System designMediumsd-g672

Subject Database connection pool proxyLevel Mid–Senior~35 minCommon in Databases & SQL interviewsIndustries Technology, Software development

Question

Design a connection-pool / proxy layer for a Postgres primary that can only handle ~500 concurrent backends, but sits behind 2,000 stateless app instances each wanting its own pool, spiking under load. Symptoms: connection exhaustion, memory pressure on the DB, and a thundering herd of reconnects after a failover. How do you pool connections, multiplex many clients onto few DB connections, and survive failover?

What a strong answer looks like

Clarify scale and constraints first. Propose a clean component breakdown, then go deep on the hard parts — data model, bottlenecks, consistency, failure modes — and name the trade-offs you are making.

Learn the concepts

Narrate your design

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Run or narrate your approach, then ask the coach.