Scaling Write-Heavy Transaction Databases

Context

As transaction volume grows in products like Revolut Card payments, bank transfers, and ledger updates, a single PostgreSQL instance can become a bottleneck on writes. Interviewers ask this to test whether you can separate techniques that improve read scale, write scale, and operational manageability.

Core question

Explain how replication, sharding, and partitioning help scale a database under heavy write loads. You should compare what each technique actually improves, where it does not help, and what trade-offs it introduces around consistency, routing, rebalancing, and operational complexity.

Scope guidance

A strong answer should distinguish vertical vs horizontal scaling, explain why replication alone usually does not solve a primary write bottleneck in PostgreSQL, show how partitioning can reduce index and maintenance overhead on large write-heavy tables, and describe when sharding is the real write-scaling step. You should also mention implications for transactional correctness and data integrity in financial systems.