SimpleDS provides a simple persistence framework for Google AppEngine that gets as little in the way as possible. It is barely a wrapper around Datastore APIs, providing mapping between Entity and Java classes.

You can explore the details about SimpleDS at our presentation for Codemotion 2012.

There is also a demo application on GitHub.

Features

• Supports Guice, Spring or plain Java configuration.
• Support for embedded classes serialized as properties or in JSON format.
• ==, <, <=, >, <=, IN, != and left like are supported.
• Easy transformations between Java Objects and Datastore Entities.
• Easy support for Cursors.
• Validations of expected ancestors
• Transaction support
• Level 1 and 2 Cache
• Cacheable queries

Examples

This is a brief comparison between SimpleDS and JPA:

// JPA retrieve by key
Model m1 = entityManager.find(Model.class, key);
Model m2 = entityManager.find(Model.class, key2);

// SimpleDS retrieve by key
Model m1 = entityManager.get(key);
Map<Key, Model> l = entityManager.get(key1, key2);

// JPA persist changes
entityManager.merge(m1);
entityManager.persist(m2);

// SimpleDS persist changes
entityManager.put(m1);
entityManager.put(ImmutableList.of(m1, m2));
Model m3 = new Model();
entityManager.put(parentKey, m3);

// JPA remove
Model m1 = entityManager.find(Model.class, key);
entityManager.remove(m1);

// SimpleDS remove
entityManager.remove(ImmutableList.of(key1, key2, key3));

Queries

// JPA
Query query = entityManager.createQuery(
   "select m from Model m where m.createdAt<=?1 and m.createdBy=?"
);
query.setParameter(1, new Date());
query.setParameter(2, userKey);
return query.getResultList();

// SimpleDS
return entityManager.createQuery(Model.class)
  .lessThan("createdAt", new Date())
  .equal("createdBy", userKey)
  .asList();

// retrieve just keys
return entityManager.createQuery(Model.class)
  .keysOnly()
  .asList();

// with limits
return entityManager.createQuery(Model.class)
  .withOffset(10)
  .withLimit(100)
  .asList();