Middleware

Middleware in Iris is a way of running code before or after the main handler of an HTTP request. For example, a middleware called "logger" could write the request details to a file or the console, before passing the control to the main handler. Middleware is very flexible and reusable within and across your applications!

A middleware is simply an iris.Handler that has the signature func(ctx iris.Context). Each middleware is executed when the previous one calls the ctx.Next() method, which can be used for authentication, i.e., if the request is not authorized, we could send an error response instead of calling this method.

Iris offers 7 (+1) different ways to register middleware, depending on your application's needs. Party.UseError, Party.Use, Party.UseOnce, Application.UseGlobal, Party.Done, Application.DoneGlobal, Party.UseRouter and Application.WrapRouter. Iris gives you the tools to have full control over the request flow.

Register A Middleware

Let's see how each one of these methods works in practice by registering middlewares in an Iris Application and Party (group of routes). Below you will see a simple example that shows the order of execution of each registered handler to the client.

package main

import (
    "net/http"
    
    "github.com/kataras/iris/v12"
)

func main() {
    app := iris.New()
    
    app.WrapRouter(routerWrapper)
    app.UseRouter(routerMiddleware)
    app.UseGlobal(globalMiddleware)
    app.Use(useMiddleware)
    app.UseError(errorMiddleware)
    // app.Done(done)
    // app.DoneGlobal(doneGlobal)
    
    // Adding a OnErrorCode(iris.StatusNotFound) causes `.UseGlobal`
    // to be fired on 404 pages without this,
    // only `UseError` will be called, and thus should
    // be used for error pages.
    app.OnErrorCode(iris.StatusNotFound, notFoundHandler)
    
    app.Get("/", mainHandler)

    app.Listen(":8080")
}

func mainHandler(ctx iris.Context) {
    ctx.WriteString("Main Handler")
}

func notFoundHandler(ctx iris.Context) {
    ctx.WriteString("404 Error Handler")
}

The `Application.WrapRouter` Method

A WrapperFunc registered in this way runs for the whole Application, before anything else. It is executed last registered first, unlike other middleware methods. A WrapperFunc is not an iris.Handler, instead its signature is: func(http.ResponseWriter, *http.Request, router http.HandlerFunc). It's the lowest-level functionality used to intercept requests and optionally change the behaviour of the router (e.g. navigate to another route than the requested path one, perform security checks before even the router is executed and more). A good example of use-case is a CORS implementation.

The `Party.UseRouter` Method

A middleware registered in this way will run under a specific Party's path prefix on all routes (whether they are matched or not, including routes that would result in an error.) All child Parties inherit them unless Party.ResetRouteFilters() or Party.Reset() is called. They are executed in the order they are registered, and they run before UseGlobal and Use on matched routes or UseError on errors.

The `Application.UseGlobal` Method

A middleware registered in this way will run on all previous and future routes, and registered error handlers (pages matched with OnErrorCode), in the whole Application (all Parties and Subdomains included.) They are also executed in the order they are registered, and they run before Use or UseError.

The `Party.Use` Method

A middleware registered in this way will run after UseGlobal under a particular Party and its children. They are also executed in the order they are registered, and they run right before the Handle itself (such as Get, Post, Patch, ...) It is not executed on errors.

The `Party.UseError` Method

A middleware registered in this way will run only when an HTTP error is encountered (such as the case with an unavailable 404 or protected 401 resources), unlike Use. They fire for all children Parties and will be called in the order they were registered.

The `Party.Done` Method

Register handler to run after routes middleware and handlers under a specific Party and its children. Requires a ctx.Next call on the last route's handler, unless Execution Rules are modified.

The `Application.DoneGlobal` Method

Same as UseGlobal but for the Done handlers. Runs on the Application instance level, after everything else.

The middlewares used for this example are included below:

func routerWrapper(w http.ResponseWriter, r *http.Request,
    router http.HandlerFunc) {

    if r.URL.Path == "/" {
        w.Write([]byte("#1 .WrapRouter\n"))
        /* Note for new Gophers:
            If we Write anything here on an error resource in the raw
            `net/http` wrapper like this one, then the response writer will
            automatically send a `200` OK status code (when we first write).
            Any error handler executed after this will not fire as expected.
            Also, when `w.WriteHeader` is called you can NOT change the
            status code later on.

            In Iris Handlers, if you write before the status code has been
            set, then it will also automatically send the 200 OK status
            code which then cannot be changed later. However, if we call
            `ctx.StatusCode` inside an Iris Handler without writing any
            content, then we can change the status code later on. When you
            need to change that behaviour, you must start the handler with
            a `ctx.Record` call.
        */
    }

    // Continue by executing the Iris Router and let it do its job.
    router(w, r)
}


func routerMiddleware(ctx iris.Context) {
    if ctx.Path() == "/" {
        ctx.WriteString("#2 .UseRouter\n")
	// The same caveat described in routerWrapper applies here as well.
    }

    ctx.Next()
}

func globalMiddleware(ctx iris.Context) {
    ctx.WriteString("#3 .UseGlobal\n")
    ctx.Next()
}

func useMiddleware(ctx iris.Context) {
    ctx.WriteString("#4 .Use\n")
    ctx.Next()
}

func errorMiddleware(ctx iris.Context) {
    ctx.WriteString("#3 .UseError\n")
    ctx.Next()
}

Run our simple application:

$ go run main.go
Now listening on: http://localhost:8080
Application started. Press CTRL+C to shut down.

Point your browser to http://localhost:8080, and the output should look exactly like this:

#1 .WrapRouter
#2 .UseRouter
#3 .UseGlobal
#4 .Use
Main Handler

And for a page that should give a 404, such as http://localhost:8080/a_not_found_resource:

#3 .UseGlobal
#3 .UseError
404 Error Handler

(Note that WrapRouter and UseRouter are still fired 1st and 2nd, just not shown in the output. Read the block comment to learn why.)

However, without an OnErrorCode:

It is important to note that without OnErrorCode registered, your UseGlobal will not be executed.

#3 .UseError
Not Found

Modify A Middleware

You can modify the behavior of any existing middleware by wrapping it.

Let's take, for example, that you want to register a middleware on UseRouter to run everywhere except on the static.example.com subdomain.

You have two ways to do that. The first one is:

package main

import (
    "github.com/kataras/iris/v12"
    "github.com/kataras/iris/v12/middleware/basicauth"
)

func main() {
    users := map[string]string{"username":"password"}
    auth := basicauth.Default(users)

    app.UseRouter(skipStaticSubdomain(auth)) // <--

    // [...]
    app.Listen(":80")
}

func skipStaticSubdomain(handler iris.Handler) iris.Handler {
    return func(ctx iris.Context) {
        if ctx.Subdomain() == "static." {
            // continue to the next or main handler and exit.
            ctx.Next()
            return
        }

        handler(ctx)   
    }
}

And the second way is by using the iris.NewConditionalHandler:

type Filter func(Context) bool

func NewConditionalHandler(filter Filter, handlers ...Handler) Handler

Here is a usage example, which checks if a subdomain exists and it is NOT the static. one:

app.UseRouter(iris.NewConditionalHandler(isNotStaticSubdomain, auth))

func isNotStaticSubdomain(ctx iris.Context) bool {
    return ctx.Subdomain() != "static."
}

That's all. When you've got the idea, it's trivial.

Transfer Data Between Handlers

Each request-response lifecycle contains an instance of iris.Context. This instance is shared across the handlers chain—the Context.Values() returns temporary memory storage which can be used to transfer data between middleware and handlers.

This storage contains many helper methods, the most important you'll probably use are:

Set(key string, value interface{}) (Entry, bool)
Get(key string) interface{}

Usage

Set a value:

func myMiddleware(ctx iris.Context) {
    ctx.Values().Set("key", value)
}

Get a value:

func myHandler(ctx iris.Context) {
    value := ctx.Values().Get("key")
}

Complete list of the memstore methods.

Writing a middleware

package main

import "github.com/kataras/iris/v12"

func main() {
    app := iris.New()
    // or app.Use(before) and app.Done(after).
    app.Get("/", before, mainHandler, after)
    app.Listen(":8080")
}

func before(ctx iris.Context) {
    shareInformation := "this is a sharable information between handlers"

    requestPath := ctx.Path()
    println("Before the mainHandler: " + requestPath)

    ctx.Values().Set("info", shareInformation)
    ctx.Next() // execute the next handler, in this case the main one.
}

func after(ctx iris.Context) {
    println("After the mainHandler")
}

func mainHandler(ctx iris.Context) {
    println("Inside mainHandler")

    // take the info from the "before" handler.
    info := ctx.Values().GetString("info")

    // write something to the client as a response.
    ctx.HTML("<h1>Response</h1>")
    ctx.HTML("<br/> Info: " + info)

    ctx.Next() // execute the "after".
}

$ go run main.go # and navigate to the http://localhost:8080
Now listening on: http://localhost:8080
Application started. Press CTRL+C to shut down.
Before the mainHandler: /
Inside mainHandler
After the mainHandler

Removing a Handler from a Route

To remove a specific handler from a specific route use the RemoveHandler method of the *Route registered by the Handle/Get/Post/Put... methods. The RemoveHandler method expects the handler name (it's the PC func, see HandlerName of the iris context subpackage) or the handler itself.

Example Code:

func middleware(ctx iris.Context) {
    // [...]
}

func main() {
    app := iris.New()

    // Register the middleware to all matched routes.
    app.Use(middleware)

    // Handlers = middleware, other
    app.Get("/", index)

    // Handlers = other
    app.Get("/other", other).RemoveHandler(middleware)
}

Execution Rules

You could also use the ExecutionRules to force Begin(UseXXX) and Finish(DoneXXX) handlers to be executed without the requirement of a ctx.Next() call, to forcibly forward them:

app.SetExecutionRules(iris.ExecutionRules{
    // Begin: ...
    // Main:  ...
    Done: iris.ExecutionOptions{Force: true},
})

Convert `http.Handler/HandlerFunc`

However, you are not limited to them - you can use any third-party middleware that works with the net/http package.

Iris, unlike others, is 100% compatible with the standards and that's why many big companies that use Go in their workflow, like a very famous US Television Network, trust Iris; it's up-to-date, and it will always follow the std net/http package which is updated by the Go Authors on each new release of the Go Programming Language.

You can use any third-party middleware that is written for net/http with Iris by using the iris.FromStd(aThirdPartyMiddleware). Remember, ctx.ResponseWriter() and ctx.Request() return the same net/http input arguments of an http.Handler.