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MSETNX

Introduction​

In Dragonfly, as well as in Redis and Valkey, the MSETNX command is used to set multiple keys to multiple values, but only if none of the keys already exist. It stands for "Multi-Set if Not Exists" and ensures atomicity—you can set multiple key-value pairs only when none of the keys are currently set in the database.

This command is particularly useful in scenarios where you need to initialize several keys at once in an atomic fashion but want to ensure no overwriting of existing keys.

Syntax​

MSETNX key1 value1 [key2 value2 ...]
  • Time complexity: O(N) where N is the number of keys to set.
  • ACL categories: @write, @string, @slow

Parameter Explanations​

  • key: The key to be set.
  • value: The value to associate with the key.
  • You can input multiple key-value pairs by repeating this [key value] pattern.

Return Values​

The command returns:

  • (integer) 1: if all keys were successfully set.
  • (integer) 0: if no keys were set because at least one key already exists.

Code Examples​

Basic Example​

Set multiple keys only if none of them exist:

dragonfly> MSETNX user:1000 "Alice" user:1001 "Bob"
(integer) 1  # Both keys were set because they didn't exist before.
dragonfly> MGET user:1000 user:1001
1) "Alice"
2) "Bob"

Example When a Key Exists​

If at least one of the keys exists, no keys are set:

dragonfly> SET user:1002 "Charlie"
OK
dragonfly> MSETNX user:1000 "Alice" user:1002 "Dave"
(integer) 0  # No keys were set because "user:1002" already exists.
dragonfly> MGET user:1000 user:1002
1) (nil)     # "user:1000" wasn't set.
2) "Charlie" # "user:1002" remains unchanged.

Using MSETNX in a Locking Mechanism​

MSETNX can be used to implement simple resource locking, ensuring nobody else can claim the resource while it's in use:

# Acquiring a distributed lock for a shared resource as well as for the specific user.
# The same user also cannot acquire the lock twice.
dragonfly> MSETNX shared:resource:lock "locked" user:1000 "locked"
(integer) 1  # Lock acquired.

# Another process trying to acquire the same shared resource for a different user.
dragonfly> MSETNX shared:resource:lock "locked" user:1001 "locked"
(integer) 0  # Lock NOT acquired.

# Original process releasing the lock for the user who acquired it.
dragonfly> DEL shared:resource:lock user:1000
(integer) 2  # Lock released.

Best Practices​

  • MSETNX ensures atomicity, so leverage it when you need to initialize several keys at once without risk of partial writes.
  • Use it for tasks like session management, distributed locking, or maintaining idempotency where creation should only occur if all related keys are absent.

Common Mistakes​

  • Using MSETNX when you need to overwrite existing values. If you intend to overwrite, you should use MSET instead.
  • Expecting the command to set only non-existent keys, even if some already exist. MSETNX sets no keys if even one existing key is detected.

FAQs​

What happens if none of the keys exist?​

If none of the keys exist, MSETNX will successfully set all keys and return 1.

Will MSETNX overwrite an existing key?​

No, MSETNX will not overwrite any key. If any key already exists, the command does nothing, and none of the keys are set.

Can I use MSETNX with multiple databases?​

MSETNX, like other commands, operates within the context of the currently selected logical database. This command sets multiple keys in the same database.