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Query-based matching

Query parameters are variables that get passed along with the URL in an HTTP request. This is what we commonly see in a REST GET request. The gorilla/mux route can match and collect query parameters. See this following URL, for example:

http://localhost:8000/articles?id=123&category=books

It has id and category as query parameters. All query parameters begin after the ? character.

Let us modify our copy of our previous example into a new one with the name queryParameters/main.go. Modify the route object to point to a new handler called QueryHandler, like this:

// Add this in your main program
r := mux.NewRouter()
r.HandleFunc("/articles", QueryHandler)

In QueryHandler, we can use request.URL.Query() to obtain query parameters from the HTTP request. QueryHandler looks like this:

// QueryHandler handles the given query parameters
func QueryHandler(w http.ResponseWriter, r *http.Request) {
  queryParams := r.URL.Query()
  w.WriteHeader(http.StatusOK)
  fmt.Fprintf(w, "Got parameter id:%s!\n", queryParams["id"][0])
  fmt.Fprintf(w, "Got parameter category:%s!", queryParams["category"][0])
}

This program is similar to the previous example, but processes query parameters instead of path parameters.

Run the new program:

go run $GOPATH/src/github.com/git-user/chapter2/queryParameters/main.go

Fire a curl request in this format in a Terminal:

curl -X GET http://localhost:8000/articles\?id\=1345\&category\=birds

We need to escape special characters in the shell. If it is in the browser, there is no problem of escaping. The output looks like this:

Got parameter id:1345! 
Got parameter category:birds!

The r.URL.Query() function returns a map with all the parameter and value pairs. They are basically strings and, in order to use them in our program logic, we need to convert the number strings to integers. We can use Go's strconv package to convert a string to an integer, and vice versa.

We have used http.StatusOK to write a successful HTTP response. Similarly, use appropriate status codes for different REST operations. For example, 404 – Not found, 500 – Server error, and so on.