Server-sent events (SSE) don't work with CIO server

Ktor Version

1.2.2

Ktor Engine Used

ktor-server-cio

JVM Version, Operating System and Relevant Context

$ java -version
java version "1.8.0_31"
Java(TM) SE Runtime Environment (build 1.8.0_31-b13)
Java HotSpot(TM) 64-Bit Server VM (build 25.31-b07, mixed mode)

Feedback

When trying to produce server-sent events, response body doesn’t get sent to client. If I start server like this:

package cz.scuk.app

import io.ktor.application.call
import io.ktor.http.CacheControl
import io.ktor.http.ContentType
import io.ktor.response.cacheControl
import io.ktor.response.respondTextWriter
import io.ktor.routing.get
import io.ktor.routing.routing
import io.ktor.server.cio.CIO
import io.ktor.server.engine.embeddedServer
import kotlinx.coroutines.delay

fun main() {
    embeddedServer(CIO, port = 8080) {
        routing {
            get("/") {
                call.response.cacheControl(CacheControl.NoCache(null))
                call.respondTextWriter(contentType = ContentType.Text.EventStream) {
                    while (true) {
                        write("data: ${System.currentTimeMillis()}\n\n")
                        flush()
                        delay(1000)
                    }
                }
            }
        }
    }.start(true)
}

And then send request with curl:

$ curl -v --no-buffer http://localhost:8080/
*   Trying ::1...
* TCP_NODELAY set
* Connected to localhost (::1) port 8080 (#0)
> GET / HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/7.54.0
> Accept: */*
> 
< HTTP/1.1 200 OK
< Cache-Control: no-cache
< Transfer-Encoding: chunked
< Content-Type: text/event-stream; charset=UTF-8
< 

The curl hangs, no response is being received.

When CIO implementation is replaced with Netty, client periodically receives data chunks. Also when delay(1000) is removed, data are also sent to the client.

[arkanovicz]

Could you please describe your use-case: why do you use SSE and not WebSockets?

Just out of curiosity, why not?

[e5l]

Fixed in 1.5.2