SSE

Hertz supports SSE, allowing the server to send events to the client through simple HTTP response.

SSE is short for Server-Sent Events, which is a server push technology that allows the server to send events to clients through simple HTTP responses.

Hertz’s implementation can be found here.

Installation

go get github.com/hertz-contrib/sse

Example Code

In the following example code, when accessing /sse, the server will push a timestamp to the client every second.

package main

import (
	"context"
	"net/http"
	"time"
	
	"github.com/hertz-contrib/sse"
	
	"github.com/cloudwego/hertz/pkg/app"
	"github.com/cloudwego/hertz/pkg/app/server"
	"github.com/cloudwego/hertz/pkg/common/hlog"
)

func main() {
	h := server.Default()
	
	h.GET("/sse", func(ctx context.Context, c *app.RequestContext) {
		lastEventID := sse.GetLastEventID(c)
		
        // The client can inform the server of the last event received through Last-Event-ID.
        hlog.CtxInfof(ctx, "last event ID: %s", lastEventID)
		
        // You must set status code and response headers before calling Render for first time.
        c.SetStatusCode(http.StatusOK)
        
        s := sse.NewStream(c)
		for t := range time.NewTicker(1 * time.Second).C {
			event := &sse.Event{
				Event: "timestamp",
				Data:  []byte(t.Format(time.RFC3339)),
			}
			err := s.Publish(event)
			if err != nil {
				return
			}
		}
    })
	
	h.Spin()
}

Configuration

NewStream

NewStream is used to create a stream for sending events. By default, Content-Type is set as text/event-stream (it’s better not to modify the Content-Type). Cache-Control is set as no-cache.

If there are any proxies between server and client, it’s recommended to set the response header of X-Accel-Buffering as no.

Function signature:

func NewStream(c *app.RequestContext) *Stream

Example code:

package main

func main() {
    h := server.Default()
    
    h.GET("/sse", func(ctx context.Context, c *app.RequestContext) {
        c.SetStatusCode(http.StatusOK)
        c.Response.Header.Set("X-Accel-Buffering", "no")
        s := sse.NewStream(c)
		// ...
    })
   
}

Publish

Publish is used to send an event message to the client. The format of the event message should be as follows:

type Event struct {
	// Name of this event.
	Event string
	
	// Data associated with this event.
	Data []byte
	
	// ID number assigned by Server Sent Events protocol.
	ID string
	
	// Retry duration time specified in milliseconds; if present then browser will attempt reconnect after that many milliseconds have elapsed since last message received from server.
	Retry time.Duration 
}

Function signature:

func (c *Stream) Publish(event *Event) error 

GetLastEventID

GetLastEventID retrieves the ID number of last received SSE message from client. If no SSE messages were sent yet returns empty string.

Function signature:

func GetLastEventID(c *app.RequestContext) string