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diff --git a/iredis/data/commands/rpoplpush.md b/iredis/data/commands/rpoplpush.md new file mode 100644 index 0000000..7a5b662 --- /dev/null +++ b/iredis/data/commands/rpoplpush.md @@ -0,0 +1,71 @@ +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. |