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SET

Introduction​

In Dragonfly, as well as in Redis and Valkey, the SET command is used to set the value of a string key. This command may also accept several options for controlling its behavior, such as setting an expiration time or only setting the value if the key doesn't already exist. It is one of the most fundamental and frequently used commands when storing string data in key-value pairs.

Syntax​

SET key value [NX | XX] [GET] [EX seconds | PX milliseconds |
  EXAT unix-time-seconds | PXAT unix-time-milliseconds | KEEPTTL]
  • Time complexity: O(1)
  • ACL categories: @write, @string, @slow

Parameter Explanations​

  • key: The key where the value will be stored.
  • value: The value to be set for the given key.
  • NX (optional): Only set the key if it does not already exist.
  • XX (optional): Only set the key if it already exists.
  • GET (optional): Return the old value stored at key before setting to the new value.
  • EX seconds | PX milliseconds (optional): Set the expiration time for the key in seconds or milliseconds, respectively.
  • EXAT timestamp | PXAT milliseconds-timestamp (optional): Set the expiration for the key based on absolute Unix timestamps (seconds or milliseconds).
  • KEEPTTL (optional): Retain the existing time-to-live (TTL) of the key, resetting only its value.

Return Values​

  • The command returns OK if the operation was successful.
  • If a conditional option was used (e.g., NX or XX), and the condition wasn't met, the return value will be (nil).
  • When the GET option is used, the return value will be the old string stored at the key, or nil if no previous value existed.

Code Examples​

Simple SET Example​

In its simplest form, the SET command assigns a string value to a given key:

dragonfly$> SET mykey "hello"
OK
dragonfly$> GET mykey
"hello"

Use with NX (Set if Not Exists)​

This will only set the value if the key does not already exist:

dragonfly$> SET mykey "hello" NX
OK
dragonfly$> SET mykey "world" NX
(nil)  # The key already exists, so the second operation doesn't change the value.
dragonfly$> GET mykey
"hello"

Use with XX (Set if Exists)​

Use SET with the XX option to only set the value if the key already exists:

dragonfly$> SET mykey "initial"
OK
dragonfly$> SET mykey "update" XX
OK
dragonfly$> GET mykey
"update"
dragonfly$> SET non_existent_key "value" XX
(nil)  # Key doesn't exist, so the value is not set.

Set TTL with EX or PX​

The EX option sets the expiration time in seconds. It is notable that since this is a single command, the expiration time is set atomically with the value:

dragonfly$> SET mykey "temporary-ex" EX 10
OK
dragonfly$> TTL mykey
(integer) 10  # Remaining time-to-live in seconds.

You can alternatively use PX for a more granular TTL setting in milliseconds:

dragonfly$> SET mykey "temporary-px" PX 5000
OK
dragonfly$> TTL mykey
(integer) 5  # Remaining time-to-live in seconds.

Use with GET (Return Previous Value)​

The command returns the previous value stored at the key before updating it:

dragonfly$> SET mykey "initial"
OK
dragonfly$> SET mykey "updated" GET
"initial"

Absolute Expiration with EXAT or PXAT​

Set an expiration as a specific Unix timestamp, instead of after a relative period:

dragonfly$> SET mykey "expiring_soon" EXAT 1700000000
OK
dragonfly$> TTL mykey
(integer) 123456789  # TTL is based on the current time and the specified timestamp.

For even more precise control, you can specify the expiration in milliseconds using PXAT.

Using KEEPTTL to Preserve Existing TTL​

This will change the value of the key but retain its original expiration time:

dragonfly$> SET mykey "initial" EX 10
OK
dragonfly$> SET mykey "new_value" KEEPTTL
OK
dragonfly$> TTL mykey
(integer) 8  # Previous TTL remains unchanged.

Best Practices​

  • Use NX or XX when you want to conditionally set values, such as when implementing locking or caching mechanisms.
  • When setting expirations, consider if you need relative (EX, PX) or absolute (EXAT, PXAT) expiration times, based on your application's needs.
  • The GET option can be instrumental for atomic get-and-set operations without requiring an additional GET command.

Common Mistakes​

  • Combining incompatible options, such as using both NX and XX. These conflict with each other as they apply opposite conditions and cannot be used together.
  • Assuming the GET option will return the current value of the key after the new value is set. It returns the old value, not the current one.

FAQs​

What happens if the key already exists and no options like NX or XX are used?​

If NX or XX options are not provided, the SET command will overwrite the existing value unconditionally.

Can I set a TTL for an existing key without changing its value?​

No, the SET command updates both the value and TTL of the key, even if the new value is the same as the old one. To update the TTL only, you should consider using the EXPIRE command or the PEXPIRE command.

What happens to keys with NX or XX if the condition isn't met?​

If NX is used and the key exists, or if XX is used and the key does not exist, the SET command will do nothing and return (nil).

Can I set both a TTL and use KEEPTTL together?​

No, KEEPTTL retains the current TTL of a key. It is incompatible with other TTL-related options like EX, PX, EXAT, and PXAT.