Go App Engine applications communicate with the outside world via a web server compatible with Go's http package. This makes writing Go App Engine applications very similar to writing stand-alone Go web applications.
Let's begin by implementing a tiny application that displays a short message.
Creating a Simple HTTP Handler
Create a directory named
All files for this application reside in this directory.
myapp directory, create a file named
hello.go, and give it the following contents:
This Go package responds to any request by sending a response containing the
Note: When writing a stand-alone Go program we would place this code in
main. The Go App Engine Runtime provides a special
main package, so you should put HTTP handler code in a package
of your choice (in this case,
Creating the Configuration File
An App Engine application has a configuration file called
Among other things, this file tells the App Engine service which runtime to use
and which URLs should be handled by our Go program.
myapp directory, create a file named
app.yaml with the following contents:
From top to bottom, this configuration file says the following about this application:
The application identifier is
helloworld. When you register your application with App Engine later in this tutorial, you will select a unique identifier, and update this value. This value can be anything during development. For now, leave it set to
This is version number
1of this application's code. If you adjust this before uploading new versions of your application software, App Engine will retain previous versions, and let you roll back to a previous version using the administrative console.
This code runs in the
goruntime environment, with API version
Every request to a URL whose path matches the regular expression
/.*(all URLs) should be handled by the Go program. The
_go_appvalue is a magic string recognized by the development web server; it is ignored by the production App Engine servers.
Note: the Go SDK does things differently than the Python and Java SDKs:
all Go packages for a given app are built into a single executable, and request
dispatch is handled by the Go program itself. This is why we call
http.HandleFunc inside the
init function to associate
handler with the web root (
However, you may still use the
app.yaml file to configure paths
that serve static files or require special permissions.
Testing the Application
hello package and configuration file mapping every URL to
the Go program, the application is complete. You can now test it with the web
server included with the App Engine SDK.
Check that you have everything in its right place. The application's directory structure should look like this:
myapp/ app.yaml hello.go
Run the following command, giving it the path to the
to compile your app and start the development web server:
/path/to/go_appengine/goapp serve myapp/
You may drop the
/path/to/go_appengine/ if you added it to your
PATH, as suggested earlier. You can also omit the path altogether
myapp is your current directory, so the command is simply:
The web server is now running, listening for requests on port 8080. Test the application by visiting the following URL in your web browser:
For more information about running the development web server, including how to
change which port it uses, see
the Go Development Server reference,
goapp help serve.
The development app server knows to watch for changes in your file. As you
update your source, it recompiles them and relaunches your local app.
There's no need to restart
Try it now: leave the web server running, then edit
Hello, world! to something else. Reload http://localhost:8080/ to see the change.
To shut down the web server, make sure the terminal window is active, then press Control-C (or the appropriate "break" key for your console).
Leave the web server running for the rest of this tutorial. If you need to stop it, you can restart it again by running the command above.
You now have a complete App Engine application! You could deploy this simple greeting right now and share it with users worldwide. But before we deploy it, let's add some features.