Atomically Pipeline Multiple Commands
Problem
You want to execute several Redis commands with a single atomic command.
Solution
Use the MULTI/EXEC command to create a queue of commands and execute them atomically.
Use the MULTI command to start the queue. Redis reponds with an OK. Then queue up
each command. Finally, use EXEC to execute the commands. Redis returns a multi-bulk
reply with each command's return value. In this basic example, we add three values to a
list, increment (by 3) a key called 'country-count' and then ask for the range
of all the the values of the list.
If necessary, the DISCARD command can be used to clear the MULTI queue and exit the queue.
redis> MULTI
OK
redis> LPUSH country_list france
QUEUED
redis> LPUSH country_list italy
QUEUED
redis> LPUSH country_list germany
QUEUED
redis> INCRBY country_count 3
QUEUED
redis> LRANGE country_list 0 -1
QUEUED
redis> EXEC
1. (integer) 1
2. (integer) 2
3. (integer) 3
4. (integer) 3
5.
1. germany
2. italy
3. france
Discussion
MULTI/EXEC, which will be added in Redis 2.0, is an extremely important component of Redis and merits a good deal of discussion. It is important to know what it does -- and what it does not do.
M/E is 'atomic' in the sense that, while a queue is being executed, no other clients will be served by the Redis server -- the queue is treated as a single operation. This is important for data integrity.
The M/E process is designed to prevent syntax errors during execution by immediately reporting syntax errors whenever a command is added to the queue. For example:
redis> MULTI
OK
redis> LPUSH country_list italy
QUEUED
redis> LPUSH country_list italy germany
Wrong number of arguments for 'lpush'
Now, M/E does not provide complete 'transactions' -- at least not in the ordinary sense -- since it does not include 'rollback' functionality. Consider the following situation. You create a relatively large M/E queue (say, with 200 commands) and run EXEC. Before all the queued commands are executed, the server crashes, or perhaps it runs out of memory -- let's say that happens at command #148. The first 148 commands are indeed executed, but the rest are not.
Indeed, M/E is only an 'all or nothing' operation before the EXEC command is run (that is, during queueing) -- not during the execution.
Redis does provide an interesting way to deal with issue: the familiar Append-Only File. In the upcoming version of Redis, the commands in the queue are only written to the AOF upon successful completion of the EXEC command. So if your server crashes mid-EXEC, you can rebuild state according to the previous, pre-EXEC state.