为何Spring Cloud Gateway会降低SSE并发连接数？

我们使用响应式Spring Boot应用为按key订阅的客户端生成Server Sent Events（SSE），核心接口代码如下：

@GetMapping(path = "/sse", produces = MediaType.TEXT_EVENT_STREAM_VALUE)
public Flux<String> sse(@RequestParam String key) {
  return Flux.interval(Duration.ofSeconds(2))
      .map((i) -> key + " Fixed delay task - " + System.currentTimeMillis() / 1000);
}

两种访问场景的差异 #

直接访问服务 #

不通过Spring Cloud Gateway访问时，该服务最多可建立10K个并发Event Source连接，超过时抛出如下异常：

2022-09-07 12:59:02.123  WARN 90517 --- [ctor-http-nio-1] io.netty.channel.DefaultChannelPipeline  : An exceptionCaught() event was fired, and it reached at the tail of the pipeline. It usually means the last handler in the pipeline did not handle the exception.

java.io.IOException: Too many open files
    at java.base/sun.nio.ch.Net.accept(Native Method) ~[na:na]
    at java.base/sun.nio.ch.ServerSocketChannelImpl.implAccept(ServerSocketChannelImpl.java:425) ~[na:na]

通过Spring Cloud Gateway访问 #

当以Spring Cloud Gateway作为前置网关时，并发连接上限降至2K，超过后服务端抛出：

2022-09-07 13:09:37.590  WARN 90707 --- [ctor-http-nio-9] reactor.netty.channel.FluxReceive        : [559c58ed-1, L:/127.0.0.1:9999 - R:/127.0.0.1:54567] An exception has been observed post termination, use DEBUG level to see the full stack: java.net.SocketException: Connection reset

同时网关端抛出：

2022-09-07 12:59:55.696 ERROR 90264 --- [ctor-http-nio-2] a.w.r.e.AbstractErrorWebExceptionHandler : [7772ec3b-4438]  500 Server Error for HTTP GET "/sse?key=2682"

java.net.SocketException: Connection reset
    at java.base/sun.nio.ch.SocketChannelImpl.throwConnectionReset(SocketChannelImpl.java:394) ~[na:na]
    Suppressed: reactor.core.publisher.FluxOnAssembly$OnAssemblyException: 
Error has been observed at the following site(s):
    *__checkpoint ⇢ org.springframework.cloud.gateway.filter.WeightCalculatorWebFilter [DefaultWebFilterChain]
    *__checkpoint ⇢ HTTP GET "/sse?key=2682" [ExceptionHandlingWebHandler]
Original Stack Trace:
        at java.base/sun.nio.ch.SocketChannelImpl.throwConnectionReset(SocketChannelImpl.java:394) ~[na:na]
        at java.base/sun.nio.ch.SocketChannelImpl.read(SocketChannelImpl.java:426) ~[na:na]
        at io.netty.buffer.PooledByteBuf.setBytes(PooledByteBuf.java:258) ~[netty-buffer-4.1.79.Final.jar:4.1.79.Final]

疑问 #

为何Spring Cloud Gateway会对SSE服务的并发连接数产生如此大的影响？

内容的提问来源于stack exchange，提问作者Toni