Replication lag monitoring in Redis - Step-by-Step Execution

Concept Flow - Replication lag monitoring

Master Redis Server

↓

Send replication stream

↓

Replica Redis Server

↓

Measure replication offset

↓

Calculate lag = Master offset - Replica offset

↓

Alert if lag > threshold

↓

Take action or monitor

This flow shows how Redis master sends data to replica, replica tracks offsets, and lag is calculated by comparing offsets to monitor replication delay.

Execution Sample

Redis

redis-cli info replication
redis-cli role
redis-cli --raw info replication | grep master_repl_offset
redis-cli --raw info replication | grep slave_repl_offset

These commands check replication info and offsets on master and replica to monitor replication lag.

Execution Table

Step	Command	Master Offset	Replica Offset	Lag Calculation	Action
1	redis-cli info replication on master	1200000	-	-	Get master offset
2	redis-cli info replication on replica	-	1199950	-	Get replica offset
3	Calculate lag	1200000	1199950	1200000 - 1199950 = 50	Lag = 50 bytes
4	Compare lag to threshold (e.g. 100)	1200000	1199950	50 < 100	No alert, replication healthy
5	Repeat monitoring periodically	Varies	Varies	Calculate lag each time	Continuous monitoring
6	If lag > threshold	Varies	Varies	Lag exceeds threshold	Trigger alert or action
7	Exit	-	-	-	Monitoring cycle ends or continues

💡 Monitoring stops or alerts when lag exceeds threshold or manual stop

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
master_offset	0	1200000	1200000	1200000	1200000	Varies
replica_offset	0	0	1199950	1199950	1199950	Varies
lag	0	0	0	50	50	Varies

Key Moments - 3 Insights

Why do we subtract replica offset from master offset to get lag?

Because master offset shows total data sent, and replica offset shows data received. Their difference is the delay in bytes, as shown in execution_table step 3.

What does it mean if lag is zero or very small?

It means replica is almost caught up with master, replication is healthy. See execution_table step 4 where lag < threshold means no alert.

Why do we monitor lag repeatedly instead of once?

Replication lag can change over time due to network or load. Continuous monitoring (step 5) helps detect issues early.

Visual Quiz - 3 Questions

Look at the execution table, what is the lag value calculated at step 3?

A 1200000

B 1199950

C 50

D 100

💡 Hint

Check the 'Lag Calculation' column in row for step 3.

At which step does the system decide no alert is needed because lag is below threshold?

A Step 2

B Step 4

C Step 6

D Step 1

💡 Hint

Look at the 'Action' column where lag is compared to threshold.

If the replica offset was 1199900 instead of 1199950, how would lag change at step 3?

A Lag would be 100

B Lag would be 50

C Lag would be 0

D Lag would be negative

💡 Hint

Lag = master offset - replica offset; check variable_tracker values.

Concept Snapshot

Redis replication lag monitoring:
- Use 'info replication' to get master and replica offsets.
- Lag = master_repl_offset - replica_repl_offset.
- Monitor lag regularly.
- Alert if lag exceeds threshold.
- Helps ensure replica is up-to-date.

Full Transcript

Replication lag monitoring in Redis involves checking the data offsets on the master and replica servers. The master offset shows how much data has been sent, and the replica offset shows how much data has been received. By subtracting the replica offset from the master offset, we get the lag, which tells us how far behind the replica is. Commands like 'redis-cli info replication' help retrieve these offsets. Monitoring this lag regularly helps detect delays in replication. If the lag exceeds a set threshold, alerts can be triggered to take action. This process ensures the replica stays synchronized with the master.