TopolearnSome updates span several keys or depend on a read, and must not interleave with other clients.
MULTI starts a transaction; subsequent commands are queued (the server replies QUEUED), and EXEC runs them all back to back with no other client's commands in between. DISCARD drops the queue.
Redis transactions are different from SQL ones:
- there is no rollback: if a command fails at runtime (say
INCRon a non-number), the others still run, - syntax errors detected while queuing make
EXECabort the whole transaction, - you can't read a value and branch on it inside
MULTI, because replies only arrive atEXEC.
For check-then-act logic use optimistic locking with WATCH. WATCH key before MULTI; read the values you need; queue the writes; call EXEC. If any watched key changed in the meantime, EXEC returns nil and nothing runs, so you retry. This is check-and-set (CAS).
In redis-py, pipeline() gives you MULTI/EXEC, and r.transaction(func, *keys) wraps the WATCH-retry loop.
For complex logic, or when contention makes WATCH retry often, a Lua script or function is usually simpler and faster: it runs atomically on the server and can read and write in one step.
Example
import redis
r = redis.Redis(decode_responses=True)
r.set("stock:pen", 3)
def buy(pipe): # called inside WATCH stock:pen
stock = int(pipe.get("stock:pen"))
if stock <= 0:
raise ValueError("sold out")
pipe.multi() # start queuing
pipe.decr("stock:pen")
pipe.rpush("orders", "pen")
for _ in range(3):
r.transaction(buy, "stock:pen") # retries automatically if stock:pen changes
print(r.get("stock:pen"), r.llen("orders")) # 0 3
# no rollback: the failing INCR doesn't undo the SET
with r.pipeline() as p:
p.set("a", "x").incr("a").set("b", "y")
print(p.execute(raise_on_error=False))
# [True, ResponseError (value is not an integer...), True]Remember
- MULTI queues, EXEC runs the queue atomically
- No rollback: runtime errors don't undo other commands
- WATCH + EXEC gives optimistic check-and-set; nil means retry
- Use Lua when logic must read and decide atomically
Watch out for
- Expecting a failed command inside EXEC to roll back the rest.