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Atomically returns and removes the last element (tail) of the list stored at
`source`, and pushes the element at the first element (head) of the list stored
at `destination`.
For example: consider `source` holding the list `a,b,c`, and `destination`
holding the list `x,y,z`. Executing `RPOPLPUSH` results in `source` holding
`a,b` and `destination` holding `c,x,y,z`.
If `source` does not exist, the value `nil` is returned and no operation is
performed. If `source` and `destination` are the same, the operation is
equivalent to removing the last element from the list and pushing it as first
element of the list, so it can be considered as a list rotation command.
@return
@bulk-string-reply: the element being popped and pushed.
@examples
```cli
RPUSH mylist "one"
RPUSH mylist "two"
RPUSH mylist "three"
RPOPLPUSH mylist myotherlist
LRANGE mylist 0 -1
LRANGE myotherlist 0 -1
```
## Pattern: Reliable queue
Redis is often used as a messaging server to implement processing of background
jobs or other kinds of messaging tasks. A simple form of queue is often obtained
pushing values into a list in the producer side, and waiting for this values in
the consumer side using `RPOP` (using polling), or `BRPOP` if the client is
better served by a blocking operation.
However in this context the obtained queue is not _reliable_ as messages can be
lost, for example in the case there is a network problem or if the consumer
crashes just after the message is received but it is still to process.
`RPOPLPUSH` (or `BRPOPLPUSH` for the blocking variant) offers a way to avoid
this problem: the consumer fetches the message and at the same time pushes it
into a _processing_ list. It will use the `LREM` command in order to remove the
message from the _processing_ list once the message has been processed.
An additional client may monitor the _processing_ list for items that remain
there for too much time, and will push those timed out items into the queue
again if needed.
## Pattern: Circular list
Using `RPOPLPUSH` with the same source and destination key, a client can visit
all the elements of an N-elements list, one after the other, in O(N) without
transferring the full list from the server to the client using a single `LRANGE`
operation.
The above pattern works even if the following two conditions:
- There are multiple clients rotating the list: they'll fetch different
elements, until all the elements of the list are visited, and the process
restarts.
- Even if other clients are actively pushing new items at the end of the list.
The above makes it very simple to implement a system where a set of items must
be processed by N workers continuously as fast as possible. An example is a
monitoring system that must check that a set of web sites are reachable, with
the smallest delay possible, using a number of parallel workers.
Note that this implementation of workers is trivially scalable and reliable,
because even if a message is lost the item is still in the queue and will be
processed at the next iteration.
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