Java Index Configuration Using YAML

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 index.yaml
  2. Index Definitions
  3. Automatic Indexes

About index.yaml

Every datastore query made by an application needs a corresponding index. Indexes for simple queries, such as queries over a single property, are created automatically. Indexes for complex queries must be defined in a configuration file named index.yaml. This file is stored in the application's WEB-INF directory to create indexes in the datastore.

If a query needs an index that does not have an appropriate entry in the configuration file, the development web server automatically adds items to this file when the application tries to execute the query. You can adjust indexes or create new ones manually by editing the file.

Tip: If, during testing, the app exercises every query it will make using the development web server, then the generated entries in index.yaml will be complete. You only need to edit the file manually to delete indexes that are no longer used, or to define indexes not created by the development web server.

For more information about indexes, see the Datastore Indexes page.

The following is an example of an index.yaml file:


- kind: Cat
  ancestor: no
  - name: name
  - name: age
    direction: desc

- kind: Cat
  - name: name
    direction: asc
  - name: whiskers
    direction: desc

- kind: Store
  ancestor: yes
  - name: business
    direction: asc
  - name: owner
    direction: asc

The syntax of index.yaml is the YAML format. For more information about this syntax, see the YAML website for more information.

Tip: The YAML format supports comments. A line that begins with a pound (#) character is ignored:
# This is a comment.

Index Definitions

index.yaml has a single list element called indexes. Each element in the list represents an index for the application.

An index element can have the following elements:


The kind of the entity for the query. The kind attribute specifies the kind of the entities to index.


A list of properties to include as columns of the index, in the order to be sorted: properties used in equality filters first, followed by the property used in inequality filters, then the sort orders and their directions.

Each element in this list has the following elements:


The datastore name of the property.


The direction to sort, either asc for ascending or desc for descending. This is only required for properties used in sort orders of the query, and must match the direction used by the query. The default is asc.


The ancestor attribute is true if the index supports a query that filters entities by the entity group parent, false otherwise.

Automatic Indexes

Determining the indexes required by your application's queries manually can be tedious and error-prone. Thankfully, when you use index.yaml, the development server automatically determines the index configuration for you.

The development server maintains the auto-generated index in a file named datastore-indexes-auto.xml in the directory WEB-INF/appengine-generated/ in your application's WAR. AppCfg uses both index.yaml and datastore-indexes-auto.xml to determine which indexes need to be built for your app on App Engine.

If you have an app running in the development server, and it attempts a datastore query for which there is no corresponding index in either index.yaml or datastore-indexes-auto.xml, then the server adds the appropriate configuration to datastore-indexes-auto.xml.