Data store pricing

Every write bills to your wallet $$$. To avoid any surprise from Google, check this out before you get your engine in full throttle.

https://developers.google.com/appengine/docs/java/datastore/entities#Java_Understanding_write_costs

https://developers.google.com/appengine/pricing

alls to the datastore API result in the following billable operations. Small datastore operations include calls to allocate datastore ids or keys-only queries, and these operations are free. This table shows how calls map to datastore operations:

API Call	Datastore Operations
Entity Get (per entity)	1 read
New Entity Put (per entity, regardless of entity size)	2 writes + 2 writes per indexed property value + 1 write per composite index value
Existing Entity Put (per entity)	1 write + 4 writes per modified indexed property value + 2 writes per modified composite index value
Entity Delete (per entity)	2 writes + 2 writes per indexed property value + 1 write per composite index value
Query*	1 read + 1 read per entity retrieved

When your application executes a Datastore put operation, the Datastore must perform a number of writes to store the entity. Your application is charged for each of these writes. You can see how many writes will be required to store an entity by looking at the data viewer in the SDK Development Console. This section explains how these write costs are calculated.

Every entity requires a minimum of two writes to store: one for the entity itself and another for the built-in EntitiesByKind index, which is used by the query planner to service a variety of queries. In addition, the Datastore maintains two other built-in indexes,EntitiesByProperty and EntitiesByPropertyDesc, which provide efficient scans of entities by single property values in ascending and descending order, respectively. Each of an entity's indexed property values must be written to each of these indexes...