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SETNX

Sets the value of a key, only if the key does not already exist.

Syntax​

SETNX key value

Parameters​

  • key: The name of the key to set.
  • value: The value to set for the specified key.

Details​

  • Available since: 1.0.0
  • Time complexity: O(1)
  • ACL categories: @write, @string, @fast

The SETNX (SET if Not eXists) command sets the value of a key only if the key does not already exist. This command is useful for implementing locks or ensuring that a key is only set if it is not already in use. If the key already exists, the command does nothing.

Examples​

Basic Usage​

To set a key only if it does not already exist:

SETNX mykey "Hello, World!"

If mykey does not exist, this command will set it to "Hello, World!" and return:

(integer) 1

If mykey already exists:

SETNX mykey "New Value"

This will return:

(integer) 0

The value of mykey will remain unchanged.

Using SETNX for Simple Locking​

SETNX can be used to implement a simple locking mechanism:

SETNX lockkey "lockvalue"

If the command returns 1, the lock was successfully acquired. If it returns 0, the lock is already held by someone else.

Edge Cases​

  • If the key already exists, the command returns 0 and does not modify the key.
  • If the key does not exist, the command sets the key and returns 1.
  • If the key exists but is not a string, an error is returned.

RESP2/RESP3 Reply​

  • Integer reply: 1 if the key was set, 0 if the key was not set.