Redis lists

Introduction to Redis lists

Redis lists are linked lists of string values. Redis lists are frequently used to:

  • Implement stacks and queues.
  • Build queue management for background worker systems.

Examples

  • Treat a list like a queue (first in, first out):
> LPUSH work:queue:ids 101
(integer) 1
> LPUSH work:queue:ids 237
(integer) 2
> RPOP work:queue:ids
"101"
> RPOP work:queue:ids
"237"
  • Treat a list like a stack (first in, last out):
> LPUSH work:queue:ids 101
(integer) 1
> LPUSH work:queue:ids 237
(integer) 2
> LPOP work:queue:ids
"237"
> LPOP work:queue:ids
"101"
  • Check the length of a list:
> LLEN work:queue:ids
(integer) 0
  • Atomically pop an element from one list and push to another:
> LPUSH board:todo:ids 101
(integer) 1
> LPUSH board:todo:ids 273
(integer) 2
> LMOVE board:todo:ids board:in-progress:ids LEFT LEFT
"273"
> LRANGE board:todo:ids 0 -1
1) "101"
> LRANGE board:in-progress:ids 0 -1
1) "273"
  • To create a capped list that never grows beyond 100 elements, you can call LTRIM after each call to LPUSH:
> LPUSH notifications:user:1 "You've got mail!"
(integer) 1
> LTRIM notifications:user:1 0 99
OK
> LPUSH notifications:user:1 "Your package will be delivered at 12:01 today."
(integer) 2
> LTRIM notifications:user:1 0 99
OK

Limits

The max length of a Redis list is 2^32 - 1 (4,294,967,295) elements.

Basic commands

  • LPUSH adds a new element to the head of a list; RPUSH adds to the tail.
  • LPOP removes and returns an element from the head of a list; RPOP does the same but from the tails of a list.
  • LLEN returns the length of a list.
  • LMOVE atomically moves elements from one list to another.
  • LTRIM reduces a list to the specified range of elements.

Blocking commands

Lists support several blocking commands. For example:

  • BLPOP removes and returns an element from the head of a list. If the list is empty, the command blocks until an element becomes available or until the specified timeout is reached.
  • BLMOVE atomically moves elements from a source list to a target list. If the source list is empty, the command will block until a new element becomes available.

See the complete series of list commands.

Performance

List operations that access its head or tail are O(1), which means they're highly efficient. However, commands that manipulate elements within a list are usually O(n). Examples of these include LINDEX, LINSERT, and LSET. Exercise caution when running these commands, mainly when operating on large lists.

Alternatives

Consider Redis streams as an alternative to lists when you need to store and process an indeterminate series of events.

Learn more

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