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A library designed to make it easy to use Google App Engine from Clojure

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README.md

The appengine-magic library attempts to abstract away the infrastructural nuts and bolts of writing a Clojure application for the Google App Engine platform.

The development environment of Google App Engine for Java expects pre-compiled classes, and generally does not fit well with Clojure's interactive development model. appengine-magic attempts to make REPL-based development of App Engine applications as natural as any other Clojure program.

  1. Programs using appengine-magic just need to include appengine-magic as a Leiningen dev-dependency.
  2. appengine-magic takes a Ring handler and makes it available as a servlet for App Engine deployment.
  3. appengine-magic is also a Leiningen plugin, and adds several tasks which simplify preparing for App Engine deployment.

Dependencies

  • Clojure 1.2.0
  • Leiningen 1.3.1
  • Google App Engine SDK 1.3.7
  • swank-clojure 1.2.1 (optional)

Overview

To use appengine-magic effectively, you need the following:

  1. The appengine-magic jar available on the project classpath.
  2. A Ring handler for your main application. You may use any Ring-compatible framework to make it. If your application does not yet have a core.clj file, then the lein appengine-new task creates one for you with a simple "hello world" Ring handler.
  3. A var defined by passing the Ring handler to the appengine-magic.core/def-appengine-app macro. This makes the application available both to interactive REPL development, and to App Engine itself.
  4. An entry point servlet. REPL development does not use it, but the standard App Engine SDK dev_appserver.sh mode and production deployment both do. This servlet must be AOT-compiled into a class file. This servlet defaults to the name app_servlet.clj, and the lein appengine-new task creates one for your project. The servlet must refer to the var defined by def-appengine-app.
  5. Web application resources. This primarily includes web application descriptors. lein appengine-new generates those and places them in the resources/war/WEB-INF directory. You should also place all static files that your application uses in resources/war.

Getting Started

Project setup

You need a copy of the Google App Engine SDK installed somewhere. appengine-magic cannot replace its dev_appserver.sh and appcfg.sh functionality.

  1. lein new
  2. Optional: rm src/<project-name>/core.clj to clean out the default core.clj file created by Leiningen. You need to do this so that appengine-magic can create a default file which correctly invokes the def-appengine-app macro.
  3. Edit project.clj:
    • add :namespaces [<project>.app_servlet] (or use the equivalent :aot directive)
    • add [appengine-magic "0.1.2"] to your dev-dependencies
  4. lein deps. This fetches appengine-magic, and makes its Leiningen plugin tasks available.
  5. lein appengine-new. This sets up four files for your project: core.clj (which has a sample Ring handler and uses the def-appengine-app macro), app_servlet.clj (the entry point for the application), resources/war/WEB-INF/web.xml (a servlet descriptor), and resources/war/WEB-INF/appengine-web.xml (an App Engine application descriptor). These files should contain reasonable starting defaults for your application.

The default .gitignore file produced by Leiningen works well with the resulting project, but do take a careful look at it. In particular, you should avoid checking in resources/war/WEB-INF/lib or resources/war/WEB-INF/classes: let Leiningen take care of managing those directories.

Development process

Launch lein swank or lein repl, whichever you normally use. Once you have a working REPL, compile your application's core.clj (or whatever other entry point file you use).

The key construct provided by appengine-magic is the appengine-magic.core/def-appengine-app macro. It takes a Ring handler and defines a new <project-name>-app var. If you want to rename this var, remember to update app_servlet.clj. That's it: you may now write your application using any framework which produces a Ring-compatible handler. Then, just pass the resulting Ring handler to def-appengine-app.

To test your work interactively, you can control a Jetty instance from the REPL using appengine-magic.core/start and appengine-magic.core/stop. Examples (assuming you are in your application's core namespace, your application is named foo, and you aliased appengine-magic.core to ae):

(ae/start foo-app)
(ae/stop)
(ae/start foo-app :port 8095)
(ae/stop)

Recompiling the functions which make up your Ring handler should produce instantaneous results.

Testing with dev_appserver.sh

  1. lein appengine-prepare. This AOT-compiles the entry point servlet, then copies the necessary classes and library dependencies to your application's resources/war/WEB-INF/classes and resources/war/WEB-INF/lib directories.
  2. Run dev_appserver.sh with a path to your application's resources/war directory.

Static resources

Just put all static files into your application's resources/war directory. If you put a file called index.html there, it will become a default welcome file.

Deployment to App Engine

  1. First of all, be careful. You must manually maintain the version field in appengine-web.xml and you should understand its implications. Refer to Google App Engine documentation for more information.
  2. lein appengine-prepare prepares the resources/war directory with the latest classes and libraries for deployment.
  3. When you are ready to deploy, just run appcfg.sh update with a path to your application's resources/war directory.

App Engine Services

appengine-magic provides convenience wrappers for using App Engine services from Clojure.

User service

The appengine-magic.services.user namespace (suggested alias: ae-user) provides the following functions for handling users.

  • current-user: returns the com.google.appengine.api.users.User for the currently logged-in user.
  • login-url (optional keyword: :destination): returns the Google authentication servlet URL, and forwards the user to the optional destination.
  • logout-url (optional keyword: :destination): performs logout, and forwards the user to the optional destination.

Limitations

When using the interactive REPL environment, some App Engine services are more limited than in dev_appserver.sh or in deployment. Because the App Engine SDK's jars are a mess, and many are not available in Maven repositories, providing the same functionality in an interactive Clojure environment is tricky and error-prone. In particular, the administration console, /_ah/admin is not available in the REPL environment.

The following Google services are not yet tested in the REPL environment:

  • Datastore
  • Blobstore
  • Images
  • Mail
  • Memcache
  • Multitenancy
  • OAuth
  • Task queues
  • URL fetch
  • XMPP

They may still work, but appengine-magic does not provide convenient Clojure interfaces for them, and may lack mappings for any necessary supporting URLs.

Warning

Google App Engine maintains a whitelist of permitted classes in Java's standard library. Other classes will cause your application to fail to deploy. Examples include threads and sockets. If you use those in your application, it will not work. This means that you cannot use Clojure's agents or futures. In addition, if one of your dependencies uses those, your application will also not work. For example, clojure.java.io (and its fore-runner, duck-streams from clojure-contrib), uses java.net.Socket, a forbidden class.

Whenever you add a new dependency, no matter how innocuous, you should make sure your app still works. dev_appserver.sh is a good place to start, but you must also test in the main App Engine. The two do not always load classes the same way.

License

appengine-magic is distributed under the MIT license.

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