#sidebar GenericCommandsSidebar

WATCH key1 key2 ... keyN (Redis >

2.1.0)=

UNWATCH

MULTI

COMMAND_1 ...

COMMAND_2 ...

COMMAND_N ...

EXEC or DISCARD

MULTI, EXEC, DISCARD and WATCH commands are the fundation of Redis Transactions. A Redis Transaction allows the execution of a group of Redis commands in a single step, with two important guarantees:

• All the commands in a transaction are serialized and executed sequentially. It can never happen that a request issued by another client is served in the middle of the execution of a Redis transaction. This guarantees that the commands are executed as a single atomic operation.
• Either all of the commands or none are processed. The EXEC command 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 MULTI command none of the operations are performed, instead if the EXEC command is called, all the operations are performed. An exception to this rule is when the Append Only File is enabled: every command that is part of a Redis transaction will log in the AOF as long as the operation is completed, so 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.

Since Redis 2.1.0, it's also possible to add a further guarantee to the above two, in the form of optimistic locking of a set of keys in a way very similar to a CAS (check and set) operation. This is documented later in this manual page.

Usage

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 to execute this commands Redis will "queue" them. All the commands are executed once EXEC is called.

Calling DISCARD instead will flush the transaction queue and will exit the transaction.

The following is an example using the Ruby client:

?> r.multi
=> "OK"
>> r.incr "foo"
=> "QUEUED"
>> r.incr "bar"
=> "QUEUED"
>> r.incr "bar"
=> "QUEUED"
>> r.exec
=> [1, 1, 2]

As it is possible to see from the session above, MULTI 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 queued.

When a Redis connection is in the context of a MULTI request, all the commands will reply with a simple string "QUEUED" if they are correct from the point of view of the syntax and arity (number of arguments) of the commaand. Some command is still allowed to fail during execution time.

This is more clear if at 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 3 
abc
+QUEUED
LPOP a
+QUEUED
EXEC
*2
+OK
-ERR Operation against a key holding the wrong kind of value

MULTI returned a two elements bulk reply in witch one of this is a +OK code and one is a -ERR reply. It's up to the client lib to find a sensible way to provide the error to the user.

IMPORTANT: even when a command will raise an error, all the other commandsin the queue will be processed. Redis will NOT stop the processing ofcommands once an error is found.

Another example, again using the write 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 at all.

The DISCARD command

DISCARD can be used in order to abort a transaction. No command will be executed, and the state of the client is again the normal one, outside

of a transaction. Example using the Ruby client:

?> r.set("foo",1)
=> true
>> r.multi
=> "OK"
>> r.incr("foo")
=> "QUEUED"
>> r.discard
=> "OK"
>> r.get("foo")
=> "1"

Check and Set (CAS) transactions using WATCH

WATCH is used in order to provide a CAS (Check and Set) behavior to Redis Transactions.

WATCHed keys are monitored in order to detect changes against this keys. If at least a watched key will be modified before the EXEC call, the whole transaction will abort, and EXEC will return a nil object (A Null Multi Bulk reply) to notify that the transaction failed.

For example imagine we have the need to atomically increment the value of a key by 1 (I know we have INCR, let's suppose we don't have it).

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 will try to increment the key about at 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".

Thanks to WATCH we are able to model the problem very well:

WATCH mykey
val = GET mykey
val = val + 1
MULTI
SET mykey $val
EXEC

Using the above code, if there are race conditions and another client modified the result of val in the time between our call to WATCH and our call to EXEC, the transaction will fail.

We'll have just to re-iterate the operation hoping this time we'll not get a new race. This form of locking is called optimistic locking and is a very powerful form of locking as in many problems there are multiple clients accessing a much bigger number of keys, so it's very unlikely that there are collisions: usually operations don't need to be performed multiple times.

WATCH explained

So what is WATCH really about? It is a command that will make the EXEC conditional: we are asking Redis to perform the transaction only if no other client modified any of the WATCHed keys. Otherwise the transaction is not entered at all. (Note that if you WATCH a volatile key and Redis expires the key after you WATCHed it, EXEC will still work. More.)

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 the moment EXEC is called.

When EXEC is called, either if it will fail or succeed, all keys are UNWATCHed. Also when a client connection is closed, everything gets UNWATCHed.

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 transactions.

WATCH used 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, that is a command that pops the element with the lower score from a sorted set in an atomic way. This is the simplest implementation:

WATCH zset
ele = ZRANGE zset 0 0
MULTI
ZREM zset ele
EXEC

If EXEC fails (returns a nil value) we just re-iterate the operation.

Return value

Multi bulk reply, specifically:

The result of a MULTI/EXEC command is a multi bulk reply where every element is the return value of every command in the atomic transaction.

If a MULTI/EXEC transaction is aborted because of WATCH detected modified keys, a Null Multi Bulk reply is returned.