Google Cloud Datastore supports transactions. A transaction is an operation or set of operations that is atomic—either all of the operations in the transaction occur, or none of them occur. An application can perform multiple operations and calculations in a single transaction.

Using transactions

A transaction is a set of Cloud Datastore operations on one or more entities. Each transaction is guaranteed to be atomic, which means that transactions are never partially applied. Either all of the operations in the transaction are applied, or none of them are applied. Transactions have a maximum duration of 60 seconds with a 10 second idle expiration time after 30 seconds.

An operation might fail when:

  • Too many concurrent modifications are attempted on the same entity group.
  • The transaction exceeds a resource limit.
  • Cloud Datastore encounters an internal error.

In all these cases, the Cloud Datastore API raises an exception.

Transactions are an optional feature of Cloud Datastore; you're not required to use transactions to perform Cloud Datastore operations.

Here is an example of updating field named vacationDays in an entity of kind Employee named Joe:

DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
Transaction txn = datastore.beginTransaction();
try {
  Key employeeKey = KeyFactory.createKey("Employee", "Joe");
  Entity employee = datastore.get(employeeKey);
  employee.setProperty("vacationDays", 10);

  datastore.put(txn, employee);

} finally {
  if (txn.isActive()) {

Note that in order to keep our examples more succinct we sometimes omit the finally block that performs a rollback if the transaction is still active. In production code it is important to ensure that every transaction is either explicitly committed or rolled back.

Entity groups

Every entity belongs to an entity group, a set of one or more entities that can be manipulated in a single transaction. Entity group relationships tell App Engine to store several entities in the same part of the distributed network. A transaction sets up Cloud Datastore operations for an entity group, and all of the operations are applied as a group, or not at all if the transaction fails.

When the application creates an entity, it can assign another entity as the parent of the new entity. Assigning a parent to a new entity puts the new entity in the same entity group as the parent entity.

An entity without a parent is a root entity. An entity that is a parent for another entity can also have a parent. A chain of parent entities from an entity up to the root is the path for the entity, and members of the path are the entity's ancestors. The parent of an entity is defined when the entity is created, and cannot be changed later.

Every entity with a given root entity as an ancestor is in the same entity group. All entities in a group are stored in the same Cloud Datastore node. A single transaction can modify multiple entities in a single group, or add new entities to the group by making the new entity's parent an existing entity in the group. The following code snippet demonstrates transactions on various types of entities:

DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
Entity person = new Entity("Person", "tom");

// Transactions on root entities
Transaction txn = datastore.beginTransaction();

Entity tom = datastore.get(person.getKey());
tom.setProperty("age", 40);
datastore.put(txn, tom);

// Transactions on child entities
txn = datastore.beginTransaction();
tom = datastore.get(person.getKey());
Entity photo = new Entity("Photo", tom.getKey());

// Create a Photo that is a child of the Person entity named "tom"
photo.setProperty("photoUrl", "");
datastore.put(txn, photo);

// Transactions on entities in different entity groups
txn = datastore.beginTransaction();
tom = datastore.get(person.getKey());
Entity photoNotAChild = new Entity("Photo");
photoNotAChild.setProperty("photoUrl", "");
datastore.put(txn, photoNotAChild);

// Throws IllegalArgumentException because the Person entity
// and the Photo entity belong to different entity groups.

Creating an entity in a specific entity group

When your application constructs a new entity, you can assign it to an entity group by supplying the key of another entity. The example below constructs the key of a MessageBoard entity, then uses that key to create and persist a Message entity that resides in the same entity group as the MessageBoard:

DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();

String messageTitle = "Some Title";
String messageText = "Some message.";
Date postDate = new Date();

Transaction txn = datastore.beginTransaction();

Key messageBoardKey = KeyFactory.createKey("MessageBoard", boardName);

Entity message = new Entity("Message", messageBoardKey);
message.setProperty("message_title", messageTitle);
message.setProperty("message_text", messageText);
message.setProperty("post_date", postDate);
datastore.put(txn, message);


Using cross-group transactions

Cross-group transactions (also called XG transactions) operate across multiple entity groups, behaving like single-group transactions described above except that cross-group transactions don't fail if code tries to update entities from more than one entity group.

Using a cross-group transaction is similar to using a single-group transaction, except that you need to specify that you want the transaction to be cross-group when you begin the transaction, using TransactionOptions:

DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
TransactionOptions options = TransactionOptions.Builder.withXG(true);
Transaction txn = datastore.beginTransaction(options);

Entity a = new Entity("A");
a.setProperty("a", 22);
datastore.put(txn, a);

Entity b = new Entity("B");
b.setProperty("b", 11);
datastore.put(txn, b);


What can be done in a transaction

Google Cloud Datastore imposes restrictions on what can be done inside a single transaction.

All Cloud Datastore operations in a transaction must operate on entities in the same entity group if the transaction is a single-group transaction, or on entities in a maximum of twenty-five entity groups if the transaction is a cross-group transaction. This includes querying for entities by ancestor, retrieving entities by key, updating entities, and deleting entities. Notice that each root entity belongs to a separate entity group, so a single transaction cannot create or operate on more than one root entity unless it is a cross-group transaction.

When two or more transactions simultaneously attempt to modify entities in one or more common entity groups, only the first transaction to commit its changes can succeed; all the others will fail on commit. Because of this design, using entity groups limits the number of concurrent writes you can do on any entity in the groups. When a transaction starts, Google Cloud Datastore uses optimistic concurrency control by checking the last update time for the entity groups used in the transaction. Upon commiting a transaction for the entity groups, Google Cloud Datastore again checks he last update time for the entity groups used in the transaction. If it has hanged since the initial check, an exception is thrown . For an explanation of entity groups, see the Cloud Datastore Overview page.

An app can perform a query during a transaction, but only if it includes an ancestor filter. An app can also get Datastore entities by key during a transaction. You can prepare keys prior to the transaction, or you can build keys inside the transaction with key names or IDs.

Isolation and consistency

Outside of transactions, Cloud Datastore's isolation level is closest to read committed. Inside of transactions, serializable isolation is enforced. This means that another transaction cannot concurrently modify the data that is read or modified by this transaction. Read the serializable isolation wiki and the Transaction Isolation article for more information on isolation levels.

In a transaction, all reads reflect the current, consistent state of Cloud Datastore at the time the transaction started. Queries and gets inside a transaction are guaranteed to see a single, consistent snapshot of Cloud Datastore as of the beginning of the transaction. Entities and index rows in the transaction's entity group are fully updated so that queries return the complete, correct set of result entities, without the false positives or false negatives described in Transaction Isolation that can occur in queries outside of transactions.

This consistent snapshot view also extends to reads after writes inside transactions. Unlike with most databases, queries and gets inside a Cloud Datastore transaction do not see the results of previous writes inside that transaction. Specifically, if an entity is modified or deleted within a transaction, a query or get returns the original version of the entity as of the beginning of the transaction, or nothing if the entity did not exist then.

Uses for transactions

This example demonstrates one use of transactions: updating an entity with a new property value relative to its current value. Because the Cloud Datastore API does not retry transactions, we can add logic for the transaction to be retried in case another request updates the same MessageBoard or any of its Messages at the same time.

int retries = 3;
while (true) {
  Transaction txn = datastore.beginTransaction();
  try {
    Key boardKey = KeyFactory.createKey("MessageBoard", boardName);
    Entity messageBoard = datastore.get(boardKey);

    long count = (Long) messageBoard.getProperty("count");
    messageBoard.setProperty("count", count);
    datastore.put(txn, messageBoard);

  } catch (ConcurrentModificationException e) {
    if (retries == 0) {
      throw e;
    // Allow retry to occur
  } finally {
    if (txn.isActive()) {

This requires a transaction because the value might be updated by another user after this code fetches the object, but before it saves the modified object. Without a transaction, the user's request uses the value of count prior to the other user's update, and the save overwrites the new value. With a transaction, the application is told about the other user's update. If the entity is updated during the transaction, then the transaction fails with a ConcurrentModificationException. The application can repeat the transaction to use the new data.

Another common use for transactions is to fetch an entity with a named key, or create it if it doesn't yet exist:

Transaction txn = datastore.beginTransaction();
Entity messageBoard;
Key boardKey;
try {
  boardKey = KeyFactory.createKey("MessageBoard", boardName);
  messageBoard = datastore.get(boardKey);
} catch (EntityNotFoundException e) {
  messageBoard = new Entity("MessageBoard", boardName);
  messageBoard.setProperty("count", 0L);
  boardKey = datastore.put(txn, messageBoard);

As before, a transaction is necessary to handle the case where another user is attempting to create or update an entity with the same string ID. Without a transaction, if the entity does not exist and two users attempt to create it, the second overwrites the first without knowing that it happened. With a transaction, the second attempt fails atomically. If it makes sense to do, the application can try again to fetch the entity and update it.

When a transaction fails, you can have your app retry the transaction until it succeeds, or you can let your users deal with the error by propagating it to your app's user interface level. You do not have to create a retry loop around every transaction.