How to write a stream API using gin-gonic server in golang? Tried c.Stream didnt work

Question

I wanted to create one streaming API using gin-gonic server in golang.

func StreamData(c *gin.Context) {
    chanStream := make(chan int, 10)
    go func() {for i := 0; i < 5; i++ {
        chanStream <- i
        time.Sleep(time.Second * 1)
    }}()
    c.Stream(func(w io.Writer) bool {
        c.SSEvent("message", <-chanStream)
        return true
    })
}

router.GET("/stream", controller.StreamData)

But when I am trying to hit the endpoint, it just stucks and no response comes. Has someone used the stream function so that he/she can point the mistake which I might be doing. Thanks!

Figured out the issue. After SSEvent, returning true is making the stream function to keep on running, after communication through channel is completed, we need to return false after that stream will get closed. — kunalag
But now the issue is, till the time stream function is running, the caller doesnt get any stream data, it gets only after the c.Stream function completes. — kunalag

mattn mattn · Accepted Answer · 2017-06-29T14:56:37

You should return false if the stream ended. And close the chan.

package main

import (
    "io"
    "time"

    "github.com/gin-gonic/contrib/static"
    "github.com/gin-gonic/gin"
    "github.com/mattn/go-colorable"
)

func main() {
    gin.DefaultWriter = colorable.NewColorableStderr()
    r := gin.Default()
    r.GET("/stream", func(c *gin.Context) {
        chanStream := make(chan int, 10)
        go func() {
            defer close(chanStream)
            for i := 0; i < 5; i++ {
                chanStream <- i
                time.Sleep(time.Second * 1)
            }
        }()
        c.Stream(func(w io.Writer) bool {
            if msg, ok := <-chanStream; ok {
                c.SSEvent("message", msg)
                return true
            }
            return false
        })
    })
    r.Use(static.Serve("/", static.LocalFile("./public", true)))
    r.Run()
}

Additional

Client code should be:

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title></title>
</head>
<body>
<script>
var stream = new EventSource("/stream");
stream.addEventListener("message", function(e){
    console.log(e.data);
});
</script>    
</body>
</html>

How to write a stream API using gin-gonic server in golang? Tried c.Stream didnt work

1 Answers