Transactions vs Lua scripts in Redis - Visual Side-by-Side Comparison

Concept Flow - Transactions vs Lua scripts

Start Transaction

↓

Queue Commands

↓

Execute All Commands

↓

Commit or Discard

↓

Start Lua Script

↓

Run All Commands Atomically

↓

Return Result

Transactions queue commands to run all at once, while Lua scripts run all commands inside a script atomically and return results immediately.

Execution Sample

Redis

MULTI
SET key1 val1
SET key2 val2
EXEC

This Redis transaction queues two SET commands and executes them atomically.

Execution Table

Step	Action	Command	State	Output
1	Start transaction	MULTI	Transaction started, commands queued	OK
2	Queue command	SET key1 val1	Command queued	QUEUED
3	Queue command	SET key2 val2	Command queued	QUEUED
4	Execute all commands	EXEC	Commands executed atomically	["OK", "OK"]
5	End	N/A	Transaction complete	N/A

💡 All queued commands executed atomically when EXEC is called

Variable Tracker

Variable	Start	After 1	After 2	After EXEC
key1	nil	nil	nil	val1
key2	nil	nil	nil	val2
transaction_state	none	started	started	none

Key Moments - 3 Insights

Why do commands inside MULTI show QUEUED instead of immediate results?

Because commands are only queued during the transaction (see execution_table rows 2 and 3). They run only when EXEC is called (row 4).

How does a Lua script ensure atomic execution?

The entire Lua script runs as one atomic operation, so all commands inside it execute together without interruption, returning results immediately.

Can a Lua script return results while a transaction only queues commands?

Yes, Lua scripts return results after running all commands atomically, but transactions only return results after EXEC runs all queued commands.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output after queuing the first SET command?

A OK

B QUEUED

C nil

D Error

💡 Hint

Check execution_table row 2, Output column

At which step do all queued commands actually run in the transaction?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

See execution_table row 4, Action column

If you want immediate results from multiple commands atomically, which should you use?

A Lua script

B Separate commands without transaction

C MULTI/EXEC transaction

D WATCH command

💡 Hint

Lua scripts run all commands atomically and return results immediately

Concept Snapshot

Transactions (MULTI/EXEC): queue commands, run all at once atomically, results after EXEC.
Lua scripts: run all commands inside script atomically, return results immediately.
Transactions show QUEUED until EXEC; Lua scripts execute fully in one step.
Use transactions for simple atomic batches; Lua scripts for complex logic atomically.

Full Transcript

This visual execution compares Redis transactions and Lua scripts. Transactions start with MULTI, queue commands showing QUEUED, then run all commands atomically with EXEC, returning results after. Lua scripts run all commands inside the script atomically and return results immediately. Variables like keys update only after EXEC in transactions, but update instantly in Lua scripts. Key confusions include why commands queue in transactions and how Lua scripts ensure atomicity. The quiz tests understanding of when commands run and outputs during transactions versus Lua scripts.