Advanced Client-side Transaction Retries

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This page has instructions for authors of database drivers and ORMs who would like to implement client-side retries in their database driver or ORM for maximum efficiency and ease of use by application developers.

Overview

To improve the performance of transactions that fail due to contention, CockroachDB includes a set of statements (listed below) that let you retry those transactions. Retrying transactions using these statements has the following benefits:

1. When you use savepoints, you "hold your place in line" between attempts. Without savepoints, you're starting from scratch every time.
2. Transactions increase their priority each time they're retried, increasing the likelihood they will succeed. This has a lesser effect than #1.

How transaction retries work

A retryable transaction goes through the process described below, which maps to the following SQL statements:

> BEGIN;                                  -- #1
> SAVEPOINT cockroach_restart;            -- #2
-- ... various transaction statements ... -- #3
> RELEASE SAVEPOINT cockroach_restart;    -- #5 (Or #4, ROLLBACK, in case of retry error)
> COMMIT;

1. The transaction starts with the BEGIN statement.

2. The SAVEPOINT 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. It must be executed after BEGIN but before the first statement that manipulates a database.

3. The statements in the transaction are executed.

4. If a statement returns a retry 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 statement to restart the transaction and increase the transaction's priority. Alternately, the original SAVEPOINT 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 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 retry error, mainly because transactions in CockroachDB only realize that they need to be restarted when they attempt to commit. If this happens, the retry error is handled as described in step 4.

Customizing the savepoint name

Set the force_savepoint_restart session variable to true to enable using a custom name for the restart savepoint (for example, because you are using an ORM that wants to use its own names for savepoints).

Once this variable is set, the SAVEPOINT statement will accept any name for the savepoint, not just cockroach_restart. This allows compatibility with existing code that uses a single savepoint per transaction as long as that savepoint occurs before any statements that access data stored in non-virtual tables.

Examples

For examples showing how to use SAVEPOINT and the other statements described on this page to implement library support for a programming language, see the following:

• Build a Java app with CockroachDB, in particular the logic in the runSQL method.
• The source code of the sqlalchemy-cockroachdb adapter for SQLAlchemy.

See also

• Transactions
• BEGIN
• COMMIT
• ROLLBACK
• SAVEPOINT
• RELEASE SAVEPOINT
• SHOW
• CockroachDB Architecture: Transaction Layer

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