Redis Transactions

14. Redis Transactions

What is a Redis Transaction?

A Redis transaction ensures a group of commands executes atomically — either all commands run, or none do (in case of a DISCARD). No other client can interrupt the sequence.

MULTI / EXEC / DISCARD Flow

flowchart TB
    Start[Client sends commands] --> Check{Want atomic<br/>execution?}
    Check -->|Yes| MULTI["MULTI<br/>Begin transaction"]
    Check -->|No| Direct["Run commands directly<br/>(not atomic)"]

    MULTI --> Queue["Queue commands<br/>SET balance:alice 500<br/>DECRBY balance:alice 100<br/>INCRBY balance:bob 100"]
    Queue --> Decision{Ready to<br/>execute?}
    Decision -->|Yes ✅| EXEC["EXEC<br/>All commands run atomically<br/>No other client can interrupt"]
    Decision -->|No ❌| DISCARD["DISCARD<br/>Queued commands thrown away<br/>Nothing happens"]

    subgraph Optimistic[Optimistic Locking with WATCH]
        Watch["WATCH balance:alice<br/>Monitor key for changes"] --> MULTI2[MULTI]
        MULTI2 --> Queue2[DECRBY balance:alice 200]
        Queue2 --> EXEC2{EXEC}
        EXEC2 -->|Key unchanged ✅| Success[Transaction succeeds]
        EXEC2 -->|Key changed by another client ❌| Fail[EXEC returns nil<br/>Retry the transaction]
    end

    style Start fill:#3b82f6,color:#fff
    style Check fill:#f59e0b,color:#fff
    style MULTI fill:#7c3aed,color:#fff
    style Queue fill:#3b82f6,color:#fff
    style Decision fill:#f59e0b,color:#fff
    style EXEC fill:#059669,color:#fff
    style DISCARD fill:#ef4444,color:#fff
    style Optimistic fill:#ec4899,color:#fff
    style Success fill:#059669,color:#fff
    style Fail fill:#ef4444,color:#fff

MULTI / EXEC / DISCARD

# Begin transaction
MULTI

# Queue commands (they don't execute yet)
SET balance:alice 500
SET balance:bob 300
DECRBY balance:alice 100
INCRBY balance:bob 100

# Execute all queued commands atomically
EXEC
# Output:
# 1) OK
# 2) OK
# 3) (integer) 400
# 4) (integer) 400

# To cancel a transaction before EXEC
MULTI
SET key "value"
DISCARD   # All queued commands are thrown away

WATCH — Optimistic Locking

WATCH monitors keys for changes. If a watched key is modified by another client before EXEC, the transaction is aborted (returns nil).

# Watch the balance key
WATCH balance:alice

# Begin transaction
MULTI

# Queue operations
DECRBY balance:alice 200

# EXEC will fail (return nil) if balance:alice was changed by another client
EXEC
# If successful: returns array of results
# If watched key changed: returns nil → retry the transaction

Important Notes

Note: Redis transactions are NOT like database transactions. They do not support rollbacks on runtime errors. If one command fails during EXEC (e.g., wrong type), other commands still run.

Concept	Redis Transactions	SQL Transactions
Atomicity	Commands execute as a group	Full ACID atomicity
Rollback on error	No	Yes
Isolation	No other client interrupts	Yes
Durability	Depends on persistence config	Yes