are published by the server randomly there is not fixed delay. But consumer throws io.netty.handler.timeout.ReadTimeoutException: null after 60 secs if there no event
Server-side events consumer code
webClient.get()
.uri("http://localhost:8080/events")
.accept(MediaType.TEXT_EVENT_STREAM)
.retrieve()
.bodyToFlux(type)
.subscribe(event -> process(event));
I need the client to be connected even if there is no event for a long time.
Full Exception
[36mr.netty.http.client.HttpClientConnect [...] The connection observed an error
io.netty.handler.timeout.ReadTimeoutException: null
reactor.Flux.MonoFlatMapMany.1 onError(org.springframework.web.reactive.function.client.WebClientRequestException: nested exception is io.netty.handler.timeout.ReadTimeoutException)
reactor.Flux.MonoFlatMapMany.1
org.springframework.web.reactive.function.client.WebClientRequestException: nested exception is io.netty.handler.timeout.ReadTimeoutException
at org.springframework.web.reactive.function.client.ExchangeFunctions$DefaultExchangeFunction.lambda$wrapException$9(ExchangeFunctions.java:141) ~[spring-webflux-5.3.5.jar:5.3.5]
Suppressed: reactor.core.publisher.FluxOnAssembly$OnAssemblyException:
Error has been observed at the following site(s):

Answer 1

In the Mozilla description for server sent events there is a note:
A colon as the first character of a line is in essence a comment, and
is ignored. Note: The comment line can be used to prevent connections
from timing out; a server can send a comment periodically to keep the
connection alive.
So periodically sending comments can keep the connection alive. So how do we send a comment?
Well spring has the class ServerSentEvent that has the function ServerSentEvent#comment. So if we use this class in combination with for instance Flux#interval we can merge in events containing only the comments keep alive.
Here is an example from a project i built a while back
#Bean
public RouterFunction<ServerResponse> foobars() {
return route()
.path("/api", builder -> builder
.GET("/foobar/{id}", accept(TEXT_EVENT_STREAM), request -> ok()
.contentType(MediaType.TEXT_EVENT_STREAM)
.header("Cache-Control", "no-transform")
.body(Flux.merge(foobarHandler.stream(request.pathVariable("id")),
Flux.interval(Duration.ofSeconds(15)).map(aLong -> ServerSentEvent.<List<FoobarResponse>>builder()
.comment("keep alive").build())), new ParameterizedTypeReference<ServerSentEvent<List<FoobarResponse>>>(){}))
.build();
}

Answer 2

Webflux use a default timeout fallback that will eventually show io.netty.handler.timeout.ReadTimeoutException: null.
It is possible to prevent this error by passing a custom timeout fallback to the timeout method(s):
public final Flux<T> timeout(Duration timeout, #Nullable Publisher<? extends T> fallback);
Additionally, you can use methods like onErrorContinue, onErrorReturn, ... to properly handle exceptions in Flux, example:
return webClient.get().uri(url).retrieve().bodyToFlux(String.class)
.timeout(timeout, Mono.error(new ReadTimeoutException("Timeout")))
.onErrorContinue((e, i) -> {
// Log the error here.
});
If you want to disable all these logs by default it is possible adding this line into file application.properties:
logging.level.reactor.netty.http.client.HttpClient=OFF