java/src/main/com/google/appengine/api/datastore/EntityCachePolicy.java

   1 package com.google.appengine.api.datastore;
   2
   3 /**
   4  * This class specifies datastore specific entity caching policies.
   5  * <p>
   6  * See {@link EntityCacheConfig} for details on how to configure entity caching for the datastore.
   7  *
   8  */
   9 enum EntityCachePolicy {
  10
  11   /**
  12    * Specifies that entities associated with this policy will be cached in the entity cache.
  13    * However, on cache misses the entities will be retrieved from the datastore. And every entity
  14    * creation, update, or deletion will also be persisted in the datastore.
  15    *
  16    * The datastore operations that benefit from this policy are {@link DatastoreService#get} and
  17    * {@link AsyncDatastoreService#get}. These operations (when executed outside of a transaction)
  18    * will first attempt to find the entities in the entity cache. And only when that fails will an
  19    * attempt be made to find the entities in the datastore. When there is a high cache hit rate
  20    * this will reduce the number of datastore read operations and also reduce the overall
  21    * entity retrieval latency.
  22    *
  23    * With this policy, the datastore updates the entity cache in a way to minimize the chances
  24    * of stale entities being returned from the cache, but it does not completely eliminate the
  25    * possibility of stale entities being returned. For this reason, any entity reads done within
  26    * a transaction bypass the cache. However, for all other entity reads the application code
  27    * should tolerate stale entities being returned when entity caching is enabled.
  28    */
  29   CACHE,
  30
  31   /**
  32    * Specifies that entities associated with this policy will only be retrieved from and
  33    * persisted to the entity cache. The following datastore operations will not be invoked
  34    * for the associated entities:
  35    * <ul>
  36    * <li>{@link DatastoreService#put}, {@link AsyncDatastoreService#put}
  37    * <li>{@link DatastoreService#get}, {@link AsyncDatastoreService#get}
  38    * <li>{@link DatastoreService#delete}, {@link AsyncDatastoreService#delete}
  39    * </ul>
  40    * Only entities with complete keys will be put into the entity cache. All other entities on
  41    * which a put operation is performed will not be written to the entity cache and the keys
  42    * for those entities will not be completed.
  43    */
  44   CACHE_ONLY
  45 }