AOF (Append Only File) in Redis - Step-by-Step Execution

Concept Flow - AOF (Append Only File)

Client sends command

↓

Redis executes command

↓

Command appended to AOF file

↓

AOF file saved on disk

↓

If Redis restarts

↓

Replay commands from AOF to restore data

Commands from clients are executed and then saved one by one to a file. On restart, Redis replays this file to restore data.

Execution Sample

Redis

SET key1 value1
INCR counter
DEL key1

Three commands are executed and appended to the AOF file in order.

Execution Table

Step	Client Command	Redis Action	AOF File Content	Resulting Data State
1	SET key1 value1	Set key1 to value1	SET key1 value1	{key1: "value1"}
2	INCR counter	Increment counter (0->1)	SET key1 value1 INCR counter	{key1: "value1", counter: 1}
3	DEL key1	Delete key1	SET key1 value1 INCR counter DEL key1	{counter: 1}
4	Redis restarts	Replay AOF file commands	SET key1 value1 INCR counter DEL key1	{counter: 1}

💡 All commands appended and replayed to restore data after restart

Variable Tracker

Variable	Start	After 1	After 2	After 3	After Restart
key1	null	"value1"	"value1"	null	null
counter	0	1	1	1	1
AOF file	""	"SET key1 value1\n"	"SET key1 value1\nINCR counter\n"	"SET key1 value1\nINCR counter\nDEL key1\n"	"SET key1 value1\nINCR counter\nDEL key1\n"

Key Moments - 3 Insights

Why does Redis append commands to the AOF file after executing them, not before?

Appending after execution ensures only successful commands are saved, so replaying the AOF file restores the exact data state (see execution_table steps 1-3).

What happens if Redis crashes before a command is appended to the AOF file?

That command is lost on restart because it was never saved in the AOF file, so replaying the file won't restore it (see execution_table step 4).

How does Redis restore data from the AOF file on restart?

Redis reads each command in the AOF file and executes them in order to rebuild the data state exactly as before (see execution_table step 4).

Visual Quiz - 3 Questions

Look at the execution_table, what is the content of the AOF file after step 2?

A "SET key1 value1\nINCR counter\n"

B "INCR counter\nDEL key1\n"

C "SET key1 value1\nDEL key1\n"

D "DEL key1\n"

💡 Hint

Check the 'AOF File Content' column at step 2 in the execution_table.

At which step does the key 'key1' get deleted from the data state?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look at the 'Resulting Data State' column in the execution_table for when 'key1' disappears.

If Redis crashes before appending the 'DEL key1' command, what will be the data state after restart?

A {key1: null, counter: 0}

B {counter: 1}

C {key1: "value1", counter: 1}

D {}

💡 Hint

Refer to the variable_tracker for 'key1' and 'counter' values and the explanation about command loss before append.

Concept Snapshot

AOF (Append Only File) saves every executed command to a file.
Commands are appended after execution to ensure data consistency.
On restart, Redis replays the AOF file to restore data.
This provides durability by logging changes step-by-step.
AOF file grows over time and can be compacted by rewriting.

Full Transcript

The Append Only File (AOF) in Redis works by saving every command executed by Redis to a file after the command runs successfully. This means when a client sends a command, Redis executes it, then appends that command to the AOF file. If Redis restarts, it reads the AOF file and replays all commands in order to restore the data exactly as it was. For example, if Redis runs SET key1 value1, then INCR counter, then DEL key1, each command is saved in the AOF file. After restart, replaying these commands rebuilds the data state. This method ensures data durability by logging changes step-by-step. However, if Redis crashes before a command is appended, that command is lost and not restored on restart. The AOF file can grow large, so Redis can rewrite it to keep it compact.