Transactions

A transaction is a set of operations on one or more entities. Each transaction is guaranteed to be atomic, meaning that transactions are never partially applied. Either all of the operations in the transaction are applied, or none of them are applied.

Using transactions

Transactions have a maximum duration of 60 seconds with a 10 second idle expiration time after 30 seconds.

An operation may fail when:

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

In all these cases, the Datastore API returns an error.

Transactions are an optional feature. You're not required to use transactions to perform database operations.

An application can execute a set of statements and operations in a single transaction, such that if any statement or operation raises an exception, none of the database operations in the set are applied. The application defines the actions to perform in the transaction.

The following snippet shows how to perform a transaction. It transfers money from one account to another.

C#

To learn how to install and use the client library for Cloud Datastore, see the Cloud Datastore Client Libraries . For more information, see the Cloud Datastore C# API reference documentation . 

private void TransferFunds(Key fromKey, Key toKey, long amount)
{
    using (var transaction = _db.BeginTransaction())
    {
        var entities = transaction.Lookup(fromKey, toKey);
        entities[0]["balance"].IntegerValue -= amount;
        entities[1]["balance"].IntegerValue += amount;
        transaction.Update(entities);
        transaction.Commit();
    }
}

Go

To learn how to install and use the client library for Cloud Datastore, see the Cloud Datastore Client Libraries . For more information, see the Cloud Datastore Go API reference documentation . 

type BankAccount struct {
	Balance int
}

const amount = 50
keys := []*datastore.Key{to, from}
tx, err := client.NewTransaction(ctx)
if err != nil {
	log.Fatalf("client.NewTransaction: %v", err)
}
accs := make([]BankAccount, 2)
if err := tx.GetMulti(keys, accs); err != nil {
	tx.Rollback()
	log.Fatalf("tx.GetMulti: %v", err)
}
accs[0].Balance += amount
accs[1].Balance -= amount
if _, err := tx.PutMulti(keys, accs); err != nil {
	tx.Rollback()
	log.Fatalf("tx.PutMulti: %v", err)
}
if _, err = tx.Commit(); err != nil {
	log.Fatalf("tx.Commit: %v", err)
}

Java

To learn how to install and use the client library for Cloud Datastore, see the Cloud Datastore Client Libraries . For more information, see the Cloud Datastore Java API reference documentation . 

void transferFunds(Key fromKey, Key toKey, long amount) {
  Transaction txn = datastore.newTransaction();
  try {
    List<Entity> entities = txn.fetch(fromKey, toKey);
    Entity from = entities.get(0);
    Entity updatedFrom =
        Entity.newBuilder(from).set("balance", from.getLong("balance") - amount).build();
    Entity to = entities.get(1);
    Entity updatedTo = Entity.newBuilder(to).set("balance", to.getLong("balance") + amount)
        .build();
    txn.put(updatedFrom, updatedTo);
    txn.commit();
  } finally {
    if (txn.isActive()) {
      txn.rollback();
    }
  }
}

Node.js

To learn how to install and use the client library for Cloud Datastore, see the Cloud Datastore Client Libraries . For more information, see the Cloud Datastore Node.js API reference documentation . 

async function transferFunds(fromKey, toKey, amount) {
  const transaction = datastore.transaction();
  await transaction.run();
  const results = await Promise.all([
    transaction.get(fromKey),
    transaction.get(toKey),
  ]);
  const accounts = results.map(result => result[0]);

  accounts[0].balance -= amount;
  accounts[1].balance += amount;

  transaction.save([
    {
      key: fromKey,
      data: accounts[0],
    },
    {
      key: toKey,
      data: accounts[1],
    },
  ]);

  return transaction.commit();
}

PHP

To learn how to install and use the client library for Cloud Datastore, see the Cloud Datastore Client Libraries . For more information, see the Cloud Datastore PHP API reference documentation . 

/**
 * Update two entities in a transaction.
 *
 * @param DatastoreClient $datastore
 * @param Key $fromKey
 * @param Key $toKey
 * @param $amount
 */
function transfer_funds(
    DatastoreClient $datastore,
    Key $fromKey,
    Key $toKey,
    $amount
) {
    $transaction = $datastore->transaction();
    // The option 'sort' is important here, otherwise the order of the result
    // might be different from the order of the keys.
    $result = $transaction->lookupBatch([$fromKey, $toKey], ['sort' => true]);
    if (count($result['found']) != 2) {
        $transaction->rollback();
    }
    $fromAccount = $result['found'][0];
    $toAccount = $result['found'][1];
    $fromAccount['balance'] -= $amount;
    $toAccount['balance'] += $amount;
    $transaction->updateBatch([$fromAccount, $toAccount]);
    $transaction->commit();
}

Python

To learn how to install and use the client library for Cloud Datastore, see the Cloud Datastore Client Libraries . For more information, see the Cloud Datastore Python API reference documentation . 

def transfer_funds(client, from_key, to_key, amount):
    with client.transaction():
        from_account = client.get(from_key)
        to_account = client.get(to_key)

        from_account['balance'] -= amount
        to_account['balance'] += amount

        client.put_multi([from_account, to_account])

Ruby

To learn how to install and use the client library for Cloud Datastore, see the Cloud Datastore Client Libraries . For more information, see the Cloud Datastore Ruby API reference documentation . 

def transfer_funds from_key, to_key, amount
  datastore.transaction do |tx|
    from = tx.find from_key
    from["balance"] -= amount
    to = tx.find to_key
    to["balance"] += amount
    tx.save from, to
  end
end

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

What can be done in a transaction

Transactions can query or lookup any number of entities. You can create, update, or delete up to 500 entities in each transaction. The maximum size of a transaction is 10 MiB.

Isolation and consistency

Serializable isolation is enforced in Datastore mode databases. This means that data read or modified by a transaction cannot be concurrently modified. For more information on isolation levels, read the Serializable Isolation and Transaction Isolation articles.

Queries and lookups in a transaction see a consistent snapshot of the state of the database. This snapshot is guaranteed to contain the effect of all transactions and writes that completed prior to the beginning of the transaction.

This consistent snapshot view also extends to reads after writes inside transactions. Unlike with most databases, queries and lookups inside a Datastore mode 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 lookup returns the original version of the entity as of the beginning of the transaction, or nothing if the entity did not exist then.

Outside of transactions, queries and lookups also have serializable isolation.

Uses for transactions

One use of transactions is updating an entity with a new property value relative to its current value. The transferFunds example above does that for two entities, by withdrawing money from one account and transferring it to another. The Datastore API does not automatically retry transactions, but you can add your own logic to retry them, for instance to handle conflicts when another request updates the same entity at the same time.

C#

To learn how to install and use the client library for Cloud Datastore, see the Cloud Datastore Client Libraries . For more information, see the Cloud Datastore C# API reference documentation . 

/// <summary>
/// Retry the action when a Grpc.Core.RpcException is thrown.
/// </summary>
private T RetryRpc<T>(Func<T> action)
{
    List<Grpc.Core.RpcException> exceptions = null;
    var delayMs = _retryDelayMs;
    for (int tryCount = 0; tryCount < _retryCount; ++tryCount)
    {
        try
        {
            return action();
        }
        catch (Grpc.Core.RpcException e)
        {
            if (exceptions == null)
                exceptions = new List<Grpc.Core.RpcException>();
            exceptions.Add(e);
        }
        System.Threading.Thread.Sleep(delayMs);
        delayMs *= 2;  // Exponential back-off.
    }
    throw new AggregateException(exceptions);
}

private void RetryRpc(Action action)
{
    RetryRpc(() => { action(); return 0; });
}

[Fact]
public void TestTransactionalRetry()
{
    int tryCount = 0;
    var keys = UpsertBalances();
    RetryRpc(() =>
    {
        using (var transaction = _db.BeginTransaction())
        {
            TransferFunds(keys[0], keys[1], 10, transaction);
            // Insert a conflicting transaction on the first try.
            if (tryCount++ == 0)
                TransferFunds(keys[1], keys[0], 5);
            transaction.Commit();
        }
    });
    Assert.Equal(2, tryCount);
}

Go

To learn how to install and use the client library for Cloud Datastore, see the Cloud Datastore Client Libraries . For more information, see the Cloud Datastore Go API reference documentation . 

type BankAccount struct {
	Balance int
}

const amount = 50
_, err := client.RunInTransaction(ctx, func(tx *datastore.Transaction) error {
	keys := []*datastore.Key{to, from}
	accs := make([]BankAccount, 2)
	if err := tx.GetMulti(keys, accs); err != nil {
		return err
	}
	accs[0].Balance += amount
	accs[1].Balance -= amount
	_, err := tx.PutMulti(keys, accs)
	return err
})

Java

To learn how to install and use the client library for Cloud Datastore, see the Cloud Datastore Client Libraries . For more information, see the Cloud Datastore Java API reference documentation . 

int retries = 5;
while (true) {
  try {
    transferFunds(fromKey, toKey, 10);
    break;
  } catch (DatastoreException e) {
    if (retries == 0) {
      throw e;
    }
    --retries;
  }
}
// Retry handling can also be configured and automatically applied using google-cloud-java.

Node.js

To learn how to install and use the client library for Cloud Datastore, see the Cloud Datastore Client Libraries . For more information, see the Cloud Datastore Node.js API reference documentation . 

async function transferFundsWithRetry() {
  const maxTries = 5;

  async function tryRequest(currentAttempt, delay) {
    try {
      await transferFunds(fromKey, toKey, 10);
    } catch (err) {
      if (currentAttempt <= maxTries) {
        // Use exponential backoff
        setTimeout(async () => {
          await tryRequest(currentAttempt + 1, delay * 2);
        }, delay);
      }
      throw err;
    }
  }

  await tryRequest(1, 100);
}

PHP

To learn how to install and use the client library for Cloud Datastore, see the Cloud Datastore Client Libraries . For more information, see the Cloud Datastore PHP API reference documentation . 

$retries = 5;
for ($i = 0; $i < $retries; $i++) {
    try {
        transfer_funds($datastore, $fromKey, $toKey, 10);
    } catch (Google\Cloud\Exception\ConflictException $e) {
        // if $i >= $retries, the failure is final
        continue;
    }
    // Succeeded!
    break;
}

Python

To learn how to install and use the client library for Cloud Datastore, see the Cloud Datastore Client Libraries . For more information, see the Cloud Datastore Python API reference documentation . 

for _ in range(5):
    try:
        transfer_funds(client, account1.key, account2.key, 50)
        break
    except google.cloud.exceptions.Conflict:
        continue
else:
    print('Transaction failed.')

Ruby

To learn how to install and use the client library for Cloud Datastore, see the Cloud Datastore Client Libraries . For more information, see the Cloud Datastore Ruby API reference documentation . 

(1..5).each do |i|
  begin
    return transfer_funds from_key, to_key, amount
  rescue Google::Cloud::Error => e
    raise e if i == 5
  end
end

This requires a transaction because the value of balance in an entity may 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 balance 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.

Another common use for transactions is to fetch an entity with a named key, or create it if it doesn't yet exist (this example builds on the TaskList example from creating an entity):

C#

To learn how to install and use the client library for Cloud Datastore, see the Cloud Datastore Client Libraries . For more information, see the Cloud Datastore C# API reference documentation . 

Entity task;
using (var transaction = _db.BeginTransaction())
{
    task = transaction.Lookup(_sampleTask.Key);
    if (task == null)
    {
        transaction.Insert(_sampleTask);
        transaction.Commit();
    }
}

Go

To learn how to install and use the client library for Cloud Datastore, see the Cloud Datastore Client Libraries . For more information, see the Cloud Datastore Go API reference documentation . 

_, err := client.RunInTransaction(ctx, func(tx *datastore.Transaction) error {
	var task Task
	if err := tx.Get(key, &task); err != datastore.ErrNoSuchEntity {
		return err
	}
	_, err := tx.Put(key, &Task{
		Category:    "Personal",
		Done:        false,
		Priority:    4,
		Description: "Learn Cloud Datastore",
	})
	return err
})

Java

To learn how to install and use the client library for Cloud Datastore, see the Cloud Datastore Client Libraries . For more information, see the Cloud Datastore Java API reference documentation . 

Entity task;
Transaction txn = datastore.newTransaction();
try {
  task = txn.get(taskKey);
  if (task == null) {
    task = Entity.newBuilder(taskKey).build();
    txn.put(task);
    txn.commit();
  }
} finally {
  if (txn.isActive()) {
    txn.rollback();
  }
}

Node.js

To learn how to install and use the client library for Cloud Datastore, see the Cloud Datastore Client Libraries . For more information, see the Cloud Datastore Node.js API reference documentation . 

async function getOrCreate(taskKey, taskData) {
  const taskEntity = {
    key: taskKey,
    data: taskData,
  };
  const transaction = datastore.transaction();

  try {
    await transaction.run();
    const [task] = await transaction.get(taskKey);
    if (task) {
      // The task entity already exists.
      transaction.rollback();
    } else {
      // Create the task entity.
      transaction.save(taskEntity);
      transaction.commit();
    }
    return taskEntity;
  } catch (err) {
    transaction.rollback();
  }
}

PHP

To learn how to install and use the client library for Cloud Datastore, see the Cloud Datastore Client Libraries . For more information, see the Cloud Datastore PHP API reference documentation . 

$transaction = $datastore->transaction();
$existed = $transaction->lookup($task->key());
if ($existed === null) {
    $transaction->insert($task);
    $transaction->commit();
}

Python

To learn how to install and use the client library for Cloud Datastore, see the Cloud Datastore Client Libraries . For more information, see the Cloud Datastore Python API reference documentation . 

with client.transaction():
    key = client.key('Task', datetime.datetime.utcnow().isoformat())

    task = client.get(key)

    if not task:
        task = datastore.Entity(key)
        task.update({
            'description': 'Example task'
        })
        client.put(task)

    return task

Ruby

To learn how to install and use the client library for Cloud Datastore, see the Cloud Datastore Client Libraries . For more information, see the Cloud Datastore Ruby API reference documentation . 

task = nil
datastore.transaction do |tx|
  task = tx.find task_key
  if task.nil?
    task = datastore.entity task_key do |t|
      t["category"] = "Personal"
      t["done"] = false
      t["priority"] = 4
      t["description"] = "Learn Cloud Datastore"
    end
    tx.save task
  end
end

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.

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.

Read-only transactions

Finally, you can use a transaction to read a consistent snapshot of the database. This can be useful when you need multiple reads to render a page or to export data that must be consistent. You can create read-only transaction for these cases.

Read-only transactions cannot modify entities, but in return, they do not contend with any other transactions and do not need to be retried. If you perform only reads in a regular, read-write transaction, then that transaction may contend with transaction that modify the same data.

C#

To learn how to install and use the client library for Cloud Datastore, see the Cloud Datastore Client Libraries . For more information, see the Cloud Datastore C# API reference documentation . 

Entity taskList;
IReadOnlyList<Entity> tasks;
using (var transaction = _db.BeginTransaction(TransactionOptions.CreateReadOnly()))
{
    taskList = transaction.Lookup(taskListKey);
    var query = new Query("Task")
    {
        Filter = Filter.HasAncestor(taskListKey)
    };
    tasks = transaction.RunQuery(query).Entities;
    transaction.Commit();
}

Go

To learn how to install and use the client library for Cloud Datastore, see the Cloud Datastore Client Libraries . For more information, see the Cloud Datastore Go API reference documentation . 

tx, err := client.NewTransaction(ctx, datastore.ReadOnly)
if err != nil {
	log.Fatalf("client.NewTransaction: %v", err)
}
defer tx.Rollback() // Transaction only used for read.

ancestor := datastore.NameKey("TaskList", "default", nil)
query := datastore.NewQuery("Task").Ancestor(ancestor).Transaction(tx)
var tasks []Task
_, err = client.GetAll(ctx, query, &tasks)

Java

To learn how to install and use the client library for Cloud Datastore, see the Cloud Datastore Client Libraries . For more information, see the Cloud Datastore Java API reference documentation . 

Entity taskList;
QueryResults<Entity> tasks;
Transaction txn = datastore.newTransaction(
    TransactionOptions.newBuilder()
        .setReadOnly(ReadOnly.newBuilder().build())
        .build()
);
try {
  taskList = txn.get(taskListKey);
  Query<Entity> query = Query.newEntityQueryBuilder()
      .setKind("Task")
      .setFilter(PropertyFilter.hasAncestor(taskListKey))
      .build();
  tasks = txn.run(query);
  txn.commit();
} finally {
  if (txn.isActive()) {
    txn.rollback();
  }
}

Node.js

To learn how to install and use the client library for Cloud Datastore, see the Cloud Datastore Client Libraries . For more information, see the Cloud Datastore Node.js API reference documentation . 

async function getTaskListEntities() {
  const transaction = datastore.transaction({readOnly: true});
  try {
    const taskListKey = datastore.key(['TaskList', 'default']);

    await transaction.run();
    const [taskList] = await transaction.get(taskListKey);
    const query = datastore.createQuery('Task').hasAncestor(taskListKey);
    const [taskListEntities] = await transaction.runQuery(query);
    await transaction.commit();
    return [taskList, taskListEntities];
  } catch (err) {
    await transaction.rollback();
  }
}

PHP

To learn how to install and use the client library for Cloud Datastore, see the Cloud Datastore Client Libraries . For more information, see the Cloud Datastore PHP API reference documentation . 

$transaction = $datastore->readOnlyTransaction();
$taskListKey = $datastore->key('TaskList', 'default');
$query = $datastore->query()
    ->kind('Task')
    ->hasAncestor($taskListKey);
$result = $transaction->runQuery($query);
$taskListEntities = [];
/* @var Entity $task */
foreach ($result as $task) {
    $taskListEntities[] = $task;
}

Python

To learn how to install and use the client library for Cloud Datastore, see the Cloud Datastore Client Libraries . For more information, see the Cloud Datastore Python API reference documentation . 

with client.transaction(read_only=True):
    task_list_key = client.key('TaskList', 'default')

    task_list = client.get(task_list_key)

    query = client.query(kind='Task', ancestor=task_list_key)
    tasks_in_list = list(query.fetch())

    return task_list, tasks_in_list

Ruby

To learn how to install and use the client library for Cloud Datastore, see the Cloud Datastore Client Libraries . For more information, see the Cloud Datastore Ruby API reference documentation . 

task_list_key = datastore.key "TaskList", "default"
datastore.read_only_transaction do |tx|
  task_list = tx.find task_list_key
  query = datastore.query("Task").ancestor(task_list)
  tasks_in_list = tx.run query
end

What's next