Transactions

Transaction commands for grouping multiple operations into atomic units with optimistic locking support.

Commands

Command	Syntax	Description
MULTI	MULTI	Start a transaction block
EXEC	EXEC	Execute all queued commands atomically
DISCARD	DISCARD	Abort transaction and discard queued commands
WATCH	WATCH key [key ...]	Watch keys for changes (optimistic locking)
UNWATCH	UNWATCH	Clear all watched keys

How Transactions Work

1. MULTI starts a transaction - commands are queued, not executed
2. Each command returns QUEUED instead of its result
3. EXEC executes all queued commands atomically and returns results
4. DISCARD aborts and clears the queue

Examples

Basic Transaction

127.0.0.1:6379> MULTI
OK
127.0.0.1:6379> SET user:1:name "Alice"
QUEUED
127.0.0.1:6379> SET user:1:email "alice@example.com"
QUEUED
127.0.0.1:6379> INCR user:count
QUEUED
127.0.0.1:6379> EXEC
1) OK
2) OK
3) (integer) 1

Abort Transaction

127.0.0.1:6379> MULTI
OK
127.0.0.1:6379> SET key1 "value1"
QUEUED
127.0.0.1:6379> SET key2 "value2"
QUEUED
127.0.0.1:6379> DISCARD
OK

# Nothing was set - transaction aborted
127.0.0.1:6379> GET key1
(nil)

Optimistic Locking with WATCH

WATCH enables optimistic locking - the transaction aborts if any watched key changes before EXEC:

# Terminal 1: Watch and modify
127.0.0.1:6379> SET balance 100
OK
127.0.0.1:6379> WATCH balance
OK
127.0.0.1:6379> GET balance
"100"
127.0.0.1:6379> MULTI
OK
127.0.0.1:6379> DECRBY balance 50
QUEUED

# Before EXEC, Terminal 2 modifies balance...

127.0.0.1:6379> EXEC
(nil)  # Transaction aborted - balance was modified!

# Terminal 2: Concurrent modification
127.0.0.1:6379> SET balance 200
OK

Check-and-Set Pattern

# Increment only if value is what we expect
127.0.0.1:6379> SET counter 10
OK
127.0.0.1:6379> WATCH counter
OK
127.0.0.1:6379> GET counter
"10"

# Only proceed if counter is still 10
127.0.0.1:6379> MULTI
OK
127.0.0.1:6379> SET counter 11
QUEUED
127.0.0.1:6379> EXEC
1) OK  # Success - counter wasn't modified

Clear Watches

127.0.0.1:6379> WATCH key1 key2 key3
OK
127.0.0.1:6379> UNWATCH
OK
# All watches cleared

Transfer Between Accounts

127.0.0.1:6379> SET account:A 1000
OK
127.0.0.1:6379> SET account:B 500
OK

# Atomic transfer of 200 from A to B
127.0.0.1:6379> WATCH account:A account:B
OK
127.0.0.1:6379> MULTI
OK
127.0.0.1:6379> DECRBY account:A 200
QUEUED
127.0.0.1:6379> INCRBY account:B 200
QUEUED
127.0.0.1:6379> EXEC
1) (integer) 800
2) (integer) 700

Library Mode (Rust)

For embedded library mode, Redlite provides SQLite-level transactions which offer even stronger guarantees:

use redlite::Db;

let db = Db::open("mydata.db")?;

// SQLite transaction - all-or-nothing
db.with_transaction(|tx| {
    tx.set("user:1:name", b"Alice")?;
    tx.set("user:1:email", b"alice@example.com")?;
    tx.incr("user:count")?;
    Ok(())
})?;

// Transaction with rollback on error
db.with_transaction(|tx| {
    tx.decrby("account:A", 200)?;

    // Check balance
    let balance = tx.get("account:A")?.unwrap_or_default();
    let balance: i64 = String::from_utf8_lossy(&balance).parse().unwrap_or(0);

    if balance < 0 {
        return Err(redlite::error::KvError::Custom("Insufficient funds".into()));
        // Transaction automatically rolls back
    }

    tx.incrby("account:B", 200)?;
    Ok(())
})?;

Important Notes

WATCH Behavior

• WATCH must be called before MULTI
• Watches persist until EXEC, DISCARD, or UNWATCH
• If any watched key is modified by another client, EXEC returns nil
• EXEC clears all watches automatically

Server Mode Only

• MULTI/EXEC/DISCARD/WATCH/UNWATCH are only available in server mode
• For embedded library mode, use db.with_transaction() for SQLite transactions

Transaction Scope

• Commands in a transaction see the database state at EXEC time
• No reads inside a transaction see uncommitted writes from the same transaction
• Use WATCH for read-modify-write patterns

Use Cases

Atomic Counter with Limit

WATCH counter
GET counter
# Check if counter < 100 in application
MULTI
INCR counter
EXEC

Inventory Management

WATCH inventory:widget
GET inventory:widget
# Check if inventory >= order_quantity
MULTI
DECRBY inventory:widget 5
LPUSH orders:pending order:123
EXEC

Session Management

MULTI
SET session:abc123 '{"user":"alice"}'
EXPIRE session:abc123 3600
SADD active_sessions "abc123"
EXEC