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Diffstat (limited to 'iredis/data/commands/rpoplpush.md')
| -rw-r--r-- | iredis/data/commands/rpoplpush.md | 51 |
1 files changed, 26 insertions, 25 deletions
diff --git a/iredis/data/commands/rpoplpush.md b/iredis/data/commands/rpoplpush.md index 2333f84..d00e8c9 100644 --- a/iredis/data/commands/rpoplpush.md +++ b/iredis/data/commands/rpoplpush.md @@ -3,16 +3,15 @@ Atomically returns and removes the last element (tail) of the list stored at 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`. +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. - -As per Redis 6.2.0, RPOPLPUSH is considered deprecated. Please prefer `LMOVE` in -new code. +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 @@ -32,22 +31,24 @@ 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. +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. +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 before it can be processed. `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. +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 +there for too much time, pushing timed out items into the queue again if needed. ## Pattern: Circular list @@ -57,17 +58,17 @@ 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: +The above pattern works even if one or both of the following conditions occur: -- There are multiple clients rotating the list: they'll fetch different - elements, until all the elements of the list are visited, and the process +* 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. +* 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. +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 |