CockroachDB supports bundling multiple SQL statements into a single all-or-nothing transaction. Each transaction guarantees ACID semantics spanning arbitrary tables and rows, even when data is distributed. If a transaction succeeds, all mutations are applied together with virtual simultaneity. If any part of a transaction fails, the entire transaction is aborted, and the database is left unchanged. CockroachDB guarantees that while a transaction is pending, it is isolated from other concurrent transactions.

Note:
For a detailed discussion of CockroachDB transaction semantics, see How CockroachDB Does Distributed Atomic Transactions and Serializable, Lockless, Distributed: Isolation in CockroachDB. Note that the explanation of the transaction model described in this blog post is slightly out of date. See the Transaction Retries section for more details.

SQL Statements

Each of the following SQL statements control transactions in some way.

Statement Function
BEGIN Initiate a transaction, as well as control its priority and isolation level.
SET TRANSACTION Control a transaction's priority and isolation level.
SAVEPOINT cockroach_restart Declare the transaction as retryable. This lets you retry the transaction if it doesn't succeed because a higher priority transaction concurrently or recently accessed the same values.
RELEASE SAVEPOINT cockroach_restart Commit a retryable transaction.
COMMIT Commit a non-retryable transaction or clear the connection after committing a retryable transaction.
ROLLBACK TO SAVEPOINT cockroach_restart Handle retryable errors by rolling back a transaction's changes and increasing its priority.
ROLLBACK Abort a transaction and roll the database back to its state before the transaction began.
SHOW Display the current transaction settings.

Syntax

In CockroachDB, a transaction is set up by surrounding SQL statements with the BEGIN and COMMIT statements.

To use client-side transaction retries, you should also include the SAVEPOINT cockroach_restart, ROLLBACK TO SAVEPOINT cockroach_restart and RELEASE SAVEPOINT statements.

> BEGIN;

> SAVEPOINT cockroach_restart;

<transaction statements>

> RELEASE SAVEPOINT cockroach_restart;

> COMMIT;

At any time before it's committed, you can abort the transaction by executing the ROLLBACK statement.

Clients using transactions must also include logic to handle retries.

Error Handling

To handle errors in transactions, you should check for the following types of server-side errors:

  • Retryable Errors: Errors with the code 40001 or string retry transaction, which indicate the transaction failed because another concurrent or recent transaction accessed the same values. To handle these errors, you should retry the transaction.

  • Ambiguous Errors: Errors with the code XX000 that are returned in response to RELEASE SAVEPOINT (or COMMIT when not using SAVEPOINT), which indicate that the state of the transaction is ambiguous, i.e., you cannot assume it either committed or failed. How you handle these errors depends on how you want to resolve the ambiguity.

For example, you might want to read values from the database to see if the transaction successfully wrote values before attempting to write the values again or, alternatively, you might write the data again without seeing if the first write attempt succeeded.

Ambiguous errors are the result of inter-node communication failures which prevent a caller from knowing with certainty whether a transaction commit succeeded. Most applications will choose to simply retry the transaction.

  • SQL Errors: All other errors, which indicate that a statement in the transaction failed. For example, violating the Unique constraint generates an 23505 error. After encountering these errors, you can either issue a COMMIT or ROLLBACK to abort the transaction and revert the database to its state before the transaction began.

If you want to attempt the same set of statements again, you must begin a completely new transaction.

Transaction Retries

Transactions in CockroachDB lock data resources that are written during their execution. In the event that a pending write from one transaction conflicts with a write of a concurrent transaction, the concurrent transaction must wait for the earlier transaction to complete before proceeding. CockroachDB implements a distributed deadlock detection algorithm to discover dependency cycles. Deadlocks are resolved by allowing transactions with higher priority to abort their dependencies. Transactions which are aborted to avoid deadlock must be retried.

Transactions executed with the serializable isolation level may also require retries if they experience read/write contention with other concurrent transactions. Note that these types of transaction retries do not occur for transactions executing with the snapshot isolation level. Consider using snapshot isolation if your use case has high contention and your clients are retrying frequently. See Isolation Levels for more details.

There are two cases for handling transaction retries:

Automatic Retries

CockroachDB automatically retries any of the following types of transactions:

  • Individual statements (which are treated as implicit transactions), such as:

    > DELETE FROM customers WHERE id = 1;
    
  • Transactions sent from the client as a single batch. Batching is controlled by your driver or client's behavior, but means that CockroachDB receives all of the statements as a single unit, instead of a number of requests.

    From the perspective of CockroachDB, a transaction sent as a batch looks like this:

    > BEGIN; DELETE FROM customers WHERE id = 1; DELETE orders WHERE customer = 1; COMMIT;
    

    However, in your application's code, batched transactions are often just multiple statements sent at once. For example, in Go, this transaction would be sent as a single batch (and automatically retried):

    db.Exec(
      "BEGIN;
    
      DELETE FROM customers WHERE id = 1;
    
      DELETE orders WHERE customer = 1;
    
      COMMIT;"
    )
    

    In these cases, CockroachDB infers there is nothing conditional about these values, so it can continue to retry the transaction with the same values it originally received.

    However, if the transaction relies on conditional logic, you should instead write your transactions to use client-side intervention. This provides an opportunity for the client to check the transaction's conditions before deciding whether or not to retry the transaction, as well as update any values.

Client-Side Intervention

Your application should include client-side retry handling when the statements are sent individually, such as:

> BEGIN;

> UPDATE products SET inventory = 0 WHERE sku = '8675309';

> INSERT INTO orders (customer, status) VALUES (1, 'new');

> COMMIT;

To indicate a transaction must be retried, CockroachDB surfaces an error with the code 40001 and an error message that begins with the string retry transaction.

To handle these types of errors you have two options:

  • Recommended: Use the SAVEPOINT cockroach_restart functions to create retryable transactions. Retryable transactions can improve performance because their priority's increased each time they are retried, making them more likely to succeed the longer they're in your system.

    For more information, see Client-Side Transaction Retries.

  • Abort the transaction using ROLLBACK, and then reissue all of the statements in the transaction. This does not automatically increase the transaction's priority, so it's possible in high-contention workloads for transactions to take an incredibly long time to succeed.

Client-Side Transaction Retries

To improve the performance of transactions that fail due to contention, CockroachDB includes a set of statements that let you retry those transactions. Retrying transactions has the benefit of increasing their priority each time they're retried, increasing their likelihood to succeed.

Retried transactions are also issued at a later timestamp, so the transaction now operates on a later snapshot of the database, so the reads might return updated data.

Implementing client-side retries requires three statements:

  • SAVEPOINT cockroach_restart declares the client's intent to retry the transaction if there are contention errors. It must be executed after BEGIN but before the first statement that manipulates a database.

  • ROLLBACK TO SAVEPOINT cockroach_restart is used when your application detects 40001 / retry transaction errors. It provides you a chance to "retry" the transaction by rolling the database's state back to the beginning of the transaction and increasing the transaction's priority.

After issuing ROLLBACK TO SAVEPOINT cockroach_restart, you must issue any statements you want the transaction to contain. Typically, this means recalculating values and reissuing a similar set of statements to the previous attempt.

  • RELEASE SAVEPOINT cockroach_restart commits the transaction. At this point, CockroachDB checks to see if the transaction contends with others for access to the same values; the highest priority transaction succeeds, and the others return 40001 / retry transaction errors.

You must also execute COMMIT afterward to clear the connection for the next transaction.

You can find examples of this in the Syntax section of this page or in our Build an App with CockroachDB tutorials.

Tip:
If you're building an application in the following languages, we have packages to make client-side retries simpler:

It's also important to note that retried transactions are restarted at a later timestamp. This means that the transaction operates on a later snapshot of the database and related reads might retrieve updated data.

For greater detail, here's the process a retryable transaction goes through.

  1. The transaction starts with the BEGIN statement.

  2. The SAVEPOINT cockroach_restart statement declares the intention to retry the transaction in the case of contention errors. Note that CockroachDB's savepoint implementation does not support all savepoint functionality, such as nested transactions.

  3. The statements in the transaction are executed.

  4. If a statement returns a retryable error (identified via the 40001 error code or retry transaction string at the start of the error message), you can issue the ROLLBACK TO SAVEPOINT cockroach_restart statement to restart the transaction. Alternately, the original SAVEPOINT cockroach_restart statement can be reissued to restart the transaction.

    You must now issue the statements in the transaction again.

    In cases where you do not want the application to retry the transaction, you can simply issue ROLLBACK at this point. Any other statements will be rejected by the server, as is generally the case after an error has been encountered and the transaction has not been closed.

  5. Once the transaction executes all statements without encountering contention errors, execute RELEASE SAVEPOINT cockroach_restart to commit the changes. If this succeeds, all changes made by the transaction become visible to subsequent transactions and are guaranteed to be durable if a crash occurs.

    In some cases, the RELEASE SAVEPOINT statement itself can fail with a retryable error, mainly because transactions in CockroachDB only realize that they need to be restarted when they attempt to commit. If this happens, the retryable error is handled as described in step 4.

Transaction Parameters

Each transaction is controlled by two parameters: its priority and its isolation level. The following two sections detail these further.

Transaction Priorities

Every transaction in CockroachDB is assigned an initial priority. By default, that priority is NORMAL, but for transactions that should be given preference in high-contention scenarios, the client can set the priority within the BEGIN statement:

> BEGIN PRIORITY <LOW | NORMAL | HIGH>;

Alternately, the client can set the priority immediately after the transaction is started as follows:

> SET TRANSACTION PRIORITY <LOW | NORMAL | HIGH>;

The client can also display the current priority of the transaction with SHOW TRANSACTION PRIORITY.

Note:
When two transactions contend for the same resources indirectly, they may create a dependency cycle leading to a deadlock situation, where both transactions are waiting on the other to finish. In these cases, CockroachDB allows the transaction with higher priority to abort the other, which must then retry. On retry, the transaction inherits the higher priority. This means that each retry makes a transaction more likely to succeed in the event it again experiences deadlock.

Isolation Levels

CockroachDB supports two transaction isolation levels: SERIALIZABLE and SNAPSHOT. By default, transactions use the SERIALIZABLE isolation level, but the client can explicitly set a transaction's isolation when starting the transaction:

> BEGIN ISOLATION LEVEL <SERIALIZABLE | SNAPSHOT>;

Alternately, the client can set the isolation level immediately after the transaction is started:

> SET TRANSACTION ISOLATION LEVEL <SERIALIZABLE | SNAPSHOT>;

The client can also display the current isolation level of the transaction with SHOW TRANSACTION ISOLATION LEVEL.

Note:
For a detailed discussion of isolation in CockroachDB transactions, see Serializable, Lockless, Distributed: Isolation in CockroachDB.

Serializable Isolation

With SERIALIZABLE isolation, a transaction behaves as though it has the entire database all to itself for the duration of its execution. This means that no concurrent writers can affect the transaction unless they commit before it starts, and no concurrent readers can be affected by the transaction until it has successfully committed. This is the strongest level of isolation provided by CockroachDB and it's the default.

Unlike SNAPSHOT, SERIALIZABLE isolation permits no anomalies. However, due to CockroachDB's transaction model, SERIALIZABLE isolation may require more transaction restarts, especially in the presence of high contention between concurrent transactions. Consider using SNAPSHOT isolation for high contention workloads.

Snapshot Isolation

With SNAPSHOT isolation, a transaction behaves as if it were reading the state of the database consistently at a fixed point in time. Unlike the SERIALIZABLE level, SNAPSHOT isolation permits the write skew anomaly, but in cases where write skew conditions are unlikely, this isolation level can be highly performant.

Comparison to ANSI SQL Isolation Levels

CockroachDB uses slightly different isolation levels than ANSI SQL isolation levels.

Aliases

  • REPEATABLE READ is an alias for SERIALIZABLE.
  • READ UNCOMMITTED and READ COMMITTED are aliases for SNAPSHOT.
Tip:
Despite similarity in names, REPEATABLE READ does not equate to SNAPSHOT in CockroachDB. We made this choice to avoid potential confusion between them and the anomalies they can introduce. REPEATABLE READ permits the phantom read anomaly, while SNAPSHOT permits the write skew anomaly.

Comparison

  • The CockroachDB SERIALIZABLE level is stronger than the ANSI SQL REPEATABLE READ level and equivalent to the ANSI SQL SERIALIZABLE level.
  • The CockroachDB SNAPSHOT level is stronger than the ANSI SQL READ UNCOMMITTED and READ COMMITTED levels.

For more information about the relationship between these levels, see this paper.

See Also



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