RDB configuration (save intervals) in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the effect of this Redis configuration snippet?

Given the Redis configuration line:
save 900 1
What does this setting do?

Redis

save 900 1

A Redis saves the DB every 900 seconds regardless of changes.

B Redis disables saving the DB automatically.

C Redis saves the DB if at least 1 key changed within 900 seconds.

D Redis saves the DB if at least 900 keys changed within 1 second.

💡 Hint

Think about the meaning of the two numbers in the 'save' directive.

The 'save' directive syntax is 'save '. It triggers a DB save if at least keys changed within seconds.

🧠 Conceptual

intermediate

Why might you configure multiple 'save' intervals in Redis?

Redis allows multiple 'save' directives like:
save 900 1
save 300 10
save 60 10000
Why would an administrator set multiple save intervals?

A To save the DB more frequently when many keys change and less frequently when fewer keys change.

B To save the DB only once at the shortest interval specified.

C To disable saving when the number of changes is below the smallest threshold.

D To save the DB only if all conditions are met simultaneously.

💡 Hint

Think about how different thresholds affect save frequency.

Multiple 'save' directives allow Redis to save more often when many keys change quickly, and less often when changes are rare, balancing performance and durability.

📝 Syntax

advanced

Identify the syntax error in this Redis save configuration line

Which option contains a syntax error in Redis 'save' configuration?

A save 900 1

B save 60

C save 300 5

D save 120 100

💡 Hint

The 'save' directive requires two arguments.

The 'save' directive must have two arguments: seconds and number of changes. Option B only has one argument, causing a syntax error.

❓ optimization

advanced

Which Redis save interval configuration best balances performance and durability for a busy server?

Consider a Redis server with frequent writes. Which save configuration is best to reduce performance impact but keep data safe?

A

save 900 1
save 300 10
save 60 10000

B

save 60 1
save 30 5
save 10 10

C save 3600 10000

D save 1 100000

💡 Hint

Think about how often saves happen and how many changes trigger them.

Option A sets multiple intervals that save more often when many keys change and less often when few keys change, balancing durability and performance.

🔧 Debug

expert

Why does this Redis server never save snapshots despite 'save' directives present?

A Redis server has these lines in redis.conf:
save 900 1
save 300 10
But it never creates RDB snapshots. What is the most likely cause?

A The server is running in cluster mode which disables RDB saving.

B The server has no keys, so no changes trigger saving.

C The server is configured with 'appendonly yes' which disables RDB snapshots.

D The 'save' directives are overridden by a 'save ""' directive disabling snapshots.

💡 Hint

Check if any configuration disables saving explicitly.

A 'save ""' directive disables all RDB snapshotting, overriding other 'save' lines. This is a common cause of no snapshots despite save directives.

Practice

1. What does the Redis save configuration option control?

easy

A. The port Redis listens on

B. The maximum memory Redis can use

C. When Redis automatically saves snapshots of the dataset

D. The password required to connect to Redis

Solution

1. Step 1: Understand the purpose of save in Redis

   The save option defines intervals for automatic snapshot saving.

2. Step 2: Identify what save controls

   It controls when Redis saves data snapshots based on time and changes.

3. Final Answer:

   When Redis automatically saves snapshots of the dataset -> Option C

4. Quick Check:

   RDB save intervals = automatic snapshot timing [OK]

Hint: Remember: save sets snapshot save timing [OK]

Common Mistakes:

• Confusing save with memory limits
• Thinking save sets connection ports
• Assuming save controls authentication

2. Which of the following is the correct syntax to save Redis data every 300 seconds if at least 10 keys changed?

easy

A. save 10 300

B. save 300 10

C. save 300,10

D. save 10;300

Solution

1. Step 1: Recall Redis save syntax

   The syntax is save <seconds> <changes> without commas or semicolons.

2. Step 2: Match the correct order

   Time in seconds comes first, then number of changes.

3. Final Answer:

   save 300 10 -> Option B

4. Quick Check:

   Correct syntax = save seconds changes [OK]

Hint: Syntax is: save seconds changes (space separated) [OK]

Common Mistakes:

• Swapping seconds and changes order
• Using commas or semicolons instead of spaces
• Omitting one of the two required values

3. Given the Redis config lines:
save 900 1
save 300 10
What happens if 5 keys change within 400 seconds?

medium

A. Redis does not save a snapshot because 10 changes were not reached

B. Redis saves a snapshot after 300 seconds because 10 changes are needed

C. Redis saves a snapshot immediately after 5 changes

D. Redis saves a snapshot after 900 seconds because 1 change triggers it

Solution

1. Step 1: Analyze each save interval

   First interval: save every 900 seconds if at least 1 change.
   Second interval: save every 300 seconds if at least 10 changes.

2. Step 2: Check conditions for 5 changes in 400 seconds

   5 changes < 10, so second interval not triggered.
   900 seconds not reached yet, so first interval not triggered either.

3. Final Answer:

   Redis does not save a snapshot because 10 changes were not reached -> Option A

4. Quick Check:

   5 changes in 400s does not meet any save condition [OK]

Hint: Check each interval separately for time and changes [OK]

Common Mistakes:

• Assuming 5 changes triggers 10-change interval
• Ignoring the time condition for the first interval
• Thinking Redis saves immediately after changes

4. You wrote this Redis config line:
save 60,5
Why does Redis ignore this setting?

medium

A. Because commas are invalid; spaces must separate values

B. Because 5 changes is too low to trigger saving

C. Because 60 is not a valid time value

D. Because save intervals must be in milliseconds

Solution

1. Step 1: Check syntax of save line

   Redis requires space-separated values, not commas.

2. Step 2: Understand why commas cause ignoring

   Commas make the line invalid, so Redis skips it.

3. Final Answer:

   Because commas are invalid; spaces must separate values -> Option A

4. Quick Check:

   Use spaces, not commas in save intervals [OK]

Hint: Use spaces, never commas, between save parameters [OK]

Common Mistakes:

• Thinking commas are allowed separators
• Believing time must be in milliseconds
• Assuming low changes disables saving

5. You want Redis to save a snapshot if either:
- 5 keys changed in 60 seconds
- OR 100 keys changed in 900 seconds
Which configuration lines achieve this?

hard

A. save 900 5\nsave 60 100

B. save 5 60\nsave 100 900

C. save 60 100\nsave 900 5

D. save 60 5\nsave 900 100

Solution

1. Step 1: Match time and changes correctly

   Format is save <seconds> <changes>.
   So 60 seconds and 5 changes is save 60 5.
   900 seconds and 100 changes is save 900 100.

2. Step 2: Verify both conditions are separate lines

   Each condition must be on its own save line.

3. Final Answer:

   save 60 5 save 900 100 -> Option D

4. Quick Check:

   Time first, changes second per line [OK]

Hint: Write each interval on its own line: save seconds changes [OK]

Common Mistakes:

• Swapping seconds and changes
• Putting both intervals on one line
• Using commas or semicolons instead of spaces