Replication Lag: Concepts, Trade-offs & Failure Modes

Replication lag is the delay between when data is written to the primary and when it's available on replicas.

What is Replication Lag?

Definition: Time difference between primary write and replica update.

Causes:

• Network latency
• Replica processing time
• High write load
• Network congestion

Impact

Stale reads: Reading from replica may return old data

Inconsistency: Different replicas may have different data

User experience: Users may not see their own writes immediately

Measuring Lag

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Handling Lag

Read Your Writes

Route user's reads to primary after their writes.

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Monotonic Reads

Ensure user always sees newer data (not older).

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Examples

Database Replication

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Common Pitfalls

• Ignoring lag: Users see stale data. Fix: Monitor and handle lag
• Not routing critical reads: Important reads go to stale replica. Fix: Route to primary
• No lag monitoring: Don't know when lag is high. Fix: Monitor continuously
• Assuming zero lag: Replicas always have some lag. Fix: Design for lag

Interview Questions

Beginner

Q: What is replication lag and why does it matter?

A: Replication lag is the delay between when data is written to the primary database and when it's available on replicas.

Why it matters:

• Stale reads: Reading from replica may return old data
• Inconsistency: Users may not see their own writes
• User experience: Confusing when data doesn't appear immediately

Example: User updates profile, but when they refresh, old data appears (read from replica that hasn't updated yet).

Intermediate

Q: How do you handle replication lag in a read replica setup?

A:

Strategies:

1. Read your writes: Route user's reads to primary after their writes
2. Monotonic reads: Ensure user always sees newer data
3. Lag-aware routing: Route critical reads to primary if lag is high
4. Wait for replication: Wait for replica to catch up before reading

Implementation:

• Track user's last write time
• If recent write, read from primary
• Otherwise, read from replica
• Monitor lag and adjust routing

Senior

Q: Design a system that handles replication lag for a social media platform. Users post content that must be visible to followers. How do you ensure consistency while maintaining performance?

A:

Design:

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Optimizations:

• Caching: Cache recent posts to reduce replica load
• Fan-out: Write to follower timelines on post (not on read)
• Eventual consistency: Accept that followers may see posts slightly later