Inconsistent behavior when adding an Entity using Google App Engine datastore

Question

I'm working on a Google App Engine project that's written in Java.

Whenever I add an entity into the datastore I use a transaction:

DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
Transaction transaction = datastore.beginTransaction();     
datastore.put(entity);
transaction.commit();

I then count the number of entities that are in the datastore:

Query query = new Query(kind);
query.setFilter(filter);

PreparedQuery preparedQuery = datastore.prepare(query); 
int count = preparedQuery.countEntities(FetchOptions.Builder.withDefaults());

System.out.println("count = " + count);

I've noticed that sometimes the datastore can be inconsistent. Like sometimes I'll add an entity, but the count doesn't show up. If I then add another entity the count goes up by 2.

Why does this happen? How do I stop it from happening?

Gwyn Howell · Accepted Answer · 2014-08-17 07:58:59Z

4

You are experiencing the effects of "Eventual Consistency", which is a side effect of using the High Replication Datastore. Basically, you are calling the query before the data has time to replicate across data centres. You can avoid this, by using "Ancestor Queries", which always results in "Strongly Consistent" queries. Suggest you read this, which explains it in detail.

answered Aug 17, 2014 at 7:58

Gwyn Howell

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2 Comments

tygerpatch Over a year ago

I think my problem was that I was creating entities without a parent entity. I'm new to the Google App Engine, though.

Gwyn Howell Over a year ago

kind of. by specifying a parent entity, you are forcing the entities to persist within the same "entity group". if you then perform an ancestor query against those entities, you will get "strongly consistent" results, which will solve your issue.

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