Redis persistence overview (RDB, AOF) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the command to save the current Redis database state to disk immediately.

Redis

[1]

Drag options to blanks, or click blank then click option'

ASAVE

BBGSAVE

CDUMP

DPERSIST

Attempts:

3 left

💡 Hint

Common Mistakes

Using BGSAVE when you need an immediate synchronous save.

Using DUMP which serializes a single key, not the entire database.

The SAVE command performs a synchronous save of the Redis dataset to disk as an RDB snapshot. It blocks the server until complete, unlike BGSAVE which runs in the background.

2fill in blank

medium

Complete the command to append every write operation to the AOF file.

Redis

CONFIG SET appendonly [1]

Drag options to blanks, or click blank then click option'

Ano

Byes

Cmaybe

Dauto

Attempts:

3 left

💡 Hint

Common Mistakes

Setting appendonly to 'no' disables AOF.

Using invalid values like 'maybe' or 'auto'.

Setting appendonly to yes enables AOF persistence, which logs every write operation.

3fill in blank

hard

Choose the correct command to rewrite the AOF file in the background.

Redis

[1]

Drag options to blanks, or click blank then click option'

ABGREWRITEAOF now

BBGREWRITEAOF

CBGREWRITEAOF force

DREWRITEAOF

Attempts:

3 left

💡 Hint

Common Mistakes

Adding arguments like 'now' or 'force' which are not valid.

Using REWRITEAOF which is not a valid Redis command.

The BGREWRITEAOF command takes no arguments. It triggers a background rewrite of the AOF file to compact it. Adding arguments like 'now' or 'force' causes errors.

4fill in blank

hard

Fill both blanks to configure Redis to save RDB snapshots every 60 seconds if at least 1000 keys changed.

Redis

CONFIG SET save "[1] [2]"

Drag options to blanks, or click blank then click option'

A60

B1000

C30

D500

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the two numbers.

Using incorrect values like 30 seconds or 500 changes.

The save configuration uses pairs of seconds and number of changes. Here, 60 seconds and 1000 changes trigger a snapshot.

5fill in blank

hard

Fill all three blanks to configure AOF to fsync every second and use the 'everysec' policy.

Redis

CONFIG SET appendfsync [1]
CONFIG SET appendonly [2]
CONFIG SET no-appendfsync-on-rewrite [3]

Drag options to blanks, or click blank then click option'

Aalways

Byes

Cno

Deverysec

Attempts:

3 left

💡 Hint

Common Mistakes

Setting appendfsync to 'always' causes more disk writes.

Disabling AOF by setting appendonly to 'no'.

Incorrectly enabling no-appendfsync-on-rewrite.

Setting appendfsync to everysec makes Redis fsync the AOF file every second. appendonly set to yes enables AOF. no-appendfsync-on-rewrite set to no disables skipping fsync during rewrite.

Practice

1. What is the main purpose of Redis persistence?

easy

A. To speed up data retrieval during queries

B. To save data so it is not lost after a restart

C. To compress data for less storage use

D. To encrypt data for security

Solution

1. Step 1: Understand Redis persistence goal

   Redis persistence is designed to keep data safe even if Redis restarts or crashes.

2. Step 2: Match purpose with options

   Only To save data so it is not lost after a restart talks about saving data across restarts, which matches the goal.

3. Final Answer:

   To save data so it is not lost after a restart -> Option B

4. Quick Check:

   Persistence means saving data = A [OK]

Hint: Persistence means data stays after restart [OK]

Common Mistakes:

• Confusing persistence with speed optimization
• Thinking persistence compresses data
• Assuming persistence encrypts data

2. Which Redis persistence method saves snapshots of the dataset at intervals?

easy

A. None of the above

B. AOF (Append Only File)

C. Both AOF and RDB

D. RDB (Redis Database)

Solution

1. Step 1: Recall persistence methods

   RDB saves snapshots of the dataset at set intervals, while AOF logs every write operation.

2. Step 2: Identify correct method for snapshots

   Only RDB matches the description of saving snapshots periodically.

3. Final Answer:

   RDB (Redis Database) -> Option D

4. Quick Check:

   Snapshots = RDB [OK]

Hint: Snapshots = RDB saves whole data periodically [OK]

Common Mistakes:

• Mixing AOF with snapshot saving
• Choosing both methods as snapshot savers
• Thinking AOF saves snapshots

3. Given Redis configured with both RDB and AOF enabled, which statement is true about data recovery after a crash?

medium

A. Redis will use the AOF log if available, otherwise the RDB snapshot

B. Redis will use only the RDB snapshot for recovery

C. Redis will use only the AOF log for recovery

D. Redis cannot recover data if both are enabled

Solution

1. Step 1: Understand recovery priority

   When both RDB and AOF are enabled, Redis prefers AOF for recovery because it has more recent data.

2. Step 2: Analyze options

   Redis will use the AOF log if available, otherwise the RDB snapshot correctly states Redis uses AOF if available, else falls back to RDB.

3. Final Answer:

   Redis will use the AOF log if available, otherwise the RDB snapshot -> Option A

4. Quick Check:

   AOF if available else RDB = B [OK]

Hint: AOF has latest data, used first for recovery [OK]

Common Mistakes:

• Assuming Redis uses only RDB always
• Thinking Redis cannot recover with both enabled
• Believing AOF is ignored if RDB exists

4. You configured Redis with AOF persistence but notice the AOF file is not growing after writes. What is the most likely cause?

medium

A. AOF rewrite is stuck and blocking writes

B. Redis is only using RDB snapshots

C. AOF persistence is disabled in the config

D. Redis does not support AOF persistence

Solution

1. Step 1: Check AOF file behavior

   If AOF file size does not increase after writes, it suggests AOF is not active or enabled.

2. Step 2: Identify cause from options

   AOF persistence is disabled in the config states AOF persistence is disabled, which explains no file growth.

3. Final Answer:

   AOF persistence is disabled in the config -> Option C

4. Quick Check:

   No AOF growth means AOF disabled = C [OK]

Hint: No AOF file growth? Check if AOF is enabled [OK]

Common Mistakes:

• Assuming Redis does not support AOF
• Thinking AOF rewrite blocks all writes
• Confusing RDB usage with AOF file size

5. You want to minimize data loss in Redis but keep fast restart times. Which persistence setup is best?

hard

A. Enable both RDB snapshots and AOF with fsync every second

B. Enable only AOF with every write logged

C. Disable persistence and rely on backups

D. Enable only RDB snapshots every hour

Solution

1. Step 1: Understand trade-offs

   RDB snapshots are fast but risk losing data between snapshots. AOF logs every write but can slow restarts.

2. Step 2: Analyze combined setup

   Enabling both with AOF fsync every second balances minimal data loss and reasonable restart speed.

3. Step 3: Evaluate other options

   Only RDB risks data loss; only AOF can slow restarts; disabling persistence risks total data loss.

4. Final Answer:

   Enable both RDB snapshots and AOF with fsync every second -> Option A

5. Quick Check:

   Best mix for safety and speed = D [OK]

Hint: Combine RDB + AOF for safety and speed [OK]

Common Mistakes:

• Choosing only RDB and ignoring data loss risk
• Choosing only AOF and ignoring restart speed
• Disabling persistence entirely