Using the local development server

You can use the local development server to simulate running your App Engine application in production as well as use it to access App Engine bundled services.

The simulated environment enforces some sandbox restrictions, such as restricted system functions and Go 1.11 module imports, but not others, like request timeouts 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. 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.

Before you begin

Since Go 1.11 has reached the end of support, you can no longer use the latest version of dev_appserver.py to locally run your applications. To download an archived version of devapp_server.py, follow these steps:

  1. From the archive, download the zipped folder that contains the dev_appserver.py server for runtimes that have reached the end of support.

  2. Extract the directory's contents to your local file system, such as to your /home directory. You can find dev_appserver.py in the google_appengine/ directory.

Set up the local development server

To run the local development server tool, you must set up the following:

  1. Verify that you have installed a Python 2 interpreter of version 2.7.12 or later.

  2. Set the DEVAPPSERVER_ROOT environment variable in your shell to the path of your Python 2 interpreter.

Run the local development server

After setting up the local development server and creating the app.yaml configuration file for your app, you can use the dev_appserver.py command to run your app locally.

To start the local development server:

  1. In the directory that contains your app.yaml configuration file, run the dev_appserver.py command.

    dev_appserver.py

    Specify the directory path to your app, for example:

    python2 DEVAPPSERVER_ROOT/google_appengine/dev_appserver.py PATH_TO_YOUR_APP

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

    python2 DEVAPPSERVER_ROOT/google_appengine/dev_appserver.py app.yaml

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

    python2 DEVAPPSERVER_ROOT/google_appengine/dev_appserver.py --port=9999 PATH_TO_YOUR_APP

    Replace DEVAPPSERVER_ROOT with the path to the folder where you extract the archived version of devapp_server.py.

    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

    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, press the following:

  • macOS or Linux: Control+C
  • Windows: Control+Break

Specify application IDs

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.

Browse 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.

Specify 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:

python2 DEVAPPSERVER_ROOT/google_appengine/dev_appserver.py --auto_id_policy=sequential

Replace:

  • DEVAPPSERVER_ROOT with the path to the folder where you extract the archived version of devapp_server.py.

  • --auto_id_policy with one of the following:

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

Clear the local Datastore

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

python2 DEVAPPSERVER_ROOT/google_appengine/dev_appserver.py --clear_datastore=yes app.yaml

Replace DEVAPPSERVER_ROOT with the path to the folder where you extract the archived version of devapp_server.py.

Change local Datastore location

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

python2 DEVAPPSERVER_ROOT/google_appengine/dev_appserver.py --datastore_path=/tmp/myapp_datastore app.yaml

Replace DEVAPPSERVER_ROOT with the path to the folder where you extract the archived version of devapp_server.py.

Use 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.

Use 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::

   python2 [DEVAPPSERVER_ROOT]/google_appengine/dev_appserver.py --smtp_host=smtp.example.com --smtp_port=25 \
    --smtp_user=ajohnson --smtp_password=k1tt3ns [PATH_TO_YOUR_APP]

Replace:

  • [DEVAPPSERVER_ROOT] with the path to the folder where you extract the archived version of devapp_server.py.
  • --smtp_host, --smtp_port, --smtp_user and --smtp_password options with your own configuration values.

Using Sendmail

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

   python2 [DEVAPPSERVER_ROOT]/google_appengine/dev_appserver.py --enable_sendmail=yes [PATH_TO_YOUR_APP]

Replace [DEVAPPSERVER_ROOT] with the path to the folder where you where you extract the archived version of devapp_server.py.

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

Use 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.