The App Engine Datastore uses indexes for every query your application makes. These indexes are updated whenever an entity changes, so the results can be returned quickly when the app makes a query. To do this, the datastore needs to know in advance which queries the application will make. You specify which indexes your app needs in a configuration file. The development server can generate the datastore index configuration automatically as you test your app.

1. About datastore-indexes.xml
2. Using automatic index configuration

About datastore-indexes.xml

You specify configuration for datastore indexes in WEB-INF/datastore-indexes.xml , in your app's war/ directory. This is an XML file whose root element is <datastore-indexes> . It contains zero or more <datastore-index> elements, one for each index that the Datastore should maintain.

As described on the Datastore Indexes page, an index is a table of values for a set of given properties for entities of a given kind. Each column of property values is sorted either in ascending or descending order. Configuration for an index specifies the kind of the entities, and the names of the properties and their sort orders.

Here is an example that specifies two indexes:

<?xml version="1.0" encoding="utf-8"?>
<datastore-indexes
  autoGenerate="true">
    <datastore-index kind="Employee" ancestor="false">
        <property name="lastName" direction="asc" />
        <property name="hireDate" direction="desc" />
    </datastore-index>
    <datastore-index kind="Project" ancestor="false">
        <property name="dueDate" direction="asc" />
    </datastore-index>
</datastore-indexes>

The <datastore-indexes> element has an autoGenerate attribute that controls whether this file should be considered along with automatically generated index configuration. See Using Automatic Index Configuration below.

Each <datastore-index> element represents an index. The kind attribute specifies the kind of the entities to index. The ancestor attribute is true if the index supports queries that filter by ancestor-key to constrain results to a single entity group, false otherwise.

The <property> elements in a <datastore-index> represent the entity properties to index. The name attribute is the property name, and the direction attribute is the sort order, either asc for ascending or desc for descending. The order of the property elements specifies the order in the index: rows are sorted by the first property, then the second property, and so on.

Using automatic index configuration

Determining the indexes required by your application's queries manually can be tedious and error-prone. Thankfully, the development server can determine the index configuration for you. To use automatic index configuration, add the attribute autoGenerate="true" to your WEB-INF/datastore-indexes.xml file's <datastore-indexes> element. Automatic index configuration is also used if your app does not have a datastore-indexes.xml file.

With automatic index configuration enabled, the development server maintains a file named WEB-INF/appengine-generated/datastore-indexes-auto.xml in your app's war/ directory. When your app, running in the development server, attempts a datastore query for which there is no corresponding index in either datastore-indexes.xml or datastore-indexes-auto.xml , the server adds the appropriate configuration to datastore-indexes-auto.xml .

If automatic index configuration is enabled when you upload your application, AppCfg uses both datastore-indexes.xml and datastore-indexes-auto.xml to determine which indexes need to be built for your app in production.

If autoGenerate="false" is in your datastore-indexes.xml , the development server and AppCfg ignore the contents of datastore-indexes-auto.xml . If the app running locally performs a query whose index is not specified in datastore-indexes.xml , the development server throws an exception, just as the production Datastore would.

It's a good idea to occasionally move index configuration from datastore-indexes-auto.xml to datastore-indexes.xml , then disable automatic index configuration and test your app in the development server. This makes it easy to maintain indexes without having to manage two files, and ensures that your testing will reproduce errors caused by missing index configuration.

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