Read‑your‑writes on replicas: PostgreSQL WAIT FOR LSN and MongoDB Causal Consistency

In databases designed for high availability and scalability, secondary nodes can fall behind the primary. Typically, a quorum of nodes is updated synchronously to guarantee durability while maintaining availability, while remaining standby instances are eventually consistent to handle partial failures. To balance availability with performance, synchronous replicas acknowledge a write only when it is durable and recoverable, even if it is not yet readable. As a result, if your application writes data and then immediately queries another node, it may still see stale data. Here’s a common anomaly: you commit an order on the primary and then try to retrieve it from a reporting system. The order is missing because the read replica has not yet applied the write. PostgreSQL and MongoDB tackle this problem in different ways: PostgreSQL 19 introduces a WAIT FOR LSN command, allowing applications to explicitly coordinate reads after writes. MongoDB provides causal consistency within sessions usi