Replication lag monitoring in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Check replication lag using Redis INFO command

You run the command INFO replication on a Redis replica. Which field indicates the current replication lag in seconds?

Redis

INFO replication

A master_last_io_seconds_ago

B slave_repl_offset

C master_link_down_since_seconds

D master_repl_offset

💡 Hint

Look for a field that shows how long ago the last interaction with the master happened.

The master_last_io_seconds_ago field shows how many seconds ago the replica last communicated with the master, which reflects replication lag.

🧠 Conceptual

intermediate

Understanding replication lag causes

Which of the following is the most common cause of replication lag in Redis?

A Replica running out of disk space

B Replica configured with a different Redis version

C Master CPU overload causing slow command processing

D Network latency between master and replica

💡 Hint

Think about what delays the data transfer from master to replica.

Network latency is the most common cause of replication lag because it delays the data sent from master to replica.

❓ Troubleshoot

advanced

Diagnose replication lag from Redis logs

You see the following log entry on a Redis replica: "SYNC failed: OOM command not allowed when used memory > 'maxmemory'." What is the most likely cause of replication lag?

A Replica is out of memory and cannot process replication commands

B Master is down and not sending data

C Network partition between master and replica

D Replica has a corrupted RDB file

💡 Hint

Look at the error about memory usage.

The error indicates the replica exceeded its memory limit and cannot accept replication commands, causing lag.

🔀 Workflow

advanced

Steps to monitor replication lag continuously

Which sequence of steps correctly describes how to monitor Redis replication lag continuously using built-in commands?

A 1,3,2,4

B 1,2,3,4

C 2,1,3,4

D 1,2,4,3

💡 Hint

Think about the logical order: get data, extract value, alert, then log.

The correct order is to run the command, extract the lag value, alert if needed, and log for history.

✅ Best Practice

expert

Choosing the best metric for replication lag alerting

Which Redis metric is the most reliable for alerting on replication lag to avoid false positives caused by temporary network glitches?

A master_last_io_seconds_ago

B master_link_down_since_seconds

C slave_repl_offset difference from master_repl_offset

D instantaneous_input_kbps on replica

💡 Hint

Consider a metric that measures actual data delay rather than connection status.

The difference between slave_repl_offset and master_repl_offset shows how far behind the replica is in bytes, which is a precise lag measure.

Practice

1. What does the master_last_io_seconds_ago field in INFO replication indicate in Redis?

easy

A. The number of seconds since the master last communicated with the replica

B. The total number of connected replicas

C. The current memory usage of the master

D. The replication backlog size in bytes

Solution

1. Step 1: Understand the INFO replication output

   This command shows replication details including communication status between master and replicas.

2. Step 2: Interpret master_last_io_seconds_ago

   This field tells how many seconds ago the master last sent data to the replica, indicating replication lag.

3. Final Answer:

   The number of seconds since the master last communicated with the replica -> Option A

4. Quick Check:

   Replication lag = master_last_io_seconds_ago [OK]

Hint: Lag time is how long since last master-replica communication [OK]

Common Mistakes:

• Confusing lag with number of replicas
• Mixing memory usage with replication info
• Thinking backlog size shows lag

2. Which Redis command correctly shows replication lag details?

easy

A. SHOW replication

B. INFO replication

C. MONITOR replication

D. REPLICA STATUS

Solution

1. Step 1: Recall Redis commands for replication info

   The INFO command with the replication section shows replication status and lag.

2. Step 2: Verify command correctness

   MONITOR shows all commands, SHOW and REPLICA STATUS are invalid commands.

3. Final Answer:

   INFO replication -> Option B

4. Quick Check:

   Correct command = INFO replication [OK]

Hint: Use INFO replication to check lag, not monitor or show [OK]

Common Mistakes:

• Using MONITOR instead of INFO
• Trying non-existent commands
• Confusing SHOW with INFO

3. Given this INFO replication snippet:

master_link_status:up
master_last_io_seconds_ago:5

What does the master_last_io_seconds_ago value tell you?

medium

A. The replica is 5 seconds behind the master

B. The master has been down for 5 seconds

C. The master last sent data 5 seconds ago

D. There are 5 replicas connected

Solution

1. Step 1: Analyze master_link_status and master_last_io_seconds_ago

   The status 'up' means the master and replica are connected. The number 5 means 5 seconds since last data sent.

2. Step 2: Interpret the lag meaning

   The value shows how many seconds ago the master last communicated, not the exact lag or downtime.

3. Final Answer:

   The master last sent data 5 seconds ago -> Option C

4. Quick Check:

   master_last_io_seconds_ago = last data sent seconds [OK]

Hint: Lag means last data sent time, not downtime or replica count [OK]

Common Mistakes:

• Confusing lag with downtime
• Assuming number means replicas count
• Thinking it shows replica delay exactly

4. You run INFO replication and see master_link_status:down and master_last_io_seconds_ago:1000. What is the best first step to fix replication lag?

medium

A. Restart the Redis master server immediately

B. Flush all data on the replica

C. Increase the replication backlog size

D. Check network connectivity between master and replica

Solution

1. Step 1: Understand the meaning of master_link_status:down

   This means the replica lost connection to the master, causing lag.

2. Step 2: Identify the cause and fix

   Network issues are the common cause; checking connectivity is the first step before restarting or changing configs.

3. Final Answer:

   Check network connectivity between master and replica -> Option D

4. Quick Check:

   Link down means network check first [OK]

Hint: Link down? Check network before restarting or flushing [OK]

Common Mistakes:

• Restarting master without checking network
• Changing backlog size without cause
• Flushing replica data unnecessarily

5. You want to alert if replication lag exceeds 10 seconds. Which approach using INFO replication is best?

hard

A. Parse master_last_io_seconds_ago and trigger alert if > 10

B. Check connected_slaves count and alert if less than 1

C. Monitor master_link_status only for 'down' status

D. Use INFO memory to check memory usage spikes

Solution

1. Step 1: Identify the lag metric

   master_last_io_seconds_ago directly shows how many seconds since last master-replica communication, ideal for lag monitoring.

2. Step 2: Set alert condition

   Trigger alert when this value exceeds 10 seconds to catch lag early.

3. Final Answer:

   Parse master_last_io_seconds_ago and trigger alert if > 10 -> Option A

4. Quick Check:

   Lag alert = master_last_io_seconds_ago > threshold [OK]

Hint: Alert on master_last_io_seconds_ago > threshold seconds [OK]

Common Mistakes:

• Using connected_slaves count for lag
• Only watching link status without lag value
• Checking memory instead of replication lag