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.