Why Do My Google App Engine Datastore Query Tests Fail Despite Working in Production?

Testing Google App Engine Datastore Queries

In attempting to unit test datastore queries, developers may encounter issues when trying to verify the results. This article delves into a common problem faced by testers.

Query Failures in Tests

When testing datastorequeries, it's not uncommon to face failure despite the code working correctly in the production environment. Queries within tests often fail to retrieve data that has been successfully inserted.

Reason for Failures: Eventual Consistency

The crux of the issue lies in the datastore's use of "eventual consistency." This means that queries are not immediately consistent and may take some time to display the effects of recent data modifications.

Data Simulator Issue

The datastore simulator used in testing mimics the latency observed in production. When inserting a new entity and immediately running a query, the query will not include the new entity because the data has not yet been fully propagated.

Resolve with Delay or Strongly Consistent Datastore

To resolve this issue, introduce a delay between the datastore.Put() and q.GetAll() calls or utilize the StronglyConsistentDatastore option. Ancestor queries, known for their strong consistency, also resolve this problem.

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