如何从Spring WebFlux中的多个Flux(WebsocketSession :: receive)正确向Sink发射值? [英] How to correctly emit values to Sink from multiple Fluxes (WebsocketSession::receive) in Spring WebFlux?
问题描述
在简化的情况下，我想将WebSocket客户端发送的消息广播给所有其他客户端.该应用程序是使用Spring的反应式Websockets构建的.
In my simplified case I want to broadcast a message sent by WebSocket client to all other clients. The application is built using reactive websockets with Spring.
我的想法是使用单身
Sink，如果从客户端收到消息，则在此接收器上发出它. WebsocketSession::send只是将此Sink发出的事件转发到连接的客户端.
My idea was to use single
Sink and if a message is received from the client, emit it on this sink. WebsocketSession::send just forwards events emitted by this Sink to connected clients.
@Component
class ReactiveWebSocketHandler(private val sink: Sinks.Many<Message>,
private val objectMapper : ObjectMapper) : WebSocketHandler {
override fun handle(session: WebSocketSession): Mono<Void> {
val input = session.receive()
.doOnNext {
sink.emitNext(fromJson(it.payloadAsText, Message::class.java), Sinks.EmitFailureHandler.FAIL_FAST)
}
.then()
val output = session.send(sink.asFlux().map { message -> session.textMessage(toJson(message)) })
return Mono.zip(input, output).then()
}
fun toJson(obj : Any) : String = objectMapper.writeValueAsString(obj)
fun <T> fromJson(json : String, clazz : Class<T>) : T{
return objectMapper.readValue(json, clazz)
}
}
此实现并不安全，因为可以从不同的线程调用Sink.emitNext.
This implementation is not safe as Sink.emitNext can be called from different threads.
我的尝试是使用publishOn并传递单线程的Scheduler，以便从单个线程调用所有WebSocketSession的onNext.然而
这是行不通的.从websocket客户端发出一项，然后所有后续的websocket客户端在连接后立即收到onClose事件:
My attempt was to use publishOn and pass a singled threaded Scheduler so that onNext for all WebSocketSessions is called from a single thread. However
this does not work. One item is emitted from a websocket client and then all subsequent websocket clients receive onClose event immediately after connection :
@Component
class ReactiveWebSocketHandler(private val sink: Sinks.Many<Message>,
private val objectMapper : ObjectMapper) : WebSocketHandler {
private val scheduler = Schedulers.newSingle("sink-scheduler")
override fun handle(session: WebSocketSession): Mono<Void> {
val input = session.receive()
.publishOn(scheduler) // publish on single threaded scheduler
.doOnNext {
sink.emitNext(fromJson(it.payloadAsText, Message::class.java), Sinks.EmitFailureHandler.FAIL_FAST)
}
.then()
...
}
}
我可以看到的另一种选择是在某个通用锁上使用synchronize，以便发出线程安全:
Another option which I could see is to synchronize on some common lock so that emission is thread safe :
@Component
class ReactiveWebSocketHandler(private val sink: Sinks.Many<Message>,
private val objectMapper : ObjectMapper) : WebSocketHandler {
private val lock = Any()
override fun handle(session: WebSocketSession): Mono<Void> {
val input = session.receive()
.doOnNext {
synchronized(lock) {
sink.emitNext(fromJson(it.payloadAsText, Message::class.java), Sinks.EmitFailureHandler.FAIL_FAST)
}
}
.then()
...
}
}
但是我不确定是否应该那样做.
However I am not sure if this should be done like that.
在这种情况下是否可以使用publishOn以便发射是线程安全的，如果不是，则该问题的其他解决方案是什么(除了像我对synchronized关键字所做的那样使用同步之外).
Is it possible to use publishOn in this case so that emission is thread safe and if not what is other solution to this problem (apart of using synchronization like I have done with synchronized keyword).
推荐答案
除了使用synchronized选项进行悲观锁定之外，您还可以创建一个与FAIL_FAST相当的EmitFailureHandler，但它会为EmitResult.NON_SERIALIZED_ACCESS返回true
Instead of pessimistic locking with the synchronized option, you could create an EmitFailureHandler comparable to FAIL_FAST except it returns true for EmitResult.NON_SERIALIZED_ACCESS.
这将导致并发的发射尝试立即重试，例如在繁忙的循环中.
This would result in the concurrent emit attempts to be immediately retried, like in a busy loop.
乐观地说，这将最终成功.如果您想对无限循环采取额外的防御措施，甚至可以使自定义处理程序引入延迟或限制其返回true的次数.
Optimistically, this will end up succeeding. You can even make the custom handler introduce a delay or limit the number of times it returns true if you want to be extra defensive against infinite loops.
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