Skip to main content

BRPOPLPUSH

Atomically removes the last element from one list and pushes it to another list, with blocking support.

Syntax​

BRPOPLPUSH source destination 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.
  • timeout: The maximum time in seconds to block waiting for an element to be available. If set to 0, the command will block indefinitely until an element is available. If set to a positive number, the command will block for that number of seconds before returning nil if no element is available.

Details​

  • Available since: 1.2.0
  • 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 BRPOPLPUSH command atomically removes the last element from the source list and pushes it to the destination list. The command supports blocking behavior, meaning it will wait until the source list contains at least one element or until the timeout expires.

If the source list is empty and the timeout expires, the command will return nil. If the timeout is set to 0, the command will block indefinitely until an element becomes available in the source list.

Examples​

Basic Usage​

Assume we have the following lists:

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

To atomically remove the last element from mylist1 and push it to mylist2:

BRPOPLPUSH mylist1 mylist2 0

After running this command:

  • mylist1 will contain: a b
  • mylist2 will contain: x y c
  • The returned value will be:
"c"

Blocking Behavior​

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

BRPOPLPUSH mylist1 mylist2 5

If an element becomes available within 5 seconds, it will be removed from mylist1 and added 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:

BRPOPLPUSH mylist1 mylist2 0

If mylist1 is empty, the command will return nil until an element is added or the process is terminated.

Empty Source List​

If mylist1 is empty and you run:

BRPOPLPUSH mylist1 mylist2 10

The command will block for up to 10 seconds waiting for an element to be added to mylist1. If no element is added within that time, the command will return nil.

Edge Cases​

  • If timeout is set to a negative value, the command will return an error.
  • If the source list does not exist, it is treated as an empty list. If the destination list does not exist, it is created as an empty list before pushing the element.
  • If the source list is empty and timeout expires, the command will return nil.

RESP2/RESP3 Reply​

  • Bulk string reply: The element that was removed from the source list and pushed to the destination list. If no element is available within the timeout period, the reply is nil.