Graceful shutdown of client requests on server shutdown

[ZIRAKrezovic]

Not sure how to put this in enough words to fit into title, so feel free to update accordingly.

While reproducing an SSE shutdown issue, I have stumbled upon WebClient handling (ungracful) server shutdown ungracefully.

Consider the reproducer: https://github.com/ZIRAKrezovic/spring-boot-issues/tree/graceful-shutdown

Remove onErrorComplete() from test https://github.com/ZIRAKrezovic/spring-boot-issues/blob/graceful-shutdown/src/test/java/com/example/demo/DemoApplicationTests.java#L52C1-L53C1

Observe the error

2026-02-19T18:42:34.573+01:00 ERROR 22880 --- [demo] [undedElastic-19] reactor.core.publisher.Operators         : Operator called default onErrorDropped

reactor.core.Exceptions$ErrorCallbackNotImplemented: org.springframework.web.reactive.function.client.WebClientResponseException: 200 OK from GET http://localhost:8080/test/sse, but response failed with cause: reactor.netty.http.client.PrematureCloseException: Connection prematurely closed DURING response
Caused by: org.springframework.web.reactive.function.client.WebClientResponseException: 200 OK from GET http://localhost:8080/test/sse, but response failed with cause: reactor.netty.http.client.PrematureCloseException: Connection prematurely closed DURING response
	at org.springframework.web.reactive.function.client.WebClientResponseException.create(WebClientResponseException.java:324) ~[spring-webflux-7.0.5.jar:7.0.5]
	at org.springframework.web.reactive.function.client.DefaultClientResponse.lambda$createException$1(DefaultClientResponse.java:214) ~[spring-webflux-7.0.5.jar:7.0.5]
	at reactor.core.publisher.FluxMap$MapSubscriber.onNext(FluxMap.java:106) ~[reactor-core-3.8.3.jar:3.8.3]
	at reactor.core.publisher.FluxOnErrorReturn$ReturnSubscriber.onError(FluxOnErrorReturn.java:196) ~[reactor-core-3.8.3.jar:3.8.3]
	at reactor.core.publisher.FluxDefaultIfEmpty$DefaultIfEmptySubscriber.onError(FluxDefaultIfEmpty.java:156) ~[reactor-core-3.8.3.jar:3.8.3]
	at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onError(FluxOnAssembly.java:545) ~[reactor-core-3.8.3.jar:3.8.3]
	at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.onError(FluxMapFuseable.java:143) ~[reactor-core-3.8.3.jar:3.8.3]
	at reactor.core.publisher.FluxContextWrite$ContextWriteSubscriber.onError(FluxContextWrite.java:123) ~[reactor-core-3.8.3.jar:3.8.3]
	at reactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.onError(FluxMapFuseable.java:341) ~[reactor-core-3.8.3.jar:3.8.3]
	at reactor.core.publisher.FluxFilterFuseable$FilterFuseableConditionalSubscriber.onError(FluxFilterFuseable.java:383) ~[reactor-core-3.8.3.jar:3.8.3]
	at reactor.core.publisher.MonoCollect$CollectSubscriber.onError(MonoCollect.java:134) ~[reactor-core-3.8.3.jar:3.8.3]
	at reactor.core.publisher.FluxMap$MapSubscriber.onError(FluxMap.java:134) ~[reactor-core-3.8.3.jar:3.8.3]
	at reactor.core.publisher.FluxPeek$PeekSubscriber.onError(FluxPeek.java:225) ~[reactor-core-3.8.3.jar:3.8.3]
	at reactor.core.publisher.FluxMap$MapSubscriber.onError(FluxMap.java:134) ~[reactor-core-3.8.3.jar:3.8.3]
	at reactor.core.publisher.Operators.error(Operators.java:198) ~[reactor-core-3.8.3.jar:3.8.3]
	at reactor.netty.channel.FluxReceive.startReceiver(FluxReceive.java:178) ~[reactor-netty-core-1.3.3.jar:1.3.3]
	at reactor.netty.channel.FluxReceive.subscribe(FluxReceive.java:148) ~[reactor-netty-core-1.3.3.jar:1.3.3]
	at reactor.core.publisher.InternalFluxOperator.subscribe(InternalFluxOperator.java:67) ~[reactor-core-3.8.3.jar:3.8.3]
	at reactor.netty.ByteBufFlux.subscribe(ByteBufFlux.java:340) ~[reactor-netty-core-1.3.3.jar:1.3.3]
	at reactor.core.publisher.Mono.subscribe(Mono.java:4569) ~[reactor-core-3.8.3.jar:3.8.3]
	at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onError(FluxOnErrorResume.java:104) ~[reactor-core-3.8.3.jar:3.8.3]
	at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onError(FluxOnAssembly.java:545) ~[reactor-core-3.8.3.jar:3.8.3]
	at reactor.core.publisher.FluxConcatMapNoPrefetch$FluxConcatMapNoPrefetchSubscriber.maybeOnError(FluxConcatMapNoPrefetch.java:326) ~[reactor-core-3.8.3.jar:3.8.3]
	at reactor.core.publisher.FluxConcatMapNoPrefetch$FluxConcatMapNoPrefetchSubscriber.onError(FluxConcatMapNoPrefetch.java:220) ~[reactor-core-3.8.3.jar:3.8.3]
	at reactor.core.publisher.FluxBufferPredicate$BufferPredicateSubscriber.onError(FluxBufferPredicate.java:345) ~[reactor-core-3.8.3.jar:3.8.3]
	at reactor.core.publisher.FluxPeekFuseable$PeekFuseableConditionalSubscriber.onError(FluxPeekFuseable.java:554) ~[reactor-core-3.8.3.jar:3.8.3]
	at reactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.onError(FluxMapFuseable.java:341) ~[reactor-core-3.8.3.jar:3.8.3]
	at reactor.core.publisher.FluxContextWrite$ContextWriteSubscriber.onError(FluxContextWrite.java:123) ~[reactor-core-3.8.3.jar:3.8.3]
	at reactor.core.publisher.FluxDoFinally$DoFinallySubscriber.onError(FluxDoFinally.java:119) ~[reactor-core-3.8.3.jar:3.8.3]
	at reactor.core.publisher.FluxConcatArray$ConcatArraySubscriber.onError(FluxConcatArray.java:190) ~[reactor-core-3.8.3.jar:3.8.3]
	at reactor.core.publisher.FluxFlattenIterable$FlattenIterableSubscriber.drainAsync(FluxFlattenIterable.java:352) ~[reactor-core-3.8.3.jar:3.8.3]
	at reactor.core.publisher.FluxFlattenIterable$FlattenIterableSubscriber.drain(FluxFlattenIterable.java:725) ~[reactor-core-3.8.3.jar:3.8.3]
	at reactor.core.publisher.FluxFlattenIterable$FlattenIterableSubscriber.onError(FluxFlattenIterable.java:264) ~[reactor-core-3.8.3.jar:3.8.3]
	at reactor.core.publisher.FluxMap$MapSubscriber.onError(FluxMap.java:134) ~[reactor-core-3.8.3.jar:3.8.3]
	at reactor.core.publisher.FluxPeek$PeekSubscriber.onError(FluxPeek.java:225) ~[reactor-core-3.8.3.jar:3.8.3]
	at reactor.core.publisher.FluxMap$MapSubscriber.onError(FluxMap.java:134) ~[reactor-core-3.8.3.jar:3.8.3]
	at reactor.netty.channel.FluxReceive.terminateReceiver(FluxReceive.java:484) ~[reactor-netty-core-1.3.3.jar:1.3.3]
	at reactor.netty.channel.FluxReceive.drainReceiver(FluxReceive.java:277) ~[reactor-netty-core-1.3.3.jar:1.3.3]
	at reactor.netty.channel.FluxReceive.onInboundError(FluxReceive.java:472) ~[reactor-netty-core-1.3.3.jar:1.3.3]
	at reactor.netty.channel.ChannelOperations.onInboundError(ChannelOperations.java:536) ~[reactor-netty-core-1.3.3.jar:1.3.3]
	at reactor.netty.http.client.HttpClientOperations.onInboundClose(HttpClientOperations.java:408) ~[reactor-netty-http-1.3.3.jar:1.3.3]
	at reactor.netty.channel.ChannelOperationsHandler.channelInactive(ChannelOperationsHandler.java:73) ~[reactor-netty-core-1.3.3.jar:1.3.3]
	at io.netty.channel.AbstractChannelHandlerContext.fireChannelInactive(AbstractChannelHandlerContext.java:251) ~[netty-transport-4.2.10.Final.jar:4.2.10.Final]
	at io.netty.channel.ChannelInboundHandlerAdapter.channelInactive(ChannelInboundHandlerAdapter.java:81) ~[netty-transport-4.2.10.Final.jar:4.2.10.Final]
	at io.netty.handler.codec.http.HttpContentDecoder.channelInactive(HttpContentDecoder.java:241) ~[netty-codec-http-4.2.10.Final.jar:4.2.10.Final]
	at io.netty.channel.AbstractChannelHandlerContext.fireChannelInactive(AbstractChannelHandlerContext.java:251) ~[netty-transport-4.2.10.Final.jar:4.2.10.Final]
	at io.netty.channel.CombinedChannelDuplexHandler$DelegatingChannelHandlerContext.fireChannelInactive(CombinedChannelDuplexHandler.java:416) ~[netty-transport-4.2.10.Final.jar:4.2.10.Final]
	at io.netty.handler.codec.ByteToMessageDecoder.channelInputClosed(ByteToMessageDecoder.java:427) ~[netty-codec-base-4.2.10.Final.jar:4.2.10.Final]
	at io.netty.handler.codec.ByteToMessageDecoder.channelInactive(ByteToMessageDecoder.java:392) ~[netty-codec-base-4.2.10.Final.jar:4.2.10.Final]
	at io.netty.handler.codec.http.HttpClientCodec$Decoder.channelInactive(HttpClientCodec.java:410) ~[netty-codec-http-4.2.10.Final.jar:4.2.10.Final]
	at io.netty.channel.CombinedChannelDuplexHandler.channelInactive(CombinedChannelDuplexHandler.java:219) ~[netty-transport-4.2.10.Final.jar:4.2.10.Final]
	at io.netty.channel.AbstractChannelHandlerContext.fireChannelInactive(AbstractChannelHandlerContext.java:251) ~[netty-transport-4.2.10.Final.jar:4.2.10.Final]
	at io.netty.channel.DefaultChannelPipeline$HeadContext.channelInactive(DefaultChannelPipeline.java:1424) ~[netty-transport-4.2.10.Final.jar:4.2.10.Final]
	at io.netty.channel.DefaultChannelPipeline.fireChannelInactive(DefaultChannelPipeline.java:876) ~[netty-transport-4.2.10.Final.jar:4.2.10.Final]
	at io.netty.channel.AbstractChannel$AbstractUnsafe$6.run(AbstractChannel.java:676) ~[netty-transport-4.2.10.Final.jar:4.2.10.Final]
	at io.netty.util.concurrent.AbstractEventExecutor.runTask(AbstractEventExecutor.java:148) ~[netty-common-4.2.10.Final.jar:4.2.10.Final]
	at io.netty.util.concurrent.AbstractEventExecutor.safeExecute$$$capture(AbstractEventExecutor.java:141) ~[netty-common-4.2.10.Final.jar:4.2.10.Final]
	at io.netty.util.concurrent.AbstractEventExecutor.safeExecute(AbstractEventExecutor.java) ~[netty-common-4.2.10.Final.jar:4.2.10.Final]
	at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:535) ~[netty-common-4.2.10.Final.jar:4.2.10.Final]
	at io.netty.channel.SingleThreadIoEventLoop.run(SingleThreadIoEventLoop.java:201) ~[netty-transport-4.2.10.Final.jar:4.2.10.Final]
	at io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:1195) ~[netty-common-4.2.10.Final.jar:4.2.10.Final]
	at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74) ~[netty-common-4.2.10.Final.jar:4.2.10.Final]
	at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30) ~[netty-common-4.2.10.Final.jar:4.2.10.Final]
	at java.base/java.lang.Thread.run(Thread.java:1583) ~[na:na]
Caused by: reactor.netty.http.client.PrematureCloseException: Connection prematurely closed DURING response
	Suppressed: The stacktrace has been enhanced by Reactor, refer to additional information below: 
Error has been observed at the following site(s):
	*__checkpoint ⇢ Body from GET http://localhost:8080/test/sse [DefaultClientResponse]
	*__checkpoint ⇢ Body from GET http://localhost:8080/test/sse [DefaultClientResponse]

I was hoping this would be handled by framework gracefully, especially since Flux creation is handled by framework. At the moment I either have to use .onErrorComplete() as shown in the test case, or write more specific variations such as

.onErrorComplete(t -> t.getCause() != null && t.getCause().getClass().getName().contains("PrematureCloseException"))

This is a bit cumbersome to get right for multiple underlying clients (Jetty, JDK, Netty, Apache Http Client, ...)

I wonder if you'd support (un)graceful completion of a streaming response when a server closes the response (due to shut down, for example).

If you don't think this should be handled by the framework, feel free to close the issue.

Thanks.

PS: I'm not really sure if "ungraceful" is the right word for "opposite of graceful" in this scenario, but that's the context I've used it with.

[bclozel]

I'm not sure what is the ask here. What is the behavior you would expect? onErrorComplete simply ignores the error, I don't think we should ignore completely such issues.

In my opinion, clients receiving streaming responses should be in charge of deciding what to do in case of errors. Should we ignore or should we reconnect and retry? It is really an application decision.

[ZIRAKrezovic]

Yes, ultimately it's up to the user to decide. But it'd be helpful if there were some helpers to detect disconnected states - similar to default HTTP Response mapping to *ClientException. Only in this case, it would be opt-in rather than opt-out. It would provide a slight convenience in the long run, but feel free to close the issue if you feel it's out of scope.

[rstoyanchev]

The SSE Flux ends with in an error signal, and it's up to the application to handle it.

WebClientRequestException and WebClientResponseException should be thrown consistently both of which extend WebClientEception, a single exception to handle. The root case PrematureCloseException is an IOExcetion. This should be enough to determine the stream ended with an I/O error.

I don't see anything more we can do.