Cloud Datastore Transactions

A transaction is a set of Datastore operations on one or more entities in up to 25 entity groups. 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.

Using transactions

Transactions have a maximum duration of 270 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 group .
• The transaction exceeds a resource limit.
• Datastore encounters an internal error.

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

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

An application can execute a set of statements and Datastore operations in a single transaction, such that if any statement or operation raises an exception, none of the Datastore 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 using the Datastore API. It transfers money from one account to another.

  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 
 

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

All Datastore operations in a transaction can operate on a maximum of twenty-five entity groups. This includes querying for entities by ancestor, retrieving entities by key, updating entities, and deleting entities.

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, Datastore uses optimistic concurrency control by checking the last update time for the entity groups used in the transaction. Upon committing a transaction for the entity groups, Datastore again checks the last update time for the entity groups used in the transaction. If it has changed since our initial check, an error is returned. For an explanation of entity groups, see Ancestor paths .

Isolation and consistency

Outside of transactions, 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 modifiedby 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 Datastore at the time the transaction started. Queries and lookups inside a transaction are guaranteed to see a single, consistent snapshot of Datastore as of the beginning of the transaction. Entities and index rows in the transaction's entity groups 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 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 lookup 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

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.

  /// <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 
 

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 ):

  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 
 

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.

Finally, you can use a transaction to read a consistent snapshot of Datastore. This can be useful when multiple reads are needed to render a page or export data that must be consistent. These kinds of transactions are often called read-only transactions, since they perform no writes. Read-only single-group transactions never fail due to concurrent modifications, so you don't have to implement retries upon failure. However, multi-entity-group transactions can fail due to concurrent modifications, so these should have retries.

  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 
 

Transactions and entity groups

An entity group is a set of entities connected through ancestry to a common root element. The organization of data into entity groups can limit what transactions can be performed:

• All the data accessed by a transaction must be contained in at most 25 entity groups.
• If you want to use queries within a transaction, your data must be organized into entity groups in such a way that you can specify ancestor filters that will match the right data.

• There is a write throughput limit of about one transaction per second within a single entity group. This limitation exists because Datastore performs masterless, synchronous replication of each entity group over a wide geographic area to provide high reliability and fault tolerance.

In many applications, it is acceptable to use eventual consistency (i.e. a non-ancestor query spanning multiple entity groups, which may at times return slightly stale data) when obtaining a broad view of unrelated data, and then to use strong consistency (an ancestor query, or a lookup of a single entity) when viewing or editing a single set of highly related data. In such applications, it is usually a good approach to use a separate entity group for each set of highly related data. For more information, see Data Consistency .

What's next

• Learn about Datastore Queries .
• Learn more about Data Consistency in Datastore.