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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();
    }
}

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)
}

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();
    }
  }
}

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 await transaction.commit();
}

/**
 * Update two entities in a transaction.
 *
 * @param string $fromKeyId
 * @param string $toKeyId
 * @param int $amount
 * @param string $namespaceId
 */
function transfer_funds(
    string $fromKeyId,
    string $toKeyId,
    int $amount,
    string $namespaceId = null
) {
    $datastore = new DatastoreClient(['namespaceId' => $namespaceId]);
    $transaction = $datastore->transaction();
    $fromKey = $datastore->key('Account', $fromKeyId);
    $toKey = $datastore->key('Account', $toKeyId);
    // 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();
}

from google.cloud import datastore

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

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])

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

/// <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);
}

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
})

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.

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);
}

$retries = 5;
for ($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;
}

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.")

(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

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

_, 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
})

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();
  }
}

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.
      await transaction.rollback();
    } else {
      // Create the task entity.
      transaction.save(taskEntity);
      await transaction.commit();
    }
    return taskEntity;
  } catch (err) {
    await transaction.rollback();
  }
}

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

from google.cloud import datastore

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

import datetime

with client.transaction():
    key = client.key(
        "Task", datetime.datetime.now(tz=datetime.timezone.utc).isoformat()
    )

    task = client.get(key)

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

    return task

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

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();
}

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)

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();
  }
}

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();
  }
}

$transaction = $datastore->readOnlyTransaction();
$taskListKey = $datastore->key('TaskList', 'default');
$query = $datastore->query()
    ->kind('Task')
    ->hasAncestor($taskListKey);
$result = $transaction->runQuery($query);
$taskListEntities = [];
$num = 0;
/* @var Entity $task */
foreach ($result as $task) {
    $taskListEntities[] = $task;
    $num += 1;
}

from google.cloud import datastore

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

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

# task_list_name = "default"
task_list_key = datastore.key "TaskList", task_list_name
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