Redis persistence overview (RDB, AOF) - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Difference between RDB and AOF persistence

Which statement correctly describes the main difference between RDB and AOF persistence in Redis?

A RDB logs every write operation to a file, while AOF saves snapshots of the dataset at intervals.

B RDB saves snapshots of the dataset at intervals, while AOF logs every write operation to a file.

C Both RDB and AOF save snapshots of the dataset at the same frequency.

D RDB and AOF are two names for the same persistence method in Redis.

💡 Hint

Think about how often data is saved and what is recorded.

RDB creates point-in-time snapshots of the dataset at specified intervals. AOF records every write operation received by the server, allowing more fine-grained recovery.

❓ query_result

intermediate

Result of Redis restart with only RDB enabled

If Redis is configured to use only RDB persistence and it crashes before the next snapshot, what data will be available after restart?

A No data will be available; Redis starts empty.

B All data including the most recent writes before the crash.

C Only the data saved in the last snapshot before the crash.

D Data will be partially corrupted and unusable.

💡 Hint

Consider when RDB saves data and what happens if Redis crashes unexpectedly.

RDB saves data only at snapshot times. If Redis crashes before the next snapshot, recent writes after the last snapshot are lost.

📝 Syntax

advanced

Correct Redis configuration for AOF persistence

Which Redis configuration snippet correctly enables AOF persistence with appendfsync set to 'everysec'?

Redis

appendonly yes
appendfsync everysec

A

appendonly yes
appendfsync everysec

B

appendonly no
appendfsync always

C

appendonly maybe
appendfsync everysec

D

appendonly yes
appendfsync no

💡 Hint

Check the valid values for appendonly and appendfsync.

To enable AOF, appendonly must be set to 'yes'. The appendfsync option controls how often Redis fsyncs the AOF file; 'everysec' is a common safe setting.

❓ optimization

advanced

Choosing persistence for minimal data loss and performance

You want Redis to minimize data loss but also maintain good performance. Which persistence setup is best?

A Disable both RDB and AOF for best performance.

B Enable only RDB snapshots every hour.

C Enable AOF with appendfsync always and disable RDB.

D Enable AOF with appendfsync everysec and RDB snapshots as backup.

💡 Hint

Think about balancing data safety and speed.

Using AOF with 'everysec' fsync reduces data loss risk while keeping performance reasonable. RDB snapshots provide a backup in case AOF is corrupted.

🔧 Debug

expert

Diagnosing Redis data loss after crash with AOF enabled

Redis is configured with AOF enabled and appendfsync set to 'everysec'. After a crash, some recent writes are missing. What is the most likely cause?

A The crash happened less than one second after the last fsync, so recent writes were not yet flushed to disk.

B RDB snapshots overwrite AOF data causing loss.

C AOF was disabled, so no writes were logged.

D appendfsync was set to 'always', causing data loss.

💡 Hint

Consider how appendfsync everysec works and timing of crashes.

With appendfsync everysec, Redis fsyncs the AOF file once per second. If a crash occurs just before fsync, recent writes in memory are lost.