How transactions work in Redis
All the commands in a transaction are serialized and executed sequentially. A request sent by another client will never be served in the middle of the execution of a Redis Transaction. This guarantees that the commands are executed as a single isolated operation.
EXECcommand triggers the execution of all the commands in the transaction, so if a client loses the connection to the server in the context of a transaction before calling the
EXECcommand none of the operations are performed, instead if the
EXECcommand is called, all the operations are performed. When using the append-only file Redis makes sure to use a single write(2) syscall to write the transaction on disk. However if the Redis server crashes or is killed by the system administrator in some hard way it is possible that only a partial number of operations are registered. Redis will detect this condition at restart, and will exit with an error. Using the
redis-check-aoftool it is possible to fix the append only file that will remove the partial transaction so that the server can start again.
Starting with version 2.2, Redis allows for an extra guarantee to the above two, in the form of optimistic locking in a way very similar to a check-and-set (CAS) operation. This is documented later on this page.
A Redis Transaction is entered using the
MULTI command. The command
always replies with
OK. At this point the user can issue multiple
commands. Instead of executing these commands, Redis will queue
them. All the commands are executed once
EXEC is called.
DISCARD instead will flush the transaction queue and will exit
The following example increments keys
> MULTI OK > INCR foo QUEUED > INCR bar QUEUED > EXEC 1) (integer) 1 2) (integer) 1
As is clear from the session above,
EXEC returns an
array of replies, where every element is the reply of a single command
in the transaction, in the same order the commands were issued.
When a Redis connection is in the context of a
all commands will reply with the string
QUEUED (sent as a Status Reply
from the point of view of the Redis protocol). A queued command is
simply scheduled for execution when
EXEC is called.
Errors inside a transaction
During a transaction it is possible to encounter two kind of command errors:
- A command may fail to be queued, so there may be an error before
EXECis called. For instance the command may be syntactically wrong (wrong number of arguments, wrong command name, ...), or there may be some critical condition like an out of memory condition (if the server is configured to have a memory limit using the
- A command may fail after
EXECis called, for instance since we performed an operation against a key with the wrong value (like calling a list operation against a string value).
Starting with Redis 2.6.5, the server will detect an error during the accumulation of commands.
It will then refuse to execute the transaction returning an error during
EXEC, discarding the transaction.
Note for Redis < 2.6.5: Prior to Redis 2.6.5 clients needed to detect errors occurring prior to
EXECby checking the return value of the queued command: if the command replies with QUEUED it was queued correctly, otherwise Redis returns an error. If there is an error while queueing a command, most clients will abort and discard the transaction. Otherwise, if the client elected to proceed with the transaction the
EXECcommand would execute all commands queued successfully regardless of previous errors.
Errors happening after
EXEC instead are not handled in a special way:
all the other commands will be executed even if some command fails during the transaction.
This is more clear on the protocol level. In the following example one command will fail when executed even if the syntax is right:
Trying 127.0.0.1... Connected to localhost. Escape character is '^]'. MULTI +OK SET a abc +QUEUED LPOP a +QUEUED EXEC *2 +OK -ERR Operation against a key holding the wrong kind of value
It's important to note that even when a command fails, all the other commands in the queue are processed – Redis will not stop the processing of commands.
Another example, again using the wire protocol with
telnet, shows how
syntax errors are reported ASAP instead:
MULTI +OK INCR a b c -ERR wrong number of arguments for 'incr' command
This time due to the syntax error the bad
INCR command is not queued
What about rollbacks?
Redis does not support rollbacks of transactions since supporting rollbacks would have a significant impact on the simplicity and performance of Redis.
Discarding the command queue
DISCARD can be used in order to abort a transaction. In this case, no
commands are executed and the state of the connection is restored to
> SET foo 1 OK > MULTI OK > INCR foo QUEUED > DISCARD OK > GET foo "1"
Optimistic locking using check-and-set
WATCH is used to provide a check-and-set (CAS) behavior to Redis
WATCHed keys are monitored in order to detect changes against them. If
at least one watched key is modified before the
EXEC command, the
whole transaction aborts, and
EXEC returns a Null reply to notify that
the transaction failed.
For example, imagine we have the need to atomically increment the value
of a key by 1 (let's suppose Redis doesn't have
The first try may be the following:
val = GET mykey val = val + 1 SET mykey $val
This will work reliably only if we have a single client performing the
operation in a given time. If multiple clients try to increment the key
at about the same time there will be a race condition. For instance,
client A and B will read the old value, for instance, 10. The value will
be incremented to 11 by both the clients, and finally
SET as the value
of the key. So the final value will be 11 instead of 12.
WATCH we are able to model the problem very well:
WATCH mykey val = GET mykey val = val + 1 MULTI SET mykey $val EXEC
We just have to repeat the operation hoping this time we'll not get a new race. This form of locking is called optimistic locking. In many use cases, multiple clients will be accessing different keys, so collisions are unlikely – usually there's no need to repeat the operation.
So what is
WATCH really about? It is a command that will
EXEC conditional: we are asking Redis to perform
the transaction only if none of the
WATCHed keys were modified. This includes
modifications made by the client, like write commands, and by Redis itself,
like expiration or eviction. If keys were modified between when they were
WATCHed and when the
EXEC was received, the entire transaction will be aborted
- In Redis versions before 6.0.9, an expired key would not cause a transaction to be aborted. More on this
- Commands within a transaction wont trigger the
WATCHcondition since they are only queued until the
WATCH can be called multiple times. Simply all the
WATCH calls will
have the effects to watch for changes starting from the call, up to
EXEC is called. You can also send any number of keys to a
It is also possible to use the
UNWATCH command (without arguments)
in order to flush all the watched keys. Sometimes this is useful as we
optimistically lock a few keys, since possibly we need to perform a
transaction to alter those keys, but after reading the current content
of the keys we don't want to proceed. When this happens we just call
UNWATCH so that the connection can already be used freely for new
Using WATCH to implement ZPOP
A good example to illustrate how
WATCH can be used to create new
atomic operations otherwise not supported by Redis is to implement ZPOP
ZPOPMAX and their blocking variants have only been added
in version 5.0), that is a command that pops the element with the lower
score from a sorted set in an atomic way. This is the simplest
WATCH zset element = ZRANGE zset 0 0 MULTI ZREM zset element EXEC
Redis scripting and transactions
Something else to consider for transaction like operations in redis are redis scripts which are transactional. Everything you can do with a Redis Transaction, you can also do with a script, and usually the script will be both simpler and faster.