Passing Parameters in Golang HTTP Context

Web
Golang
Gin-Gonic
HTTP


When developing HTTP APIs, we may have to process the same request-specific data throughout middlewares. Since it’s a quite common pattern, I decide to figure it out and share how I solve it.

Example

Here we are using Gin, a web framework written in golang featuring performance and good productivity.

Suppose we have an api endpoint /api/hello handled by HelloHandleFunc. For each incoming request, we want to authenticate its JWT (in Authentication header) and pass the decoded user ID to HelloHandleFunc. This website clearly illustrates what JWT is.

The main idea is to make use of context.Context in http.Request. The context of each request controls the entire request life cycle. Thus, we can set request-specific data (key-value pairs) in the context using context.WithValue() and retrives it using Context.Value().

The following code shows the implementation of AddUserID() middleware, which summarizes the above procedure:

func extractToken(r *http.Request) string {
	bearToken := r.Header.Get(conf.JWTAuthHeader)
	strArr := strings.Split(bearToken, " ")
	if len(strArr) == 2 {
		return strArr[1]
	}
	return ""
}
func AddUserID() gin.HandlerFunc {
	return func(c *gin.Context) {
        accessToken := extractToken(c.Request)
        if accessToken == "" {
		    c.AbortWithStatus(http.StatusUnauthorized)
			return
		}
        userID := DecodeJWT(accessToken)
		c.Request = c.Request.WithContext(context.WithValue(c.Request.Context(), "user_id", userID))
		c.Next()
	}
}

Retrieve user_id from the context in HelloHandleFunc:

func HelloHandleFunc(c *gin.Context) {
	customerID, ok := c.Request.Context().Value("user_id").(string)
	if !ok {
		c.Abort()
		return
	}
	c.String(http.StatusOK, fmt.Printf("hi, %s!", customerID))
	return
}

Register the route with AddUserID() middleware enabled:

router := gin.New()
group := router.Group("/api/hello")
group.Use(AddUserID())
group.GET("/hello", HelloHandleFunc)

Start the server:

router.Run(":8080")

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