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Hi, I’m trying to implement a simple <SSE server> ...
# ktor
k
Hi, I’m trying to implement a simple SSE server with ktor 3.0.3:
Copy code
import io.ktor.server.application.install
import io.ktor.server.engine.embeddedServer
import io.ktor.server.netty.Netty
import io.ktor.server.routing.routing
import io.ktor.server.sse.SSE
import io.ktor.server.sse.sse
import io.ktor.sse.ServerSentEvent
import kotlinx.coroutines.delay

fun main() {
    embeddedServer(Netty, port = 8080) {
        install(SSE)
        routing {
            sse("/sse") {
                repeat(6) {
                    println("Sending SSE #$it")
                    send(ServerSentEvent("this is SSE #$it"))
                    delay(1000)
                }
            }
        }
    }.start(wait = true)
}
When I run it and try to consume, I get an empty response:
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$curl -N -v -w "\nTotal time: %{time_total}s\n" <http://localhost:8080/sse>

* Host localhost:8080 was resolved.
* IPv6: ::1
* IPv4: 127.0.0.1
*   Trying 127.0.0.1:8080...
* Connected to localhost (127.0.0.1) port 8080
> GET /sse HTTP/1.1
> Host: localhost:8080
> User-Agent: Mozilla/5.0 (Windows NT 6.1; rv:45.0) Gecko/20100101 Firefox/45.0
> Accept: */*
> 
* Request completely sent off
* Empty reply from server
* Closing connection
curl: (52) Empty reply from server
I see the server route was triggered, but only 2 events were sent, but neither HTTP headers nor SSE events were received:
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[main] INFO io.ktor.server.Application - Responding at <http://0.0.0.0:8080>
Sending SSE #0
Sending SSE #1
What am I missing?
👀 1
m
Hi! I created a new empty Ktor project with IDEA generator and pasted your code. I can see the next output:
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curl -N -v -w "\nTotal time: %{time_total}s\n" <http://localhost:8080/sse>
*   Trying 127.0.0.1:8080...
* Connected to localhost (127.0.0.1) port 8080 (#0)
> GET /sse HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/7.86.0
> Accept: */*
> 
* Mark bundle as not supporting multiuse
< HTTP/1.1 200 OK
< Content-Type: text/event-stream
< Cache-Control: no-store
< Connection: keep-alive
< X-Accel-Buffering: no
< transfer-encoding: chunked
< 
data: this is SSE #0

data: this is SSE #1

data: this is SSE #2

data: this is SSE #3

data: this is SSE #4

data: this is SSE #5

* Connection #0 to host localhost left intact

Total time: 6.161408s
And for server:
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2025-01-28 15:45:27.208 [main] INFO  io.ktor.server.Application - Application started in 0.223 seconds.
2025-01-28 15:45:27.335 [main] INFO  io.ktor.server.Application - Responding at <http://0.0.0.0:8080>
Sending SSE #0
Sending SSE #1
Sending SSE #2
Sending SSE #3
Sending SSE #4
Sending SSE #5
Can you please provide more details?
👍 1
k
Thank you, Maria. I will review my project dependencies carefully. I can say that my project is using latest ktor, Kotlin, coroutines and also okhttp3
Hey, @Maria Skripchenko I was able to reduce the scope of the problem. It turns out that just adding the 
ktor-serialization-kotlinx-json-jvm
dependency alone breaks things. Adding 
ktor-serialization-jackson-jvm
does not hurt. Pom.xml:
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<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="<http://maven.apache.org/POM/4.0.0>" xmlns:xsi="<http://www.w3.org/2001/XMLSchema-instance>"
         xsi:schemaLocation="<http://maven.apache.org/POM/4.0.0> <http://maven.apache.org/xsd/maven-4.0.0.xsd>">
    <modelVersion>4.0.0</modelVersion>
    <groupId>com.example</groupId>
    <artifactId>ktor-sample</artifactId>
    <version>0.0.1</version>
    <name>ktor-sample</name>
    <description>ktor-sample</description>
    <properties>
        <kotlin.code.style>official</kotlin.code.style>
        <kotlin_version>2.1.10</kotlin_version>
        <ktor_version>3.0.3</ktor_version>
        <logback_version>1.4.14</logback_version>
        <slf4j_version>2.0.9</slf4j_version>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <kotlin.compiler.incremental>true</kotlin.compiler.incremental>
        <main.class>com.example.ApplicationKt</main.class>
    </properties>
    <repositories>
    </repositories>
    <dependencies>
        <dependency>
            <groupId>io.ktor</groupId>
            <artifactId>ktor-server-core-jvm</artifactId>
            <version>${ktor_version}</version>
        </dependency>
        <dependency>
            <groupId>io.ktor</groupId>
            <artifactId>ktor-server-sse-jvm</artifactId>
            <version>${ktor_version}</version>
        </dependency>
        <dependency>
            <groupId>io.ktor</groupId>
            <artifactId>ktor-server-content-negotiation-jvm</artifactId>
            <version>${ktor_version}</version>
        </dependency>
        <!-- FIXME: this dependency breaks
        <dependency>
            <groupId>io.ktor</groupId>
            <artifactId>ktor-serialization-kotlinx-json-jvm</artifactId>
            <version>${ktor_version}</version>
        </dependency>
        -->
        <dependency>
            <groupId>io.ktor</groupId>
            <artifactId>ktor-server-netty-jvm</artifactId>
            <version>${ktor_version}</version>
        </dependency>
        <dependency>
            <groupId>ch.qos.logback</groupId>
            <artifactId>logback-classic</artifactId>
            <version>${logback_version}</version>
        </dependency>
        <dependency>
            <groupId>org.slf4j</groupId>
            <artifactId>slf4j-api</artifactId>
            <version>${slf4j_version}</version>
        </dependency>
    </dependencies>
    <build>
        <sourceDirectory>${project.basedir}/src/main/kotlin</sourceDirectory>
        <testSourceDirectory>${project.basedir}/src/test/kotlin</testSourceDirectory>
        <resources>
            <resource>
                <directory>${project.basedir}/src/main/resources</directory>
            </resource>
        </resources>

        <plugins>
            <plugin>
                <artifactId>kotlin-maven-plugin</artifactId>
                <groupId>org.jetbrains.kotlin</groupId>
                <version>${kotlin_version}</version>
                <configuration>
                    <jvmTarget>17</jvmTarget>
                </configuration>
                <executions>
                    <execution>
                        <id>compile</id>
                        <phase>compile</phase>
                        <goals>
                            <goal>compile</goal>
                        </goals>
                    </execution>
                    <execution>
                        <id>test-compile</id>
                        <phase>test-compile</phase>
                        <goals>
                            <goal>test-compile</goal>
                        </goals>
                    </execution>
                </executions>
            </plugin>
            <plugin>
                <groupId>org.codehaus.mojo</groupId>
                <artifactId>exec-maven-plugin</artifactId>
                <version>1.2.1</version>
                <executions>
                    <execution>
                        <goals>
                            <goal>java</goal>
                        </goals>
                    </execution>
                </executions>
                <configuration>
                    <mainClass>${main.class}</mainClass>
                </configuration>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-assembly-plugin</artifactId>
                <version>2.6</version>
                <configuration>
                    <descriptorRefs>
                        <descriptorRef>jar-with-dependencies</descriptorRef>
                    </descriptorRefs>
                    <archive>
                        <manifest>
                            <addClasspath>true</addClasspath>
                            <mainClass>${main.class}</mainClass>
                        </manifest>
                    </archive>
                </configuration>
                <executions>
                    <execution>
                        <id>assemble-all</id>
                        <phase>package</phase>
                        <goals>
                            <goal>single</goal>
                        </goals>
                    </execution>
                </executions>
            </plugin>
            <plugin>
                <groupId>org.jetbrains.kotlin</groupId>
                <artifactId>kotlin-maven-plugin</artifactId>
                <version>${kotlin_version}</version>
                <executions>
                    <execution>
                        <id>compile</id>
                        <phase>compile</phase>
                        <goals>
                            <goal>compile</goal>
                        </goals>
                    </execution>
                </executions>
                <configuration>
                    <compilerPlugins>
                        <plugin>kotlinx-serialization</plugin>
                    </compilerPlugins>
                </configuration>
                <dependencies>
                    <dependency>
                        <groupId>org.jetbrains.kotlin</groupId>
                        <artifactId>kotlin-maven-serialization</artifactId>
                        <version>${kotlin_version}</version>
                    </dependency>
                </dependencies>
            </plugin>
        </plugins>
    </build>
</project>
code:
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import io.ktor.http.CacheControl
import io.ktor.http.ContentType
import io.ktor.server.engine.embeddedServer
import io.ktor.server.netty.Netty
import io.ktor.server.response.cacheControl
import io.ktor.server.response.respondTextWriter
import io.ktor.server.routing.get
import io.ktor.server.routing.routing
import io.ktor.sse.ServerSentEvent
import kotlinx.coroutines.delay

fun main() {
    embeddedServer(Netty, port = 8080) {
        routing {
            get("{...}") {
                call.response.cacheControl(CacheControl.NoCache(null))
                call.respondTextWriter(contentType = ContentType.Text.EventStream) {
                    repeat(6) {
                        println("Sending SSE #$it")
                        write(ServerSentEvent("this is SSE #$it").toString())
                        flush()
                        delay(1000)
                    }
                }
            }
        }
    }.start(wait = true)
}
output:
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$ curl -N -v -w "\nTotal time: %{time_total}s\n" <http://localhost:8080/sse>
* Host localhost:8080 was resolved.
* IPv6: ::1
* IPv4: 127.0.0.1
*   Trying 127.0.0.1:8080...
* Connected to localhost (127.0.0.1) port 8080
> GET /sse HTTP/1.1
> Host: localhost:8080
> User-Agent: Mozilla/5.0 (Windows NT 6.1; rv:45.0) Gecko/20100101 Firefox/45.0
> Accept: */*
> 
* Request completely sent off
< HTTP/1.1 200 OK
< Cache-Control: no-cache
< Content-Type: text/event-stream; charset=UTF-8
< transfer-encoding: chunked
< 
data: this is SSE #0
data: this is SSE #1
data: this is SSE #2
data: this is SSE #3
data: this is SSE #4
data: this is SSE #5
* Connection #0 to host localhost left intact

Total time: 6.046663s
JDK: amazon-corretto-23. MacOS 15.2 M1
a
I've reproduced the problem and filed an issue to address it.
👍 1
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