System designHardsd-g296

Subject MvccLevel Senior–Staff~50 minCommon in Distributed systems interviewsIndustries Technology, Software development

Question

An MVCC database (think Postgres-style) is accumulating dead row versions: tables and indexes keep growing far beyond the live data, sequential scans get slower, and the autovacuum/GC process can't keep up. Investigation shows occasional long-running read transactions (analytics queries, a stuck connection holding a snapshot) that run for tens of minutes. Design the version garbage-collection / vacuum subsystem and explain precisely why long-running readers cause unbounded bloat and how you bound it without breaking snapshot reads.

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.