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
myapp
.
All files for this application reside in this directory.
Inside the
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
message
Hello, world!
.
Note:
When writing a stand-alone Go program we would place this code in
package
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,
hello
).
Creating the Configuration File
An App Engine application has a configuration file called
app.yaml
.
Among other things, this file tells the App Engine service which runtime to use
and which URLs should be handled by our Go program.
Inside the
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 tohelloworld
. -
This is version number
1
of 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
go
runtime environment, with API versiongo1
. -
Every request to a URL whose path matches the regular expression
/.*
(all URLs) should be handled by the Go program. The_go_app
value 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
our
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.
The syntax of this file is YAML . For a complete list of configuration options, see the Go Application Configuration page.
Testing the Application
With the
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
myapp
directory,
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
if
myapp
is your current directory, so the command is simply:
goapp serve
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 Development Server reference
,
or run
goapp help serve
.
Iterative Development
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
goapp
.
Try it now: leave the web server running, then edit
hello.go
to
change
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.
Next…
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.