How to do it...

The following steps cover the writing and running of your application:

1. From your Terminal or console application, create a new directory called ~/projects/go-programming-cookbook/chapter8/middleware, and navigate to this directory.
2. Run the following command:

$ go mod init github.com/PacktPublishing/Go-Programming-Cookbook-Second-Edition/chapter8/middleware

You should see a file called go.mod that contains the following:

module github.com/PacktPublishing/Go-Programming-Cookbook-Second-Edition/chapter8/middleware    

3. Copy the tests from ~/projects/go-programming-cookbook-original/chapter8/middleware, or use this as an exercise to write some of your own code!
4. Create a file called middleware.go with the following contents:

        package middleware

        import (
            "log"
            "net/http"
            "time"
        )

        // Middleware is what all middleware functions will return
        type Middleware func(http.HandlerFunc) http.HandlerFunc

        // ApplyMiddleware will apply all middleware, the last 
        // arguments will be the
        // outer wrap for context passing purposes
        func ApplyMiddleware(h http.HandlerFunc, middleware 
        ...Middleware) http.HandlerFunc {
            applied := h
            for _, m := range middleware {
                applied = m(applied)
            }
            return applied
        }

        // Logger logs requests, this will use an id passed in via
        // SetID()
        func Logger(l *log.Logger) Middleware {
            return func(next http.HandlerFunc) http.HandlerFunc {
                return func(w http.ResponseWriter, r *http.Request) {
                    start := time.Now()
                    l.Printf("started request to %s with id %s", r.URL, 
                    GetID(r.Context()))
                    next(w, r)
                    l.Printf("completed request to %s with id %s in
                    %s", r.URL, GetID(r.Context()), time.Since(start))
                }
            }
        }

5. Create a file called context.go with the following contents:

        package middleware

        import (
            "context"
            "net/http"
            "strconv"
        )

        // ContextID is our type to retrieve our context
        // objects
        type ContextID int

        // ID is the only ID we've defined
        const ID ContextID = 0

        // SetID updates context with the id then
        // increments it
        func SetID(start int64) Middleware {
            return func(next http.HandlerFunc) http.HandlerFunc {
                return func(w http.ResponseWriter, r *http.Request) {
                    ctx := context.WithValue(r.Context(), ID, 
                    strconv.FormatInt(start, 10))
                    start++
                    r = r.WithContext(ctx)
                    next(w, r)
                }
            }
        }

        // GetID grabs an ID from a context if set
        // otherwise it returns an empty string
        func GetID(ctx context.Context) string {
            if val, ok := ctx.Value(ID).(string); ok {
                return val
            }
            return ""
        }

6. Create a file called handler.go with the following contents:

        package middleware

        import (
            "net/http"
        )

        // Handler is very basic
        func Handler(w http.ResponseWriter, r *http.Request) {
            w.WriteHeader(http.StatusOK)
            w.Write([]byte("success"))
        }

7. Create a new directory named example and navigate to it.
8. Create a file called main.go with the following contents:

        package main

        import (
            "fmt"
            "log"
            "net/http"
            "os"

            "github.com/PacktPublishing/
             Go-Programming-Cookbook-Second-Edition/
             chapter8/middleware"
        )

        func main() {
            // We apply from bottom up
            h := middleware.ApplyMiddleware(
            middleware.Handler,
            middleware.Logger(log.New(os.Stdout, "", 0)),
            middleware.SetID(100),
            ) 
            http.HandleFunc("/", h)
            fmt.Println("Listening on port :3333")
            err := http.ListenAndServe(":3333", nil)
            panic(err)
        }

9. Run go run main.go.
10. You could also run the following commands:

$ go build
$ ./example

You should see the following output:

$ go run main.go
Listening on port :3333

11. In a separate Terminal, run the following curl command several times:

$ curl http://localhost:3333

You should see the following output:

$ curl http://localhost:3333
success

$ curl http://localhost:3333
success

$ curl http://localhost:3333
success

12. In the original main.go, you should see the following:

Listening on port :3333
started request to / with id 100
completed request to / with id 100 in 52.284µs
started request to / with id 101
completed request to / with id 101 in 40.273µs
started request to / with id 102

13. The go.mod file may be updated and the go.sum file should now be present in the top-level recipe directory.
14. If you copied or wrote your own tests, go up one directory and run go test. Ensure that all tests pass.