The Go Development Server

The Google Cloud SDK and App Engine SDK for Go each include a local development server that you can run locally to simulate your application running in production App Engine. The simulated environment enforces some sandbox restrictions, such as restricted system functions and Go module imports, but not others, like request time-outs or quotas.

The local development server also simulates the services provided by the libraries in the SDK for App Engine, including Datastore, Memcache, and Task Queues, by performing their tasks locally. Note that when your application is running in the development server, you can still make remote API calls to the production infrastructure using Google APIs HTTP endpoints.

Running the local development server

After you create the app.yaml configuration file for your app, you can start the local development server with the dev_appserver.py command to run your app locally.

To start the local development server:

  1. Run the dev_appserver.py command as follows from the directory that contains your app's app.yaml configuration file:
    dev_appserver.py

    Specify the directory path to your app, for example:

    dev_appserver.py [PATH_TO_YOUR_APP]

    Alternatively, you can specify the configuration file of a specific service, for example:

    dev_appserver.py app.yaml

    To change the port, you include the --port option:

    dev_appserver.py --port=9999 [PATH_TO_YOUR_APP]
    goapp serve

    The goapp serve command is included with the original App Engine SDK for Go and wraps the underlying dev_appserver.py Python tool. To start the local development server, you run:

    goapp serve

    Tip: Using the Python tool directly as dev_appserver.py provides you with greater control over the local development server.

    dev_appserver.py not working?

    To learn more about the dev_appserver.py command options, see Local Development Server Options.

  2. The local development server is now running and listening for requests. You can visit http://localhost:8080/ in your web browser to see the app in action.

    If you specified a custom port with the --port option, remember to open your browser to that port.

To stop the local server from the command line, you press the following:

  • Mac OS X or Linux: Control-C
  • Windows: Control-Break

Specifying application IDs

If you want to use application IDs with the local development server, don't use the App ID from the APPLICATION_ID environment variable. The development server differs from production App Engine service in that the local server prepends the string dev~ to the APPLICATION_ID environment variable.

Instead, if you need to access your App ID in the local server, for example to spoof an email address, use the appengine.AppID function. To get the hostname of the running app, use the appengine.DefaultVersionHostname function.

Detecting application runtime environment

To determine whether your code is running in production or in the local development server, you can call the IsDevAppServer() method. When this is true, your code is running in the local development environment; otherwise, you're running in production.

package devserver

import (
	"fmt"
	"net/http"

	"google.golang.org/appengine"
)

func init() {
	http.HandleFunc("/", handler)
}

func handler(w http.ResponseWriter, r *http.Request) {
	fmt.Fprintf(w, "IsDevAppServer: %v", appengine.IsDevAppServer())
}

Using the local Datastore

The local development server simulates the App Engine datastore using a local file that persists between invocations of the local server.

For more information on indexes and index.yaml, see the Datastore Indexes and Datastore Index Configuration pages.

Browsing the local Datastore

If your app has written data to your local Datastore using the local development server, you can browse it in the local development console.

To browse local Datastore:

  1. Start the development server.

  2. Access the Datastore Viewer in the local development console. (The URL is http://localhost:8000/datastore.)

  3. View your local Datastore contents.

Specifying the ID allocation policy

For production App Engine, you can set the Datastore to automatically generate entity IDs.

Although the auto ID assignment policies for the production server are completely different than those used by the development server, you can also set the automatic ID allocation policy for the local server.

To specify the automatic ID assignment policy, use the --auto_id_policy option:

dev_appserver.py --auto_id_policy=sequential

where --auto_id_policy can be one of the following:

  • scattered: (default) IDs are assigned from a non-repeating sequence of approximately uniformly distributed integers.
  • sequential: IDs are assigned from the sequence of consecutive integers.

Clearing the local Datastore

To clear the local datastore for an application, invoke the local development server as follows:

dev_appserver.py --clear_datastore=yes app.yaml

Changing local Datastore location

To change the location used for the datastore file, use the --datastore_path option:

dev_appserver.py --datastore_path=/tmp/myapp_datastore app.yaml

Using the Users service

App Engine provides a Users Service to simplify authentication and authorization for your application. The local development server simulates the behavior of Google Accounts with its own sign-in and sign-out pages. While running under the local development server, the LoginURL and LogoutURL functions return URLs for /_ah/login and /_ah/logout on the local server.

Using Mail

The local development server can send email for calls to the App Engine mail service using either an SMTP server or a local installation of Sendmail.

Using SMTP

To enable mail support with an SMTP server, invoke dev_appserver.py as follows::

dev_appserver.py --smtp_host=smtp.example.com --smtp_port=25 \
    --smtp_user=ajohnson --smtp_password=k1tt3ns [PATH_TO_YOUR_APP]

where you set the --smtp_host, --smtp_port, --smtp_user and --smtp_password options with your own values.

Using Sendmail

To enable mail support with Sendmail, invoke dev_appserver.py as follows:

dev_appserver.py --enable_sendmail=yes [PATH_TO_YOUR_APP]

The local server will use the sendmail command to send email messages with your installation's default configuration.

Using URL Fetch

When your application uses the URL fetch API to make an HTTP request, the local development server makes the request directly from your computer. The URL Fetch behavior on the local server may differ from production App Engine if you use a proxy server for accessing websites.

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App Engine standard environment for Go