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BLMOVE

Atomically pops an element from one list and pushes it to another list, with blocking support.

Syntax​

BLMOVE source destination where where timeout

Parameters​

  • source: The key of the list from which the element will be removed.
  • destination: The key of the list to which the element will be added.
  • where: Specifies the direction to pop from the source list. Can be LEFT or RIGHT.
  • where: Specifies the direction to push to the destination list. Can be LEFT or RIGHT.
  • timeout: The maximum time in seconds to block waiting for an element to be available. If set to 0, the command will return immediately if no element is available.

Details​

  • Available since: 6.2.0
  • Time complexity: O(1) for each pop and push operation. The blocking time complexity depends on the timeout value.
  • ACL categories: @read, @write, @list, @slow

The BLMOVE command atomically moves an element from one list to another while supporting blocking behavior. This is particularly useful for scenarios where you want to wait for an element to become available in a list before performing the move operation.

The command will block until an element becomes available in the source list or until the timeout expires. If an element is available, it is removed from the source list and added to the destination list according to the specified directions.

Examples​

Basic Usage​

Assume we have the following lists:

RPUSH mylist1 "a" "b" "c"
RPUSH mylist2 "x" "y"

To move an element from the end of mylist1 to the beginning of mylist2:

BLMOVE mylist1 mylist2 RIGHT LEFT 0

After running this command:

  • mylist1 will contain: a b
  • mylist2 will contain: c x y

The returned value will be:

"c"

Blocking Behavior​

To block until an element is available in mylist1 and move it to mylist2, with a timeout of 5 seconds:

BLMOVE mylist1 mylist2 RIGHT LEFT 5

If an element becomes available within 5 seconds, it will be moved from mylist1 to mylist2. If no element becomes available within the timeout period, the command will return nil.

Immediate Return​

To return immediately if no element is available, use a 0 timeout:

BLMOVE mylist1 mylist2 RIGHT LEFT 0

If mylist1 is empty, the command will return nil.

Edge Cases​

  • If either source or destination does not exist, it will be treated as an empty list. If the source is empty or becomes empty, the command will block until an element becomes available or the timeout expires.
  • If the timeout is set to a negative value, the command will return an error.

RESP2/RESP3 Reply​

  • Bulk string reply: the element that was moved, or nil if no element was available within the timeout period.