How Transactions Work in Redict #

Redict Transactions allow the execution of a group of commands in a single step, they are centered around the commands MULTI, EXEC, DISCARD and WATCH. Redict Transactions make two important guarantees:

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 Redict Transaction. This guarantees that the commands are executed as a single isolated operation.
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 EXEC command none of the operations are performed, instead if the EXEC command is called, all the operations are performed. When using the append-only file Redict makes sure to use a single write(2) syscall to write the transaction on disk. However if the Redict 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. Redict will detect this condition at restart, and will exit with an error. Using the redis-check-aof tool 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, Redict 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.

Usage #

A Redict 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, Redict 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 example increments keys foo and bar atomically.

> 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 Redict connection is in the context of a MULTI request, all commands will reply with the string QUEUED (sent as a Status Reply from the point of view of the Redict 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 EXEC is 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 maxmemory directive).
A command may fail after EXEC is 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 Redict 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 Redict < 2.6.5: Prior to Redict 2.6.5 clients needed to detect errors occurring prior to EXEC by checking the return value of the queued command: if the command replies with QUEUED it was queued correctly, otherwise Redict 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 EXEC command 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
-WRONGTYPE Operation against a key holding the wrong kind of value

EXEC returned two-element bulk string reply where one is an OK code and the other an error reply. It’s up to the client library to find a sensible way to provide the error to the user.

It’s important to note that even when a command fails, all the other commands in the queue are processed – Redict 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 at all.

What about rollbacks? #

Redict does not support rollbacks of transactions since supporting rollbacks would have a significant impact on the simplicity and performance of Redict.

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 normal.

> 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 Redict transactions.

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 Redict doesn’t have INCR).

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.

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 modifies the result of val in the time between our call to WATCH and our call to EXEC, the transaction will fail.

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.

WATCH explained #

So what is WATCH really about? It is a command that will make the EXEC conditional: we are asking Redict 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 Redict 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 instead.

NOTE

In Redict versions before 6.0.9, an expired key would not cause a transaction to be aborted. More on this
Commands within a transaction won’t trigger the WATCH condition since they are only queued until the EXEC is sent.

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. You can also send any number of keys to a single WATCH call.

When EXEC is called, all keys are UNWATCHed, regardless of whether the transaction was aborted or not. 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.

Using WATCH to implement ZPOP #

A good example to illustrate how WATCH can be used to create new atomic operations otherwise not supported by Redict is to implement ZPOP (ZPOPMIN, 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 implementation:

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

If EXEC fails (i.e. returns a Null reply) we just repeat the operation.

Redict scripting and transactions #

Something else to consider for transaction like operations in Redict are Redict scripts which are transactional. Everything you can do with a Redict Transaction, you can also do with a script, and usually the script will be both simpler and faster.