TL;DR — Database Replication Lag

You run a social platform with 20M DAU. The database architecture is one PostgreSQL primary in us-east-1 handling all writes, and 4 read replicas (2 in us-east-1, 2 in eu-west-1) handling reads. The application routes all SELECT queries to replicas using a connection proxy. Replication is asynchronous streaming replication. Key features: Users always see their own latest writes immediately. Other users can see updates with a small delay (eventual consistency is fine).