LMOVE

Introduction and Use Case(s)

The LMOVE command in Redis atomically moves an element from one list to another, supporting both left and right directions. It’s particularly useful for implementing queues or stacks where elements need to be transferred across different lists without race conditions.

Syntax

LMOVE source destination LEFT|RIGHT LEFT|RIGHT

Parameter Explanations

• source: The key of the list from which the element will be moved.
• destination: The key of the list to which the element will be moved.
• LEFT|RIGHT: Direction to pop from the source list. LEFT pops the first element (head), while RIGHT pops the last element (tail).
• LEFT|RIGHT: Direction to push to the destination list. LEFT pushes the element to the front (head), while RIGHT pushes it to the back (tail).

Return Values

If successful, LMOVE returns the element that was moved. If either the source list is empty or does not exist, it returns nil.

Examples of Possible Outputs

1. Element successfully moved:

   "element_value"

2. Source list is empty/non-existent:

   (nil)

Code Examples

dragonfly> RPUSH mylist "one" "two" "three"
(integer) 3
dragonfly> LRANGE mylist 0 -1
1) "one"
2) "two"
3) "three"
dragonfly> LMOVE mylist myotherlist RIGHT LEFT
"three"
dragonfly> LRANGE mylist 0 -1
1) "one"
2) "two"
dragonfly> LRANGE myotherlist 0 -1
1) "three"

Best Practices

• When using LMOVE to implement a task queue, ensure that both the source and destination keys are properly managed to avoid accidental data loss.
• Consider using BLMOVE if you need blocking behavior when the source list is empty.

Common Mistakes

• Forgetting to specify both directions (LEFT|RIGHT) for popping and pushing can lead to syntax errors.
• Using non-list types as source or destination can result in unexpected errors.

FAQs

What happens if the source list is empty?

LMOVE returns nil and no element is moved.

Can LMOVE be used with non-list data types?

No, both the source and destination must be lists.