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Data Consistency in Datastore Queries | App Engine standard environment for Java 8

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Datastore queries can deliver their results at either of two consistency levels:

• Strongly consistent queries guarantee the freshest results, but may take longer to complete.
• Eventually consistent queries generally run faster, but may occasionally return stale results.

In an eventually consistent query, the indexes used to gather the results are also accessed with eventual consistency. Consequently, such queries may sometimes return entities that no longer match the original query criteria, while strongly consistent queries are always transactionally consistent.

Queries return their results with different levels of consistency guarantee, depending on the nature of the query:

• Ancestor queries (those within an entity group) are strongly consistent by default, but can instead be made eventually consistent by setting the Datastore read policy (see below).
• Non-ancestor queries are always eventually consistent.

Fetching an entity by key, which is also called "lookup by key", is strongly consistent.

To improve performance, you can set the Datastore read policy so that all reads and queries are eventually consistent. (The API also allows you to explicitly set a strong consistency policy, but this setting will have no practical effect, since non-ancestor queries are always eventually consistent regardless of policy.)

You can also set the Datastore

, which is the maximum time, in seconds, that the application will wait for Datastore to return a result before aborting with an error. The default deadline is 60 seconds; it is not currently possible to set it higher, but you can adjust it downward to ensure that a particular operation fails quickly (for instance, to return a faster response to the user).

To set the Datastore read policy in Java, you construct a

(

), using the nested helper class

, and pass it an instance of class

. The following example shows how to set the read policy, the call deadline, or both:

• Learn how to specify what a query returns and further control query results.
• Learn the common restrictions for queries on Datastore.
• Learn about query cursors, which allow an application to retrieve a query's results in convenient batches.
• Learn the basic syntax and structure of queries for Datastore.

Except as otherwise noted, the content of this page is licensed under the Creative Commons Attribution 4.0 License, and code samples are licensed under the Apache 2.0 License. For details, see the Google Developers Site Policies. Java is a registered trademark of Oracle and/or its affiliates.

Last updated 2025-08-07 UTC.

[[["Easy to understand","easyToUnderstand","thumb-up"],["Solved my problem","solvedMyProblem","thumb-up"],["Other","otherUp","thumb-up"]],[["Hard to understand","hardToUnderstand","thumb-down"],["Incorrect information or sample code","incorrectInformationOrSampleCode","thumb-down"],["Missing the information/samples I need","missingTheInformationSamplesINeed","thumb-down"],["Other","otherDown","thumb-down"]],["Last updated 2025-08-07 UTC."],[[["This API supports first-generation runtimes and is relevant when upgrading to second-generation runtimes, while migration to Java 11/17 requires a separate migration guide."],["Datastore queries offer two consistency levels: strongly consistent queries, which guarantee the freshest results but may be slower, and eventually consistent queries, which are generally faster but may return stale results."],["Ancestor queries are strongly consistent by default but can be set to eventually consistent, while non-ancestor queries are always eventually consistent."],["The Datastore read policy can be set to eventually consistent to improve performance, and the call deadline can be adjusted to control the maximum wait time for a Datastore operation."],["Developers can use the `DatastoreServiceConfig` to set the read policy and/or the call deadline for Datastore operations, using the `ReadPolicy` for consistency and `withDeadline` for the time limit, and can use the `DatastoreServiceFactory` to get a `DatastoreService` instance with the desired configuration."]]],[]]

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