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PSETEX

Introduction​

In Dragonfly, as well as in Redis and Valkey, the PSETEX command is used to set a value in a key and simultaneously set an expiration time for that key in milliseconds. This command is similar to the SETEX command, with the key difference being that the expiration time is specified in milliseconds instead of seconds. Use cases include situations where precise control over key expiration is required, such as fine-grained cache management or coordination between nodes in a distributed system. Note that both SETEX and PSETEX commands can be replaced by the SET command with the EX or PX options, respectively.

Syntax​

PSETEX key milliseconds value
  • Time complexity: O(1)
  • ACL categories: @write, @string, @slow

Parameter Explanations​

  • key: The key where the value will be set.
  • milliseconds: The expiration time in milliseconds.
  • value: The string value to associate with the key.

Return Values​

The command returns OK if the operation was successful.

Code Examples​

Basic Example​

In this example, we set the key mykey with the value "hello" and an expiration time of 2000 milliseconds (2 seconds):

dragonfly> PSETEX mykey 2000 "hello"
OK

To verify the key expires after 2 seconds:

dragonfly> GET mykey
"hello"
# Wait for more than 2 seconds
dragonfly> GET mykey
(nil)

Expire for Inactivity​

Imagine an e-commerce system where carts expire after 10 minutes of inactivity. You can set each user's cart data with a precise expiration time using the PSETEX command. In the meantime, if the user interacts with the cart, you can update the expiration time to prevent it from expiring. As you can see in the example below, PSETEX can be replaced by the SET command with the PX option.

# Set cart data to expire after 10 minutes (600,000 milliseconds).
dragonfly> PSETEX cart:user123 600000 "cart_payload"
OK

# Adding an item to the cart updates the expiration time to 10 minutes from now.
dragonfly> SET cart:user123 "updated_cart_payload" PX 600000

# After 10 minutes, the cart data is automatically removed.
dragonfly> GET cart:user123
(nil)

Best Practices​

  • Use PSETEX when you need millisecond precision to expire temporary data. It is especially useful in distributed environments where low latency network communication requires precise control over key expiration times.
  • For keys that need expiration times above a second but for which millisecond precision is not strictly necessary, consider using the SETEX command to keep your code simpler.
  • Consider using the SET command with PX option to replace PSETEX.

Common Mistakes​

  • Providing an invalid number of milliseconds (for example, a negative or non-integer value). This will result in an error.
  • Not considering the possible effects of network delays when setting very short expiration times. If you're setting an expiration time in single-digit milliseconds, network latency may cause unexpected behavior where the key expires sooner than expected.

FAQs​

Can I modify the expiration time of an existing key?​

Yes, PSETEX will overwrite the existing key's value and expiration time with the new value and expiration time. If you want to change the expiration time only, without modifying the value, consider using the PEXPIRE or EXPIRE commands.

What happens if the key already exists?​

When you run PSETEX, the value in the key is overwritten, and the expiration time is updated to the specified milliseconds. The previous value and its expiration time (if any) are lost.

Does an expiration of 0 milliseconds work?​

No, attempting to set an expiration of 0 or a negative value results in an error.