Redis Transactions
Redis transactions allow executing multiple commands as a single unit. This lesson covers transaction usage and caveats.
What is a Transaction?
A transaction is a collection of commands that are executed sequentially without interruption by other commands.
MULTI → Command 1 → Command 2 → Command 3 → EXEC
Basic Commands
MULTI: Start a Transaction
REDIS
MULTI
OK
EXEC: Execute the Transaction
REDIS
MULTI
OK
SET name "Alice"
QUEUED
SET age 25
QUEUED
EXEC
1) OK
2) OK
DISCARD: Cancel the Transaction
REDIS
MULTI
OK
SET name "Bob"
QUEUED
DISCARD
OK
Transaction Properties
Command Queue
Commands in a transaction are not executed immediately but placed in a queue:
REDIS
MULTI
OK
SET a 1
QUEUED
SET b 2
QUEUED
GET a
QUEUED
EXEC
1) OK
2) OK
3) "1"
Atomicity
Commands in a transaction are either all executed or none executed:
REDIS
MULTI
OK
SET a 1
QUEUED
SET b 2
QUEUED
EXEC
1) OK
2) OK
⚠️ Note: Redis transactions do not support rollback. If a command fails within a transaction, other commands still execute.
WATCH: Optimistic Locking
WATCH monitors keys. If any watched key is modified before the transaction executes, the transaction is canceled.
Basic Usage
REDIS
# Watch a key
WATCH mykey
OK
# Start a transaction
MULTI
OK
# Modify the key
SET mykey "new_value"
QUEUED
# Execute the transaction
EXEC
(nil) # Returns nil if the transaction was canceled (key was modified)
Implementing Optimistic Locking
Example: Optimistic Lock
BASH
# Client 1
WATCH balance
GET balance
MULTI
SET balance 900
EXEC
# Client 2 (before Client 1 executes EXEC)
SET balance 800
Use Cases
1. Inventory Deduction
Example: Decrease Stock
BASH
# Decrease stock (atomic operation)
MULTI
DECR stock:product:123
INCR sold:product:123
EXEC
2. Money Transfer
Example: Transfer
BASH
# Transfer: send 100 from user A to user B
MULTI
DECRBY balance:user:a 100
INCRBY balance:user:b 100
EXEC
3. Batch Operations
Example: Batch Set
BASH
# Set multiple keys
MULTI
SET user:1:name "Alice"
SET user:1:age 25
SET user:1:email "alice@example.com"
EXEC
Transactions vs Scripts
Lua Scripts (Recommended)
Lua scripts are more powerful than transactions, supporting conditional logic and loops:
REDIS
# Atomic operation: deduct only if balance is sufficient
EVAL "
local balance = tonumber(redis.call('GET', KEYS[1]) or 0)
if balance >= tonumber(ARGV[1]) then
redis.call('DECRBY', KEYS[1], ARGV[1])
return 1
else
return 0
end
" 1 balance:user:a 100
Transaction vs Script Comparison
| Feature | Transaction | Lua Script |
|---|---|---|
| Atomicity | ✅ | ✅ |
| Conditional logic | ❌ | ✅ |
| Loops | ❌ | ✅ |
| Performance | Good | Better |
| Complexity | Simple | More complex |
Important Notes
⚠️ No rollback: Redis transactions do not support rollback. If a command fails, other commands still execute.
⚠️ Command errors: Syntax errors cancel the entire transaction. Runtime errors do not affect other commands.
💡 Lua scripts recommended: For complex atomic operations, Lua scripts are recommended.
❓ FAQ
Q Do Redis transactions support rollback?
A No. Redis transactions do not support rollback. Use Lua scripts if you need rollback behavior.
Q What does WATCH do?
A WATCH implements optimistic locking. It monitors keys and cancels the transaction if any watched key is modified.
Q Are commands in a transaction executed atomically?
A Yes. Commands in a transaction are executed sequentially without interruption by other commands.
📖 Summary
- MULTI starts a transaction, EXEC executes it, DISCARD cancels it
- Commands are queued in the transaction and executed sequentially on EXEC
- WATCH provides optimistic locking, monitoring keys for changes
- Redis transactions do not support rollback
- For complex atomic operations, Lua scripts are recommended
📝 Exercises
- Transaction practice: Use MULTI/EXEC to execute multiple commands
- Optimistic lock practice: Use WATCH to implement a simple optimistic lock
- Lua script practice: Use EVAL to execute a Lua script
Next Lesson
In the next lesson, we will learn Redis Persistence, covering RDB and AOF persistence methods.