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Diffstat (limited to 'iredis/data/commands/lmove.md')
| -rw-r--r-- | iredis/data/commands/lmove.md | 56 |
1 files changed, 30 insertions, 26 deletions
diff --git a/iredis/data/commands/lmove.md b/iredis/data/commands/lmove.md index 9100315..ec62ced 100644 --- a/iredis/data/commands/lmove.md +++ b/iredis/data/commands/lmove.md @@ -1,17 +1,19 @@ Atomically returns and removes the first/last element (head/tail depending on -the `wherefrom` argument) of the list stored at `source`, and pushes the element -at the first/last element (head/tail depending on the `whereto` argument) of the -list stored at `destination`. +the `wherefrom` argument) of the list stored at `source`, and pushes the +element at the first/last element (head/tail depending on the `whereto` +argument) 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 `LMOVE source destination RIGHT LEFT` -results in `source` holding `a,b` and `destination` holding `c,x,y,z`. +holding the list `x,y,z`. +Executing `LMOVE source destination RIGHT LEFT` 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 first/last element from the list and pushing it as -first/last element of the list, so it can be considered as a list rotation -command (or a no-op if `wherefrom` is the same as `whereto`). +performed. +If `source` and `destination` are the same, the operation is equivalent to +removing the first/last element from the list and pushing it as first/last +element of the list, so it can be considered as a list rotation command (or a +no-op if `wherefrom` is the same as `whereto`). This command comes in place of the now deprecated `RPOPLPUSH`. Doing `LMOVE RIGHT LEFT` is equivalent. @@ -35,19 +37,21 @@ 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 +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. -`LMOVE` (or `BLMOVE` 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. +`LMOVE` (or `BLMOVE` 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 @@ -55,22 +59,22 @@ again if needed. ## Pattern: Circular list -Using `LMOVE` 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 +Using `LMOVE` 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 +* 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. +* 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. +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 |