Transaction execution model in Redis - Step-by-Step Execution

Concept Flow - Transaction execution model

Start Transaction: MULTI

↓

Queue Commands

↓

Execute Commands: EXEC

↓

All Commands Run Atomically

↓

Transaction Ends

↓

If DISCARD: Cancel Transaction

This flow shows how Redis transactions start with MULTI, queue commands, then execute all atomically with EXEC or cancel with DISCARD.

Execution Sample

Redis

MULTI
SET key1 value1
INCR counter
EXEC

This Redis transaction queues two commands and executes them atomically.

Execution Table

Step	Command	Action	State Change	Output
1	MULTI	Start transaction mode	Commands queued = 0	OK
2	SET key1 value1	Queue command	Commands queued = 1	QUEUED
3	INCR counter	Queue command	Commands queued = 2	QUEUED
4	EXEC	Execute all queued commands atomically	Commands executed = 2	["OK", 1]
5	-	Transaction ends	Commands queue cleared	-

💡 After EXEC, all queued commands run atomically and transaction ends.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
Commands queued	0	0	1	2	0	0
key1	null	null	null	null	value1	value1
counter	null	null	null	null	1	1

Key Moments - 3 Insights

Why do commands return QUEUED after MULTI but not their usual output?

Because after MULTI, commands are only queued, not executed yet. See execution_table rows 2 and 3 where output is QUEUED.

What happens if EXEC is not called after MULTI?

Commands remain queued and are not executed. The transaction stays open until EXEC or DISCARD is called.

Can commands outside MULTI/EXEC run during a transaction?

Yes, but they run separately. The transaction commands run atomically only after EXEC.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output of the INCR command before EXEC?

A QUEUED

B 1

C OK

D Error

💡 Hint

Check the output column at step 3 in the execution_table.

At which step does the transaction actually execute the queued commands?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look for the EXEC command in the execution_table.

If DISCARD was called instead of EXEC at step 4, what would happen to the commands queued?

A They would run partially

B They would execute anyway

C They would be canceled and not run

D They would run after DISCARD

💡 Hint

Recall the concept_flow where DISCARD cancels the transaction.

Concept Snapshot

Redis transactions start with MULTI to queue commands.
Commands return QUEUED until EXEC runs them all atomically.
EXEC executes all queued commands together.
DISCARD cancels queued commands without running.
Commands outside transaction run immediately.
Transactions ensure atomic execution of multiple commands.

Full Transcript

In Redis, a transaction begins with the MULTI command, which tells Redis to start queuing commands instead of running them immediately. Each command after MULTI returns QUEUED to show it is stored for later. When EXEC is called, all queued commands run together atomically, meaning either all succeed or none do. After EXEC, the transaction ends and the queue clears. If DISCARD is called instead, the queued commands are canceled and not executed. Commands outside the transaction run normally. This model ensures multiple commands can be executed safely as one unit.