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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.
Create a directory named myapp
.
All files for this application reside in this directory.
Inside the myapp
directory, create another directory named
hello
. This will contain the Go source files for our
hello
package.
Inside the hello
directory, create a file named hello.go
, and give it the following contents:
package hello import ( "fmt" "http" ) func init() { http.HandleFunc("/", handler) } func handler(w http.ResponseWriter, r *http.Request) { fmt.Fprint(w, "Hello, world!") }
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
).
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:
application: helloworld version: 1 runtime: go api_version: 3 handlers: - url: /.* script: _go_app
From top to bottom, this configuration file says the following about this application:
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 helloworld
.
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.
go
runtime environment,
with API version 3
.
/.*
(all URLs) should be handled by the Go program. The
_go_app
value is a magic string recognized by the
dev_appserver.py
;
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 app.yaml reference.
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/ 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/google_appengine/dev_appserver.py myapp/
(You may drop the /path/to/google_appengine/
if you added it to
your PATH
, as suggested earlier.)
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 Dev Web Server reference,
or run the command with the option --help
.
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 dev_appserver.py
.
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.
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.
Continue to Using the Users Service.