Transactions

MULTI/EXEC — Transactions

Redis transactions let you queue a series of commands and execute them atomically. No other client can interleave commands between the queued operations.

MULTI and EXEC

MULTI
SET account:1 1000
SET account:2 500
EXEC

Output:

OK
QUEUED
QUEUED
1) OK
2) OK

• MULTI — begin the transaction
• Each command responds with QUEUED (not yet executed)
• EXEC — execute all queued commands atomically; returns an array of results

DISCARD — Cancel the Transaction

MULTI
SET temp "value"
DISCARD
GET temp

Output:

OK
QUEUED
OK
(nil)

DISCARD cancels the transaction and discards all queued commands.

Atomicity in Redis

Redis transactions guarantee that:

1. All commands in a MULTI/EXEC block run sequentially
2. No other client's commands can interleave
3. Either all commands execute or none do (after EXEC)

Note: Redis does not roll back on command errors within a transaction. If a command fails (e.g., INCR on a string), other commands still execute. Use DISCARD to explicitly cancel.

Transfer Example

Atomically transfer 100 from account A to account B:

SET balance:alice 1000
SET balance:bob 500

MULTI
DECRBY balance:alice 100
INCRBY balance:bob 100
EXEC

GET balance:alice
GET balance:bob

Output:

OK
OK
OK
QUEUED
QUEUED
1) (integer) 900
2) (integer) 600
"900"
"600"

WATCH — Optimistic Locking

WATCH key [key ...] monitors keys for changes. If a watched key is modified before EXEC, the entire transaction is aborted (EXEC returns nil):

WATCH balance:alice
MULTI
DECRBY balance:alice 100
EXEC

This is useful for compare-and-swap operations without locking.

Your Task

Use MULTI/EXEC to atomically:

1. SET inventory to 100
2. DECRBY inventory by 10

Then GET the final value.