why editReply return discord4j.rest.http.client.ClientException *** 404 Not Found ?

[jqnote]

pom.xml

<dependency>
    <groupId>com.discord4j</groupId>
    <artifactId>discord4j-core</artifactId>
    <version>3.2.3</version>
</dependency>

exception

discord4j.rest.http.client.ClientException: POST /interactions/10558398405806*****/aW50ZX*****lZa05mWg/callback returned 404 Not Found with response {code=10062, message=Unknown interaction}
	at discord4j.rest.http.client.ClientResponse.clientException(ClientResponse.java:171)
	Suppressed: reactor.core.publisher.FluxOnAssembly$OnAssemblyException: 
Error has been observed at the following site(s):
	*__checkpoint ⇢ Request to POST /interactions/10558398405806*****/aW50****/callback [RequestStream]
	*__checkpoint ⇢ Request to POST /interactions/10558398405806*****/aW50****/callback [DefaultRouter]
Original Stack Trace:
		at discord4j.rest.http.client.ClientResponse.clientException(ClientResponse.java:171)
		at discord4j.rest.http.client.ClientResponse.lambda$createException$13(ClientResponse.java:149)
		at reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:125)
		at reactor.core.publisher.Operators$MonoSubscriber.complete(Operators.java:1816)
		at reactor.core.publisher.MonoFlatMap$FlatMapInner.onNext(MonoFlatMap.java:249)
		at reactor.core.publisher.FluxMap$MapSubscriber.onNext(FluxMap.java:122)
		at reactor.core.publisher.FluxHandle$HandleSubscriber.onNext(FluxHandle.java:126)
		at reactor.core.publisher.FluxPeekFuseable$PeekConditionalSubscriber.onNext(FluxPeekFuseable.java:854)
		at reactor.core.publisher.FluxPeekFuseable$PeekConditionalSubscriber.onNext(FluxPeekFuseable.java:854)
		at reactor.core.publisher.FluxPeekFuseable$PeekConditionalSubscriber.onNext(FluxPeekFuseable.java:854)
		at reactor.core.publisher.FluxMap$MapConditionalSubscriber.onNext(FluxMap.java:224)
		at reactor.core.publisher.FluxDoFinally$DoFinallySubscriber.onNext(FluxDoFinally.java:113)
		at reactor.core.publisher.FluxHandleFuseable$HandleFuseableSubscriber.onNext(FluxHandleFuseable.java:191)
		at reactor.core.publisher.FluxContextWrite$ContextWriteSubscriber.onNext(FluxContextWrite.java:107)
		at reactor.core.publisher.Operators$MonoSubscriber.complete(Operators.java:1816)
		at reactor.core.publisher.MonoCollectList$MonoCollectListSubscriber.onComplete(MonoCollectList.java:129)
		at reactor.core.publisher.FluxPeek$PeekSubscriber.onComplete(FluxPeek.java:260)
		at reactor.core.publisher.FluxMap$MapSubscriber.onComplete(FluxMap.java:144)
		at reactor.netty.channel.FluxReceive.onInboundComplete(FluxReceive.java:400)
		at reactor.netty.channel.ChannelOperations.onInboundComplete(ChannelOperations.java:419)
		at reactor.netty.channel.ChannelOperations.terminate(ChannelOperations.java:473)
		at reactor.netty.http.client.HttpClientOperations.onInboundNext(HttpClientOperations.java:702)
		at reactor.netty.channel.ChannelOperationsHandler.channelRead(ChannelOperationsHandler.java:113)
		at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:444)
		at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420)
		at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:412)
		at io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:103)
		at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:444)
		at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420)
		at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:412)
		at io.netty.channel.CombinedChannelDuplexHandler$DelegatingChannelHandlerContext.fireChannelRead(CombinedChannelDuplexHandler.java:436)
		at io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:336)
		at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:308)
		at io.netty.channel.CombinedChannelDuplexHandler.channelRead(CombinedChannelDuplexHandler.java:251)
		at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:442)
		at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420)
		at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:412)
		at io.netty.handler.ssl.SslHandler.unwrap(SslHandler.java:1373)
		at io.netty.handler.ssl.SslHandler.decodeNonJdkCompatible(SslHandler.java:1247)
		at io.netty.handler.ssl.SslHandler.decode(SslHandler.java:1287)
		at io.netty.handler.codec.ByteToMessageDecoder.decodeRemovalReentryProtection(ByteToMessageDecoder.java:519)
		at io.netty.handler.codec.ByteToMessageDecoder.callDecode(ByteToMessageDecoder.java:458)
		at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:280)
		at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:444)
		at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420)
		at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:412)
		at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1410)
		at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:440)
		at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420)
		at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:919)
		at io.netty.channel.epoll.AbstractEpollStreamChannel$EpollStreamUnsafe.epollInReady(AbstractEpollStreamChannel.java:800)
		at io.netty.channel.epoll.EpollEventLoop.processReady(EpollEventLoop.java:499)
		at io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:397)
		at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:997)
		at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
		at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
		at java.lang.Thread.run(Thread.java:748)

confusion

only the case b threws out the ClientException, but case a and case c work good. why?

code

[quanticc]

This is likely a case of methodThatTakesALongTime running before deferReply is sent, taking more than the time required by Discord to reply (3 seconds). If you reply after that, the interaction is deleted and you get 404.

Why does it run before deferReply:

Before the Mono pipeline you return is subscribed to, Java calls the methods in lines 157-160 that assemble the sequence. By that point it's only an async description of what should run once subscribed. In Reactor this is called "assembly time".

However if significant work is done during assembly time, that can take valuable time towards your 3 second limit.

The fix would be to implement methodThatTakesALongTime inside a lambda so Java doesn't actually run the method until it's subscribed, Reactor calls that "subscription time".

public Mono<Void> methodThatTakesALongTime() {
	return Mono.fromCallable(() -> {
		// your previous code
	});
}

or wrap the outer call with defer

.then(Mono.defer(() -> methodThatTakesALongTime(...)))

Depending on how is the method implemented

Why are A and C working?

A works because all methods in the chain are compliant with reactor where no significant work should happen before you subscribe. methodThatTakesALongTime can be adapted to respect this too.

C works because you explicitly subscribe so the HTTP defer call is done immediately. However this removes proper error handling as you won't be able to handle errors if the HTTP call fails.

Hope this helps and sorry for the delayed response!