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.
If you are an application developer who needs to implement an application-level retry loop, see the Client-side intervention example.
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:
- When you use savepoints, you "hold your place in line" between attempts. Without savepoints, you're starting from scratch every time.
- 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;
The transaction starts with the
SAVEPOINTstatement shown here is a retry savepoint; that is, it declares the intention to retry the transaction in the case of contention errors. It must be executed after
BEGIN, but before the first statement that manipulates a database. Although nested transactions are supported in versions of CockroachDB 20.1 and later, a retry savepoint must be the outermost savepoint in a transaction.
The statements in the transaction are executed.
If a statement returns a retry error (identified via the
40001error code or
"retry transaction"string at the start of the error message), you can issue the
ROLLBACK TO SAVEPOINTstatement to restart the transaction and increase the transaction's priority. Alternately, the original
SAVEPOINTstatement 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 issue
ROLLBACKat 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.
Once the transaction executes all statements without encountering contention errors, execute
RELEASE SAVEPOINTto 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 SAVEPOINTstatement 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.
A savepoint defined with the name
cockroach_restart is a "retry savepoint" and is used to implement advanced client-side transaction retries. A retry savepoint differs from a savepoint for nested transactions as follows:
- It must be the outermost savepoint in the transaction.
- After a successful
RELEASE, a retry savepoint does not allow further use of the transaction. The next statement must be a
- It cannot be nested. Issuing
SAVEPOINT cockroach_restarttwo times in a row only creates a single savepoint marker (this can be verified with
SHOW SAVEPOINT STATUS). Issuing
ROLLBACK TO SAVEPOINT cockroach_restartreuses the marker instead of creating a new one.
Note that you can customize the retry savepoint name to something other than
cockroach_restart with a session variable if you need to.
Customizing the retry savepoint name
Once this variable is set, the
SAVEPOINT statement will accept any name for the retry savepoint, not just
cockroach_restart. In addition, it causes every savepoint name to be equivalent to
cockroach_restart, therefore disallowing the use of nested transactions.
This feature exists to support applications that want to use the advanced client-side transaction retry protocol, but cannot customize the name of savepoints to be
cockroach_restart. For example, this may be necessary because you are using an ORM that requires its own names for savepoints.
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
- The source code of the sqlalchemy-cockroachdb adapter for SQLAlchemy.
- CockroachDB Architecture: Transaction Layer