Slow log for performance analysis in Redis - Mini Project: Build & Apply

Using Redis Slow Log for Performance Analysis

📖 Scenario: You are managing a Redis database that supports a busy web application. Sometimes, the application feels slow, and you want to find out which Redis commands are taking too long to execute.Redis has a feature called the slow log that records commands that exceed a certain execution time threshold. You will learn how to enable and configure the slow log, retrieve its entries, and analyze them to improve performance.

🎯 Goal: Build a Redis setup that enables the slow log with a specific time threshold, then retrieve and analyze slow log entries to identify slow commands.

📋 What You'll Learn

Enable the Redis slow log with a threshold of 10000 microseconds (10 milliseconds).

Set the slow log maximum length to 128 entries.

Retrieve the slow log entries using the appropriate Redis command.

Clear the slow log after analysis.

💡 Why This Matters

🌍 Real World

In real-world applications, slow Redis commands can cause delays and degrade user experience. Using the slow log helps identify and fix these performance issues.

💼 Career

Database administrators and backend developers use Redis slow log to monitor and optimize Redis performance, ensuring fast and reliable applications.

1

Enable the slow log with a threshold

Use the Redis command CONFIG SET slowlog-log-slower-than 10000 to set the slow log threshold to 10000 microseconds (10 milliseconds).

Redis

# Use the Redis CLI command to set slow log threshold
# Your code here

Hint

Use the CONFIG SET slowlog-log-slower-than 10000 command to set the threshold in microseconds.

2

Set the slow log maximum length

Use the Redis command CONFIG SET slowlog-max-len 128 to set the maximum length of the slow log to 128 entries.

Redis

CONFIG SET slowlog-log-slower-than 10000
# Use the Redis CLI command to set slow log max length
# Your code here

Hint

Use CONFIG SET slowlog-max-len 128 to limit the slow log size.

3

Retrieve slow log entries

Use the Redis command SLOWLOG GET to retrieve the current slow log entries.

Redis

CONFIG SET slowlog-log-slower-than 10000
CONFIG SET slowlog-max-len 128
# Use the Redis CLI command to get slow log entries
# Your code here

Hint

Use SLOWLOG GET to see the slow commands recorded.

4

Clear the slow log

Use the Redis command SLOWLOG RESET to clear all slow log entries after analysis.

Redis

CONFIG SET slowlog-log-slower-than 10000
CONFIG SET slowlog-max-len 128
SLOWLOG GET
# Use the Redis CLI command to reset the slow log
# Your code here

Hint

Use SLOWLOG RESET to clear the slow log entries.

Practice

1. What is the primary purpose of the Redis slow log?

easy

A. To clear the Redis cache automatically

B. To store all commands executed in Redis

C. To record commands that take longer than a set time to execute

D. To backup Redis data periodically

Solution

1. Step 1: Understand what slow log tracks

   The slow log records commands that exceed a certain execution time threshold.

2. Step 2: Differentiate from other Redis features

   It does not store all commands, clear cache, or backup data; it only logs slow commands.

3. Final Answer:

   To record commands that take longer than a set time to execute -> Option C

4. Quick Check:

   Slow log = records slow commands [OK]

Hint: Slow log tracks only slow commands, not all commands [OK]

Common Mistakes:

• Thinking slow log stores all commands
• Confusing slow log with cache clearing
• Assuming slow log is for backups

2. Which Redis command correctly retrieves the slow log entries?

easy

A. SLOWLOG LIST

B. SLOWLOG FETCH

C. SLOWLOG SHOW

D. SLOWLOG GET

Solution

1. Step 1: Recall the Redis slow log retrieval command

   The official command to get slow log entries is SLOWLOG GET.

2. Step 2: Verify other options

   Commands like SLOWLOG FETCH, SLOWLOG SHOW, and SLOWLOG LIST do not exist in Redis.

3. Final Answer:

   SLOWLOG GET -> Option D

4. Quick Check:

   Retrieve slow log = SLOWLOG GET [OK]

Hint: Use SLOWLOG GET to view slow log entries [OK]

Common Mistakes:

• Using non-existent commands like SLOWLOG FETCH
• Confusing with LIST or SHOW commands
• Misspelling the command

3. Given the following Redis commands executed in order:

SLOWLOG RESET
SLOWLOG LEN
SLOWLOG GET 2

What will be the output of SLOWLOG LEN immediately after the reset?

medium

A. 0

B. 2

C. An error message

D. The last 2 slow log entries

Solution

1. Step 1: Understand SLOWLOG RESET effect

   The SLOWLOG RESET command clears all slow log entries, so the log becomes empty.

2. Step 2: Check SLOWLOG LEN after reset

   Since the log is empty, SLOWLOG LEN returns 0, indicating no entries.

3. Final Answer:

   0 -> Option A

4. Quick Check:

   Reset clears log, length = 0 [OK]

Hint: Reset clears log; length after reset is zero [OK]

Common Mistakes:

• Expecting old entries after reset
• Confusing SLOWLOG LEN with SLOWLOG GET output
• Thinking reset causes error

4. You run SLOWLOG GET but receive an empty list even though you expect slow commands. What is the most likely cause?

medium

A. The slow log threshold is set too high, so no commands qualify

B. The Redis server is down

C. You need to restart Redis to enable slow log

D. The slow log is full and cannot record more entries

Solution

1. Step 1: Understand slow log threshold role

   Redis only logs commands slower than the configured threshold (default 10ms). If threshold is too high, no commands get logged.

2. Step 2: Eliminate other causes

   Redis server down would cause connection errors, not empty logs. Restart is not needed to enable slow log. Slow log size is limited but overwrites old entries, so it won't be empty due to fullness.

3. Final Answer:

   The slow log threshold is set too high, so no commands qualify -> Option A

4. Quick Check:

   High threshold = no slow commands logged [OK]

Hint: Check slow log threshold if no entries appear [OK]

Common Mistakes:

• Assuming Redis must restart to enable slow log
• Thinking slow log fills up and stops logging
• Confusing server down with empty slow log

5. You want to analyze slow commands that took longer than 50 milliseconds and keep only the last 5 entries. Which sequence of Redis commands achieves this?

hard

A. CONFIG SET slowlog-log-slower-than 50
SLOWLOG RESET
SLOWLOG GET 5

B. CONFIG SET slowlog-max-len 5
CONFIG SET slowlog-log-slower-than 50000
SLOWLOG RESET
SLOWLOG GET 5

C. CONFIG SET slowlog-log-slower-than 50000
SLOWLOG RESET
SLOWLOG GET 5

D. CONFIG SET slowlog-max-len 5
CONFIG SET slowlog-log-slower-than 50
SLOWLOG RESET
SLOWLOG GET 5

Solution

1. Step 1: Set threshold to 50 milliseconds

   Redis uses microseconds, so 50 ms = 50000 μs. Use CONFIG SET slowlog-log-slower-than 50000. A and C correct; B and D use 50 μs (too low).

2. Step 2: Limit slow log length to 5 entries

   Use CONFIG SET slowlog-max-len 5. Only A and D do this; B and C do not.

3. Step 3: Reset slow log and get last 5 entries

   SLOWLOG RESET clears old entries; SLOWLOG GET 5 retrieves last 5. All options include this.

4. Step 4: Choose correct option

   Only A sets both max-len to 5 and threshold to 50000 μs (50 ms).

5. Final Answer:

   CONFIG SET slowlog-max-len 5 CONFIG SET slowlog-log-slower-than 50000 SLOWLOG RESET SLOWLOG GET 5 -> Option B

6. Quick Check:

   Threshold 50ms = 50000μs, max len 5, reset, get 5 [OK]

Hint: Set threshold in microseconds and max length before reset [OK]

Common Mistakes:

• Confusing milliseconds with microseconds in threshold
• Not setting max length to limit entries
• Forgetting to reset slow log before analysis