Redis persistence overview (RDB, AOF) - Step-by-Step Execution

Concept Flow - Redis persistence overview (RDB, AOF)

Start Redis Server

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Data in Memory

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RDB Snapshot

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Save to Disk

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Data Persistence

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Recovery on Restart

Redis keeps data in memory and saves it to disk using two methods: RDB snapshots and AOF logs, which help recover data after restart.

Execution Sample

Redis

SET key1 value1
SET key2 value2
SAVE
APPEND command: SET key3 value3
RESTART
LOAD data from RDB and AOF

This sequence shows setting keys, saving snapshot (RDB), appending commands (AOF), restarting Redis, and loading data from persistence files.

Execution Table

Step	Action	RDB State	AOF State	Result
1	SET key1 value1	Memory updated	AOF empty	key1 in memory
2	SET key2 value2	Memory updated	AOF empty	key2 in memory
3	SAVE (RDB snapshot)	RDB file saved with key1, key2	AOF empty	Disk has snapshot
4	APPEND command: SET key3 value3	RDB unchanged	AOF file has SET key3	AOF logs new command
5	RESTART Redis	Load RDB file	Replay AOF commands	Memory restored with key1, key2, key3
6	END	Data loaded	Data loaded	Redis ready with all keys

💡 Redis stops loading after replaying all AOF commands and loading RDB snapshot

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	Final
Memory Data	{}	{"key1":"value1"}	{"key1":"value1","key2":"value2"}	{"key1":"value1","key2":"value2"}	{"key1":"value1","key2":"value2","key3":"value3"}	{"key1":"value1","key2":"value2","key3":"value3"}	{"key1":"value1","key2":"value2","key3":"value3"}
RDB File	None	None	Snapshot with key1,key2	Snapshot with key1,key2	Snapshot with key1,key2	Loaded snapshot	Loaded snapshot
AOF File	Empty	Empty	Empty	Contains SET key3	Contains SET key3	Replayed commands	Replayed commands

Key Moments - 2 Insights

Why does Redis load both RDB and AOF files on restart?

Redis first loads the RDB snapshot to restore a baseline, then replays AOF commands to apply recent changes not in the snapshot, as shown in execution_table steps 5 and 6.

What happens if SAVE is not called before appending AOF commands?

Without SAVE, the RDB file is outdated or missing, so Redis relies on AOF to restore data. The execution_table shows RDB unchanged at step 4, meaning AOF holds newer data.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what does the RDB file contain?

A Snapshot with key1 and key2

B No data, it's empty

C Snapshot with key3 only

D All keys including key3

💡 Hint

Check the 'RDB State' column at step 3 in execution_table

At which step does the AOF file start containing data?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'AOF State' column in execution_table to see when it changes from empty

If Redis restarts without an AOF file, what will happen according to the flow?

A Data will be lost completely

B Data will be restored from RDB snapshot only

C Redis will fail to start

D Redis will create a new empty database

💡 Hint

Refer to concept_flow where RDB snapshot is loaded first on restart

Concept Snapshot

Redis persistence saves data from memory to disk.
RDB creates snapshots at intervals.
AOF logs every write command.
On restart, Redis loads RDB then replays AOF.
RDB is fast but less frequent.
AOF is slower but more complete.

Full Transcript

Redis stores data in memory for fast access. To keep data safe after a restart, it uses two methods: RDB snapshots and AOF logs. RDB snapshots save the whole dataset at once, like taking a photo. AOF logs every change command, like a diary. When Redis restarts, it loads the snapshot first, then replays the commands from the log to recover the latest data. This way, Redis balances speed and data safety.