Skip to main content

WATCH

Monitors one or more keys for changes. If any of the watched keys are modified before the transaction is executed, the transaction will be aborted.

Syntax​

WATCH key [key ...]

Parameters​

  • key: One or more keys to be monitored for changes. Each key is specified separately.

Details​

  • Available since: 1.0.0
  • Time complexity: O(N) where N is the number of keys being watched.
  • ACL categories: @transaction, @write

The WATCH command is used to monitor one or more keys for changes. When a key is watched, if any of the watched keys are modified (i.e., updated) before a transaction is executed, the transaction will be aborted. This mechanism helps ensure that transactions are executed only if the watched keys remain unchanged, which is useful for preventing conflicts and maintaining data consistency.

Examples​

Watching Keys​

To start watching one or more keys:

WATCH key1 key2

The keys key1 and key2 will be monitored for any changes. If either key is modified, the transaction that follows will be aborted.

Using WATCH with Transactions​

  1. Watch keys:

    WATCH key1 key2

  2. Start a transaction:

    MULTI

  3. Queue commands:

    SET key1 "value1"
    SET key2 "value2"

  4. Execute the transaction:

    EXEC

If any of key1 or key2 were modified by another client between the WATCH and EXEC commands, the transaction will be aborted, and EXEC will return a nil reply.

Handling Transactions with WATCH​

If no modifications occur to the watched keys, the queued commands will be executed:

WATCH key1
MULTI
SET key1 "newvalue"
EXEC

If key1 was not modified by other clients, the SET command will be executed, and key1 will be updated to "newvalue".

Unwatching Keys​

To stop monitoring the keys, use:

UNWATCH

This will remove all keys from the watch list and reset the monitoring state.

RESP2/RESP3 Reply​

  • Simple string reply: OK

Example reply for WATCH:

"OK"

This indicates that the specified keys are now being watched for changes.

Notes​

  • WATCH is used in combination with MULTI and EXEC to ensure that a transaction is only executed if no watched keys have been modified.
  • The WATCH command is useful for implementing optimistic concurrency control in Redis transactions.
  • To cancel watching keys without executing the transaction, use the UNWATCH command.