Redis configuration file - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this Redis command?

Given the Redis configuration file has maxmemory 100mb set, what will the command CONFIG GET maxmemory return?

Redis

CONFIG GET maxmemory

A ["maxmemory", "104857600"]

B ["maxmemory", "100mb"]

C ["maxmemory", "100"]

D ["maxmemory", "100000000"]

💡 Hint

Redis stores memory sizes in bytes internally.

The maxmemory value is returned in bytes. 100mb equals 100 * 1024 * 1024 = 104857600 bytes.

📝 Syntax

intermediate

Identify the syntax error in this Redis config line

Which option shows a syntax error in Redis configuration file for setting the port?

A port=6379

B port 6379

C port 0

D port 6380

💡 Hint

Redis config lines use space, not equal sign.

Redis configuration lines use space to separate directive and value. Using '=' causes syntax error.

❓ optimization

advanced

Which configuration optimizes Redis persistence for minimal data loss?

To minimize data loss in Redis, which configuration setting is best?

A save "900 1"

B appendfsync no

C appendfsync everysec

D appendonly yes

💡 Hint

Consider how often Redis writes data to disk.

Setting 'appendfsync everysec' makes Redis sync the append-only file every second, balancing performance and data safety.

🔧 Debug

advanced

Why does Redis fail to start with this config line?

Given the config line maxclients -100, why does Redis fail to start?

Redis

maxclients -100

A maxclients must be a string, not a number

B maxclients cannot be set in config file

C maxclients must be zero or positive

D Negative maxclients value is invalid

💡 Hint

Check allowed value ranges for maxclients.

maxclients must be a positive integer; negative values cause startup failure.

🧠 Conceptual

expert

What is the effect of setting protected-mode no in Redis config?

Choose the correct effect of protected-mode no in Redis configuration.

A Enables password authentication for all clients

B Disables security protections, allowing external connections without authentication

C Limits Redis to accept connections only from localhost

D Enables encryption for all Redis traffic

💡 Hint

Protected mode is a safety feature for Redis servers exposed to networks.

Setting protected-mode to no disables the default safety feature that blocks external connections without authentication.

Practice

1. What is the main purpose of the Redis configuration file?

easy

A. To write Redis commands

B. To store user data permanently

C. To display Redis logs

D. To control how Redis runs and behaves

Solution

1. Step 1: Understand the role of configuration files

   Configuration files set options that control how software behaves.

2. Step 2: Apply this to Redis

   The Redis configuration file sets how Redis runs, including memory use and security.

3. Final Answer:

   To control how Redis runs and behaves -> Option D

4. Quick Check:

   Redis config file = controls Redis behavior [OK]

Hint: Config files set behavior, not data or commands [OK]

Common Mistakes:

• Thinking config stores data
• Confusing config with command input
• Assuming config shows logs

2. Which line is the correct syntax to set the Redis port to 6380 in the configuration file?

easy

A. set port=6380

B. port 6380

C. port: 6380

D. port=6380

Solution

1. Step 1: Recall Redis config syntax

   Redis config uses simple lines: setting_name followed by value, separated by space.

2. Step 2: Check each option

   port 6380 matches this syntax: 'port 6380'. Others use invalid symbols or keywords.

3. Final Answer:

   port 6380 -> Option B

4. Quick Check:

   Redis config syntax = setting value [OK]

Hint: Use space between setting and value, no equals or colons [OK]

Common Mistakes:

• Using '=' instead of space
• Adding 'set' keyword
• Using colon ':' instead of space

3. Given this Redis config snippet:

save 900 1
save 300 10
save 60 10000
port 6379
maxmemory 256mb

What does the save 300 10 line mean?

medium

A. Save the DB if at least 10 keys changed in 300 seconds

B. Save the DB every 300 seconds regardless of changes

C. Save the DB if 300 keys changed in 10 seconds

D. Save the DB only once after 300 seconds

Solution

1. Step 1: Understand Redis save syntax

   The 'save' command uses two numbers: seconds and number of changes to trigger saving.

2. Step 2: Interpret 'save 300 10'

   This means: if 10 or more keys changed in 300 seconds, Redis saves the DB.

3. Final Answer:

   Save the DB if at least 10 keys changed in 300 seconds -> Option A

4. Quick Check:

   save seconds changes = save if changes in seconds [OK]

Hint: First number = seconds, second = changes to trigger save [OK]

Common Mistakes:

• Mixing order of seconds and changes
• Thinking save triggers every fixed time
• Ignoring the number of changes

4. You added this line to your Redis config file:
maxmemory 512mb
But Redis still uses more memory than 512MB. What is the likely problem?

medium

A. Redis needs to be restarted to apply config changes

B. The maxmemory setting requires a unit in uppercase (MB), not lowercase

C. maxmemory only limits disk usage, not RAM

D. The config file must have 'set maxmemory 512mb'

Solution

1. Step 1: Check Redis config behavior

   Redis reads config file only at startup or when reloaded.

2. Step 2: Understand applying changes

   Changing config file alone does not affect running Redis until restart or reload.

3. Final Answer:

   Redis needs to be restarted to apply config changes -> Option A

4. Quick Check:

   Config changes require restart or reload [OK]

Hint: Restart Redis after config edits to apply changes [OK]

Common Mistakes:

• Assuming lowercase units are invalid
• Thinking maxmemory limits disk space
• Using 'set' keyword in config file

5. You want Redis to save data only if at least 5 keys change within 600 seconds, and limit memory to 1GB. Which two lines should you add to the config file?

hard

A. save 600 5
maxmemory 1mb

B. save 5 600
maxmemory 1024mb

C. save 600 5
maxmemory 1024mb

D. save 5 600
maxmemory 1gb

Solution

1. Step 1: Write correct save syntax

   Save syntax is 'save <seconds> <changes>', so 'save 600 5' means save if 5 keys change in 600 seconds.

2. Step 2: Write correct maxmemory value

   For 1GB (1024MB), 'maxmemory 1024mb' is correct (A). C uses '1mb' (only 1MB, too small).

3. Step 3: Choose correct combined lines

   save 600 5 maxmemory 1024mb uses correct save syntax and valid maxmemory value.

4. Final Answer:

   save 600 5 maxmemory 1024mb -> Option C

5. Quick Check:

   save seconds changes + maxmemory in mb or gb [OK]

Hint: Save uses seconds then changes; maxmemory accepts mb or gb [OK]

Common Mistakes:

• Swapping seconds and changes in save
• Using invalid memory units
• Mixing order of lines