RDB snapshots (point-in-time) in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Understanding RDB Snapshot Timing

If a Redis server is configured to save an RDB snapshot every 60 seconds, what will be the state of the data in the snapshot file if the server crashes after 45 seconds of the last snapshot?

A The snapshot contains all data up to the last 60-second save point, missing changes made in the last 45 seconds.

B The snapshot contains all data including the last 45 seconds before the crash.

C The snapshot file will be empty because the server crashed before the next save.

D The snapshot contains only the data from the last 10 seconds before the crash.

💡 Hint

Think about when Redis saves snapshots and what happens if it crashes before the next save.

Redis saves snapshots at configured intervals. If it crashes before the next scheduled save, the snapshot file only contains data up to the last successful save point, so recent changes are lost.

🧠 Conceptual

intermediate

RDB Snapshot Configuration Impact

Which Redis configuration setting controls how often RDB snapshots are saved to disk?

A timeout

B appendonly

C maxmemory-policy

D save

💡 Hint

Look for the setting related to snapshot intervals.

The 'save' configuration defines the frequency of RDB snapshots by specifying time and number of changes.

🔧 Debug

advanced

Diagnosing Missing Data After Crash

A Redis server is configured to save snapshots every 30 seconds. After a crash, the RDB file is loaded but recent data is missing. Which of the following is the most likely cause?

A The append-only file (AOF) was corrupted and Redis ignored it.

B The server crashed before the next snapshot was saved, so recent changes were lost.

C The snapshot file was overwritten with an empty file during the crash.

D The Redis server was configured to never save snapshots.

💡 Hint

Consider how RDB snapshots work and when data is persisted.

RDB snapshots only save data at intervals. If the server crashes before the next save, recent changes are lost because they were not persisted.

📝 Syntax

advanced

Correct Redis CLI Command to Trigger Manual RDB Save

Which Redis CLI command triggers an immediate RDB snapshot save to disk?

A SAVEALL

B BGSAVE

C SAVE

D SNAPSHOT

💡 Hint

One command saves synchronously, the other asynchronously.

BGSAVE triggers an asynchronous background save of the RDB snapshot, allowing the server to continue processing commands. SAVE triggers a synchronous save, blocking the server until complete.

❓ optimization

expert

Optimizing RDB Snapshot Frequency for Data Safety and Performance

You want to configure Redis to balance data safety and server performance by saving RDB snapshots only when at least 100 changes have occurred within 60 seconds. Which 'save' configuration line achieves this?

A save 60 100

B save 100 60

C save 60 1

D save 100 60 1

💡 Hint

The 'save' directive format is: save

The 'save 60 100' means Redis saves a snapshot if 100 changes happen within 60 seconds.

Practice

1. What is the primary purpose of an RDB snapshot in Redis?

easy

A. To increase the speed of Redis commands

B. To save the current state of the database at a specific point in time

C. To permanently delete all data from Redis

D. To replicate data to another Redis instance in real-time

Solution

1. Step 1: Understand what RDB snapshots do

   RDB snapshots capture the exact data stored in Redis at a certain moment, creating a backup file.

2. Step 2: Compare options with snapshot purpose

   Only To save the current state of the database at a specific point in time describes saving the database state at a point in time, which matches the snapshot function.

3. Final Answer:

   To save the current state of the database at a specific point in time -> Option B

4. Quick Check:

   RDB snapshot = point-in-time save [OK]

Hint: Snapshots save data state at one moment, like a photo [OK]

Common Mistakes:

• Confusing snapshots with real-time replication
• Thinking snapshots delete data
• Assuming snapshots speed up commands

2. Which Redis command creates an RDB snapshot immediately and blocks the server until done?

easy

A. SAVE

B. BGSAVE

C. RESTORE

D. FLUSHDB

Solution

1. Step 1: Recall Redis commands for snapshots

   SAVE blocks Redis and saves the snapshot immediately; BGSAVE saves in background without blocking.

2. Step 2: Match command to blocking immediate save

   Only SAVE matches the description of immediate blocking snapshot creation.

3. Final Answer:

   SAVE -> Option A

4. Quick Check:

   Immediate blocking snapshot = SAVE [OK]

Hint: SAVE blocks Redis to save snapshot immediately [OK]

Common Mistakes:

• Choosing BGSAVE which is non-blocking
• Confusing RESTORE with saving snapshots
• Thinking FLUSHDB saves data

3. Given the following Redis commands executed in order:

BGSAVE
SET key1 value1
SAVE
SET key2 value2

Which keys will be present in the RDB snapshot created by the SAVE command?

medium

A. Neither key1 nor key2

B. Only key2

C. Only key1

D. Both key1 and key2

Solution

1. Step 1: Understand timing of BGSAVE and SAVE

   BGSAVE starts background save but does not block; SET key1 happens after BGSAVE starts, so key1 may or may not be in that snapshot.

2. Step 2: SAVE blocks and saves immediately after SET key1

   SAVE runs after SET key1 but before SET key2, so the snapshot from SAVE includes key1 but not key2.

3. Final Answer:

   Only key1 -> Option C

4. Quick Check:

   SAVE snapshot = keys before SAVE command [OK]

Hint: SAVE snapshot includes keys set before it runs [OK]

Common Mistakes:

• Assuming BGSAVE snapshot includes key1 for sure
• Thinking SAVE snapshot includes key2 set after it
• Confusing background and immediate saves

4. You run the command BGSAVE but receive an error: "Background save already in progress". What is the best way to fix this?

medium

A. Run FLUSHALL to clear all data and then run BGSAVE

B. Restart the Redis server to clear the background save

C. Run SAVE instead to block and save immediately

D. Wait for the current background save to finish before running BGSAVE again

Solution

1. Step 1: Understand the error cause

   The error means a background save is already running; Redis does not allow multiple concurrent BGSAVE commands.

2. Step 2: Choose the proper fix

   Waiting for the current BGSAVE to finish is the correct approach; running SAVE blocks but is not necessary, restarting or flushing data is extreme and unnecessary.

3. Final Answer:

   Wait for the current background save to finish before running BGSAVE again -> Option D

4. Quick Check:

   Only one BGSAVE at a time allowed [OK]

Hint: Only one BGSAVE runs at once; wait before retrying [OK]

Common Mistakes:

• Restarting server unnecessarily
• Running SAVE without need
• Flushing data to fix save error

5. You want to ensure Redis creates an RDB snapshot every 5 minutes only if at least 100 keys changed. Which configuration snippet correctly sets this behavior in redis.conf?

hard

A. save 300 100

B. save 100 300

C. save 5 100

D. save 300 5

Solution

1. Step 1: Understand Redis save syntax

   The syntax is save <seconds> <changes>, meaning save if at least <changes> keys changed in <seconds> seconds.

2. Step 2: Match 5 minutes and 100 changes

   5 minutes = 300 seconds, so save 300 100 means save every 300 seconds if 100 keys changed.

3. Final Answer:

   save 300 100 -> Option A

4. Quick Check:

   save seconds changes = save 300 100 [OK]

Hint: save syntax: save seconds changes (time then count) [OK]

Common Mistakes:

• Swapping seconds and changes values
• Using minutes instead of seconds
• Misunderstanding save triggers