Combined RDB + AOF in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is RDB in Redis?

RDB (Redis Database) is a snapshotting method that saves the dataset to disk at specific intervals. It creates a point-in-time copy of the data.

beginner

What does AOF stand for and what is its purpose in Redis?

AOF stands for Append Only File. It logs every write operation received by the server, allowing Redis to rebuild the dataset by replaying these commands.

intermediate

Why combine RDB and AOF in Redis?

Combining RDB and AOF gives both fast recovery from snapshots (RDB) and a more complete, fine-grained log of changes (AOF), improving durability and performance.

intermediate

How does Redis use RDB and AOF together during restart?

Redis loads the RDB snapshot first for a quick dataset load, then replays the AOF log to apply any changes made after the snapshot, ensuring data is up-to-date.

advanced

What is a key advantage of using AOF with the 'every second' fsync policy?

It balances durability and performance by syncing the AOF file to disk every second, reducing data loss risk while keeping Redis fast.

What does RDB do in Redis?

A Creates snapshots of data at intervals

B Logs every write operation

C Deletes old data automatically

D Encrypts data on disk

RDB creates snapshots of the dataset at specific times to save data to disk.

What is the main benefit of AOF over RDB?

A Faster snapshot creation

B More complete data recovery by logging every write

C Uses less disk space

D Automatically compresses data

AOF logs every write operation, allowing more complete recovery than periodic snapshots.

When Redis restarts using combined RDB + AOF, what happens first?

A RDB snapshot is loaded first

B AOF log is replayed first

C Both load simultaneously

D Data is lost and rebuilt from scratch

Redis loads the RDB snapshot first for a quick start, then replays the AOF log.

Which fsync policy for AOF balances durability and performance?

A fsync always

B fsync never

C fsync every minute

D fsync every second

Fsync every second reduces data loss risk while keeping Redis fast.

Why might you use both RDB and AOF in Redis?

A To encrypt data twice

B To save disk space

C To improve data durability and recovery speed

D To avoid backups

Using both improves durability and speeds up recovery by combining snapshot and log methods.

Explain how Redis uses RDB and AOF together to ensure data durability and fast recovery.

Think about what happens during normal operation and after a restart.

Describe the advantages and disadvantages of using only RDB or only AOF for Redis persistence.

Consider data loss risk and recovery speed.

Practice

1. What is the main advantage of using both RDB and AOF persistence together in Redis?

easy

A. It disables data persistence to improve speed.

B. It combines fast startup with high data durability.

C. It reduces memory usage significantly.

D. It automatically compresses all data files.

Solution

1. Step 1: Understand RDB persistence

   RDB creates snapshots periodically, allowing Redis to start quickly by loading a compact file.

2. Step 2: Understand AOF persistence

   AOF logs every write operation, ensuring data durability by replaying commands after a restart.

3. Final Answer:

   It combines fast startup with high data durability. -> Option B

4. Quick Check:

   Combined RDB + AOF = Fast startup + durability [OK]

Hint: Remember: RDB = snapshots, AOF = logs every write [OK]

Common Mistakes:

• Thinking combined persistence disables data saving
• Confusing RDB with AOF roles
• Assuming combined persistence reduces memory usage

2. Which Redis configuration option enables both RDB snapshots and AOF persistence?

easy

A. appendonly yes and save ""

B. appendonly no and save ""

C. save no and appendonly no

D. appendonly yes and save 900 1

Solution

1. Step 1: Identify RDB snapshot setting

   The 'save 900 1' means save snapshot if at least 1 change in 900 seconds.

2. Step 2: Identify AOF setting

   'appendonly yes' enables AOF persistence logging every write.

3. Final Answer:

   appendonly yes and save 900 1 -> Option D

4. Quick Check:

   Enable both RDB and AOF with save + appendonly yes [OK]

Hint: Look for appendonly yes plus save with time/count [OK]

Common Mistakes:

• Setting appendonly no disables AOF
• Empty save disables RDB snapshots
• Confusing syntax for save option

3. Given Redis is configured with both RDB and AOF enabled, what happens if Redis crashes after some writes but before the next RDB snapshot?

medium

A. Data is restored from AOF log, including recent writes after last snapshot.

B. Data is restored from the last snapshot only, losing recent writes.

C. Redis starts empty with no data restored.

D. Redis merges partial snapshot and AOF, causing data duplication.

Solution

1. Step 1: Understand RDB snapshot timing

   RDB snapshots happen periodically, so recent writes after last snapshot are not saved there.

2. Step 2: Understand AOF recovery

   AOF logs every write, so Redis replays AOF to restore all writes including recent ones.

3. Final Answer:

   Data is restored from AOF log, including recent writes after last snapshot. -> Option A

4. Quick Check:

   AOF recovers recent writes missed by RDB [OK]

Hint: AOF logs all writes; RDB snapshots periodically [OK]

Common Mistakes:

• Assuming only RDB restores data
• Thinking Redis starts empty after crash
• Believing Redis merges snapshot and AOF causing duplicates

4. You enabled both RDB and AOF in Redis, but after a restart, some recent data is missing. What is the most likely cause?

medium

A. Redis does not support combined persistence.

B. RDB snapshot interval was set to zero.

C. AOF file was corrupted or disabled during restart.

D. The Redis server was started with wrong port.

Solution

1. Step 1: Check AOF role in recovery

   AOF logs all writes and is used to restore recent data after restart.

2. Step 2: Identify cause of missing recent data

   If AOF is corrupted or disabled, recent writes after last snapshot are lost.

3. Final Answer:

   AOF file was corrupted or disabled during restart. -> Option C

4. Quick Check:

   Missing recent data usually means AOF problem [OK]

Hint: Check AOF file integrity if recent data missing after restart [OK]

Common Mistakes:

• Assuming Redis can't combine RDB and AOF
• Ignoring AOF file corruption possibility
• Blaming wrong port for data loss

5. You want to optimize Redis persistence by combining RDB snapshots every 15 minutes and AOF with 'everysec' fsync policy. Which statement is true about this setup?

hard

A. This setup balances fast recovery and durability with minimal performance impact.

B. This setup disables AOF fsync, risking data loss on crash.

C. RDB snapshots every 15 minutes cause Redis to block all writes during save.

D. AOF with 'everysec' fsync writes data to disk after every command synchronously.

Solution

1. Step 1: Understand RDB snapshot interval

   Snapshots every 15 minutes reduce blocking frequency and provide periodic backups.

2. Step 2: Understand AOF 'everysec' fsync

   'everysec' fsync writes AOF to disk every second asynchronously, balancing durability and performance.

3. Step 3: Combine effects

   This combination offers fast recovery from snapshots and durability from AOF with low performance cost.

4. Final Answer:

   This setup balances fast recovery and durability with minimal performance impact. -> Option A

5. Quick Check:

   RDB + AOF everysec = balanced durability and speed [OK]

Hint: Use RDB for snapshots, AOF everysec for durability and speed [OK]

Common Mistakes:

• Thinking 'everysec' fsync is synchronous after every command
• Assuming RDB snapshots block Redis for long every time
• Believing this setup disables AOF durability