YugabyteDB's YSQL supports explicit row-level locking, similar to PostgreSQL. Explicit row-locks ensure that two transactions can never hold conflicting locks on the same row. When two transactions try to acquire conflicting lock modes, the semantics are dictated by YugabyteDB's concurrency control policies.
The following types of row locks are supported:
FOR NO KEY UPDATE
FOR KEY SHARE
The following example uses the
FOR UPDATE row lock with the fail-on-conflict concurrency control policy. First, a row is selected for update, thereby locking it, and subsequently updated. A concurrent transaction should not be able to abort this transaction by updating the value of that row after the row is locked.
Before you start
The examples will run on any YugabyteDB universe.
To create a universe, see Set up YugabyteDB universe.
Create an sample table and populate it with sample data, as follows:
yugabyte=# CREATE TABLE t (k VARCHAR, v VARCHAR); yugabyte=# INSERT INTO t VALUES ('k1', 'v1');
Next, connect to the universe using two independent
ysqlsh instances. You can connect both session
ysqlsh instances to the same server or to different servers.
Begin a transaction in the first session and perform a
SELECT FOR UPDATE on the row in the table
t. This locks the row for an update as a part of a transaction that has a very high priority (that is, in the
high priority bucket, as explained in Transaction priorities):
yugabyte=# BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ;
yugabyte=# SELECT * from t WHERE k='k1' FOR UPDATE;
k | v ----+---- k1 | v1 (1 row)
Before completing the transaction, try to update the same key in your other session using a basic update statement, as follows:
yugabyte=# UPDATE t SET v='v1.1' WHERE k='k1';
ERROR: All transparent retries exhausted. Operation failed. Try again: bb3aace4-5de2-41f9-981e-d9ca06671419 Conflicts with higher priority transaction: d4dadbf8-ca81-4bbd-b68c-067023f8ee6b
This operation fails because it conflicts with the row-level lock and as per
Fail-on-Conflict concurrency control policy, the transaction aborts itself because it has a lower priority.
Note that the error message appears after all best-effort statement retries have been exhausted.
Finally, in the first session, update the row and commit the transaction, as follows:
yugabyte=# UPDATE t SET v='v1.2' WHERE k='k1';
This should succeed.