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   private LocalServiceTestHelper helper =          new LocalServiceTestHelper(new   LocalDatastoreServiceTestConfig().setBackingStoreLocation(DS_PATH)); You could copy the WEB-INF/appegine-generated/local_db.bin file with   your data next to your test case and in a @Before method, delete the   old copy and copy a new one to DS_PATH.  You might also want to check   out LocalDatastoreServiceTestConfig().setNoStrage(true)

You might want to instead use datastore's built in ability to   efficiently create unique long ids and then when sending them to a   client "muddle" them using a reversible hash function so they don't   appear predictable.  You will find that using long ids results in   shorter Keys which saves space in the datastore. You can only guarantee a unique value in the datastore by creating an   Entity with the value in its key.  This does not need to be your   "main" entity - just a special UniqueName type.  Then you can

Entity groups are a simple concept. Entities in App Engine have ancestors. For e.g. you could model Books and Authors. Author: name->X, and Author: name->Y are two entities in the Author KIND. Book is another KIND (KIND is just a logical grouping of entities). The relationship between Books and Authors can be modeled as a ancestor-child relationship. E.g. Book: name->B1, B2 could have been written by Author: name->X. So you modeled them as: Author: name->X is a parent of both Book: name->B1, B2. Similarly, Book: name->B3 is written by Author: name->Y and so could model that relationship as Author:name->Y is a parent of Book:name->B3. When you try to transact on Books kind, you cannot transact on B1,B3 and B3 together. As they participate in different ancestor-child relationships. Every ancestor child relationship is an Entity group. You can only "lock" on one entity group at a time. Hope this makes things a little clear.

I use slf4j! use slf4j-log4j when development! use slf4j-jdk log when GAE Server!(change slf4j-log4j to slf4j-jdk-log before GAE appcfg update war)

for Java developers, there is a remote API handler available; adding following to your web xml file should work (disclaimer: I have not yet personally tested this.) <servlet>   <servlet-name>remoteapi</servlet-name>   <servlet-class>com.google.apphosting.utils.remoteapi.RemoteApiServlet</servlet-class> </servlet> <servlet-mapping>   <servlet-name>remoteapi</servlet-name>   <url-pattern>/remote_api</url-pattern> </servlet-mapping> <security-constraint>   <web-resource-collection>     <web-resource-name>remoteapi</web-resource-name>     <url-pattern>/remote_api</url-pattern>   </web-resource-collection>   <auth-constraint>     <role-name>admin</role-name>   </auth-constraint> </security-constraint>

If using the datastore API directly, a common pattern of usage is: // Get a handle on the datastore itself DatastoreService datastore = DatastoreServiceFactory.getDatastoreService(); // Lookup data by known key name Entity userEntity = datastore.get(KeyFactory.createKey("UserInfo", email)); // Or perform a query Query query = new Query("Task", userEntity); query.addFilter("dueDate", Query.FilterOperator.LESS_THAN, today); for (Entity taskEntity : datastore.prepare(query).asIterable()) { if ("done".equals(taskEntity.getProperty("status"))) { datastore.delete(taskEntity); } else { taskEntity.setProperty("status", "overdue"); datastore.put(taskEntity); } }

Tip: If the app creates a new object and gives it the same string ID as another object of the same kind (and the same entity group parent), saving the new object overwrites the other object in the datastore. To detect whether a string ID is already in use prior to creating a new object, you can use a transaction to attempt to get an entity with a given ID, then create one if it doesn't exist. See Transactions.

There are 4 types of primary key fields:

the necessary changes to appengine-web.xml to get Sitebricks to run in development mode:         <sessions-enabled>true</sessions-enabled>         <system-properties>                 <property name="java.util.logging.config.file" value="WEB-INF/logging.properties"/>                 <property name="mvel2.disable.jit" value="true"/>         </system-properties>

the necessary changes to appengine-web.xml to get Sitebricks to run in development mode:         <sessions-enabled>true</sessions-enabled>         <system-properties>                 <property name="java.util.logging.config.file" value="WEB-INF/logging.properties"/>                 <property name="mvel2.disable.jit" value="true"/>         </system-properties>

We had a GaeFlashCache built specifically for appengine. Try doing: bind(FlashCache.class).to(GaeFlashCache.class).in(Singleton.class);

Just my 2 cents: I was "forced" to use an "unowned relationship" for my application because I had very large objects: My parent object "Journey" contained thousands of "JourneyPoints" in a List element. A call to "makePersistent" constantly hit the 30s deadline. By modeling it as an unowned relationship and distributing the parent key by hand to the JourneyPoints everything was fine again and the performance was very good.