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

   1 // Copyright 2008 Google Inc. All Rights Reserved.
   2 package com.google.appengine.api.datastore;
   3
   4 import java.util.ConcurrentModificationException;
   5
   6 /**
   7  * Describes the various policies the datastore can follow for implicit
   8  * transaction management.  When deciding which policy to use, keep the
   9  * following in mind: The datastore will automatically retry operations
  10  * that fail due to concurrent updates to the same entity group if the
  11  * operation is not part of a transaction.  The datastore will not retry
  12  * operations that fail due to concurrent updates to the same entity group
  13  * if the operation is part of a transaction, and will instead immediately
  14  * throw a {@link ConcurrentModificationException}.  If your application
  15  * needs to perform any sort of intelligent merging when concurrent attempts
  16  * are made to update the same entity group you probably want {@link #AUTO},
  17  * otherwise {@link #NONE} is probably acceptable.
  18  *
  19  * See {@link DatastoreService} for a list of operations that perform implicit
  20  * transaction management.
  21  *
  22  */
  23 public enum ImplicitTransactionManagementPolicy {
  24
  25   /**
  26    * If a current transaction exists, use it, otherwise execute without a
  27    * transaction.
  28    */
  29   NONE,
  30
  31   /**
  32    * If a current transaction exists, use it, otherwise create one.
  33    * The transaction will be committed before the method returns if the
  34    * datastore operation completes successfully and rolled back if the
  35    * datastore operation does not complete successfully.  No matter the type
  36    * or quantity of entities provided, only one transaction will be created.
  37    * This means that if you pass entities that belong to multiple entity groups
  38    * into one of these methods and you have this policy enabled, you will
  39    * receive an exception because transactions do not function across entity
  40    * groups.
  41    */
  42   AUTO,
  43 }