What is Read-Replica Lag?

Read-replica lag is the delay between when a primary database processes a write and when that update is visible on its read replicas, sometimes causing replicas to serve stale data.

When to Use

Read replicas are used to handle read-heavy workloads, such as analytics dashboards, reporting tools, or news feeds, where slightly stale data is acceptable and improves scalability.

Example

If you add a comment on a post and instantly refresh a feed powered by a read replica, you might not see your comment right away due to lag.

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Why Is It Important

Read-replica lag matters because it impacts data consistency. High lag can confuse users, as they may see outdated values after making changes.

Interview Tips

In interviews, define read-replica lag clearly, explain the consistency trade-off, and mention solutions like using the primary database for critical reads or monitoring lag metrics.

Trade-offs

You gain performance and scalability but lose immediate consistency. This trade-off is central to many system design discussions.

Pitfalls

Common mistakes include assuming replicas are always up to date or not setting up monitoring/alerts for lag, which can lead to bugs in production.