Retry

Retry a transaction.

Explore further

For detailed documentation that includes this code sample, see the following:

• Cloud Datastore Transactions
• Transactions

Code sample

C#

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

To authenticate to Datastore mode, set up Application Default Credentials. For more information, see Set up authentication for a local development environment.

/// <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 Datastore mode, see Datastore mode client libraries. For more information, see the Datastore mode Go API reference documentation.

To authenticate to Datastore mode, set up Application Default Credentials. For more information, see Set up authentication for a local development environment.

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 Datastore mode, see Datastore mode client libraries. For more information, see the Datastore mode Java API reference documentation.

To authenticate to Datastore mode, set up Application Default Credentials. For more information, see Set up authentication for a local development environment.

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 Datastore mode, see Datastore mode client libraries. For more information, see the Datastore mode Node.js API reference documentation.

To authenticate to Datastore mode, set up Application Default Credentials. For more information, see Set up authentication for a local development environment.

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 Datastore mode, see Datastore mode client libraries. For more information, see the Datastore mode PHP API reference documentation.

To authenticate to Datastore mode, set up Application Default Credentials. For more information, see Set up authentication for a local development environment.

$retries = 5;
for ($i = 0; $i < $retries; $i++) {
    try {
        require_once __DIR__ . '/transfer_funds.php';
        transfer_funds($fromKeyId, $toKeyId, 10, $namespaceId);
    } catch (\Google\Cloud\Core\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 Datastore mode, see Datastore mode client libraries. For more information, see the Datastore mode Python API reference documentation.

To authenticate to Datastore mode, set up Application Default Credentials. For more information, see Set up authentication for a local development environment.

from google.cloud import datastore

# For help authenticating your client, visit
# https://cloud.google.com/docs/authentication/getting-started
client = datastore.Client()

import google.cloud.exceptions

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 Datastore mode, see Datastore mode client libraries. For more information, see the Datastore mode Ruby API reference documentation.

To authenticate to Datastore mode, set up Application Default Credentials. For more information, see Set up authentication for a local development environment.

(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

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