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BLMPOP

Removes and returns one or more elements from the head or tail of one or more lists, with blocking support.

Syntax​

BLMPOP timeout [LEFT|RIGHT] key [key ...]

Parameters​

  • 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.
  • [LEFT|RIGHT]: Specifies the direction to pop from the lists. Can be LEFT to pop from the head or RIGHT to pop from the tail.
  • key: One or more keys representing the lists from which elements will be removed.

Details​

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

The BLMPOP command atomically removes and returns one or more elements from the specified lists. The command supports blocking behavior, which means it will wait until at least one of the specified lists contains elements or until the timeout expires.

The command returns a list of the removed elements and their corresponding list keys. The lists are examined in the order they are specified, and the first list with available elements is chosen based on the specified direction (LEFT or RIGHT).

Examples​

Basic Usage​

Assume we have the following lists:

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

To remove and return one element from the end of mylist1:

BLMPOP 0 RIGHT mylist1

After running this command:

  • mylist1 will contain: a b
  • The returned value will be:
1) "mylist1"
2) "c"

Blocking Behavior​

To block until an element becomes available in mylist1 or mylist2 and remove it from the end of the lists, with a timeout of 5 seconds:

BLMPOP 5 RIGHT mylist1 mylist2

If an element becomes available within 5 seconds, it will be removed from one of the lists. 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:

BLMPOP 0 RIGHT mylist1 mylist2

If both mylist1 and mylist2 are empty, the command will return nil.

Multiple Keys​

To remove and return one element from the end of multiple lists, mylist1 and mylist2, with a timeout of 10 seconds:

BLMPOP 10 RIGHT mylist1 mylist2

The command will examine both lists and return the first available element from the end of either list within 10 seconds.

Edge Cases​

  • If none of the specified lists have elements available, the command will block until an element becomes available or the timeout expires.
  • If timeout is set to a negative value, the command will return an error.

RESP2/RESP3 Reply​

  • Array reply: A list containing the removed element and the key of the list from which it was removed. If no element is available within the timeout period, the reply is nil.