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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.

Using appengine-magic still requires familiarity with Google App Engine. This README file tries to describe everything you need to know to use App Engine with Clojure, but does not explain the details of App Engine semantics. Please refer to Google's official documentation for details.


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


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 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 and 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 "0.2.0"] 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/start foo-app :port 8095)

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

Testing with

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

  • current-user: returns the 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.

Memcache service

The namespace (suggested alias: ae-memcache) provides the following functions for the App Engine memcache. See App Engine documentation for detailed explanations of the underlying Java API.

  • statistics: returns the current memcache statistics.
  • clear-all: wipes the entire cache for all namespaces.
  • contains? <key> (optional keyword: :namespace): checks if the given key exists in the cache.
  • delete <key> (optional keywords: :namespace, :millis-no-readd): removes the key from the cache, optionally refraining from adding it for the given number of milliseconds. If the key argument is sequential, deletes all the named keys.
  • get <key> (optional keyword: :namespace): returns the value for the given key, but if the key argument is sequential, returns a map of key-value pairs for each supplied key.
  • put <key> <value> (optional keywords: :namespace, :expiration, :policy): saves the given value under the given key; expiration is an instance of; policy is one of :always (the default), :add-if-not-present, or :replace-only.
  • put-map <key-value-map> (optional keywords: :namespace, :expiration, :policy): writes the key-value-map into the cache. Other keywords same as for put.
  • increment <key> <delta> (optional keywords: :namespace, :initial): atomically increments long integer values in the cache; if key is sequential, it increments all keys by the given delta.
  • increment-map <key-delta-map> (optional keywords: :namespace, :initial): atomically increments long integer values by deltas given in the argument map.


The namespace (suggested alias: ae-ds) provides a fairly complete interface for the App Engine datastore.

A few simple examples:

(ae-ds/defentity Author [^:key name, birthday])
(ae-ds/defentity Book [^:key isbn, title, author])

;; Writes three authors to the datastore.
(let [will (Author. "Shakespeare, William" nil)
      geoff (Author. "Chaucer, Geoffrey" "1343")
      oscar (Author. "Wilde, Oscar" "1854-10-16")]
  ;; First, just write Will, without a birthday.
  (ae-ds/save! will)
  ;; Now overwrite Will with an entity containing a birthday, and also
  ;; write the other two authors.
  (ae-ds/save! [(assoc will :birthday "1564"), geoff, oscar]))

;; Retrieves two authors and writes book entites.
(let [will (first (ae-ds/query :kind Author :filter (= :name "Shakespeare, William")))
      geoff (first (ae-ds/query :kind Author :filter [(= :name "Chaucer, Geoffrey")
                                                      (= :birthday "1343")]))]
  (ae-ds/save! (Book. "0393925870" "The Canterbury Tales" geoff))
  (ae-ds/save! (Book. "143851557X" "Troilus and Criseyde" geoff))
  (ae-ds/save! (Book. "0393039854" "The First Folio" will)))

;; Retrieves all Chaucer books in the datastore, sorting by descending title and
;; then by ISBN.
(let [geoff (ae-ds/retrieve Author "Chaucer, Geoffrey")]
  (ae-ds/query :kind Book
               :filter (= :author geoff)
               :sort [[title :dsc] :isbn]))

;; Deletes all books by Chaucer.
(let [geoff (ae-ds/retrieve Author "Chaucer, Geoffrey")]
  (ae-ds/delete! (ae-ds/query :kind Book :filter (= :author geoff))))
  • defentity (optional keyword: :kind): defines an entity record type suitable for storing in the App Engine datastore. These entities work just like Clojure records. Internally, they implement an additional protocol, EntityProtocol, which provides the save! method. When defining an entity, you may specify ^:key metadata on any one field of the record, and the datastore will use this as the primary key. Omitting the key will make the datastore assign an automatic primary key to the entity. Specifying the optional :kind keyword (a string value) causes App Engine to save the entity under the given "kind" name — like a datastore table. This allows kinds to remain disjoint from entity record types.
  • new*: instantiates a datastore entity record. You may also use standard Clojure conventions to instantiate entity records, but creating entities destined for entity groups requires using new*. To put the new entity into a group, use the :parent keyword with the parent entity. Instantiating an entity does not automatically write it to the datastore.
  • get-key-object: this returns the primary Key object of the given entity. For a newly-instantiated entity lacking an explicit primary key, this method returns nil. Entities properly brought under entity groups using new* will have hierarchical keys. You should rarely need to use this explicitly.
  • save!: calling this method on an entity writes it to the datastore, using the primary key returned by calling get-key-object on the entity. May be called on a sequence of entities.
  • delete!: removes an entity. May be called on a sequence of entities.
  • retrieve <entity-record-type> <primary-key> (optional keywords: :parent, :kind): this is a low-level entity retrieval function. It returns a record of the given type with the given primary key value. If the target entity belongs to an entity group, specify the parent using the optional keyword. If the target entity was stored with a different kind from the entity record type, specify the actual kind using the optional keyword.
  • query (optional keywords: :kind, :ancestor, :filter, :sort, :keys-only?, :count-only?, :in-transaction?, :limit, :offset, :prefetch-size, :chunk-size, :entity-record-type): runs a query with the given parameters.
    • :kind: primarily identifies the App Engine entity kind. If given as an entity record type (recommended), the query returns a sequence of entity records of that type. If given as a string, it then checks to see if :entity-record-type is given, and uses that type if so; otherwise, the query returns generic EntityBase records.
    • :filter: one filter clause, or a list of clauses. Each consists of a symbol specifying the filter operation, a property name, and a target property value. See example.
    • :sort: one sort criterion, or a list of criteria. Each specified criterion defaults to ascending sort order, but may also sort in descending order.
  • with-transaction <body>: wraps the body in a transaction. Can be nested. (Keep the limitations of App Engine's transaction system in mind when using this.)
  • init-datastore-service: not normally needed. Only use this method if you want to modify the the read consistency and implicit transaction policies of the datastore service.


When using the interactive REPL environment, some App Engine services are more limited than in 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:

  • Blobstore
  • Images
  • Mail
  • 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.


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, (and its fore-runner, duck-streams from clojure-contrib), uses, a forbidden class.

Whenever you add a new dependency, no matter how innocuous, you should make sure your app still works. 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.


appengine-magic is distributed under the MIT license.

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