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Interview Cheatsheet — Network Partitions#

When does partition come up in an interview?

The moment you add a second node. Any distributed system will partition. The interviewer will ask "what happens during a network failure?" — this cheatsheet is your answer.

The One-Line Definition#

"A network partition is when nodes are alive but can't communicate. Not a crash — both sides are running. The question is what each side does while isolated."

Moment 1 — "What happens if two nodes can't communicate?"#

"During a partition each isolated node must choose — serve potentially stale data or refuse requests entirely. For a payment system I'd refuse — wrong balance could cause financial loss. For a social feed I'd serve stale — slightly old content is better than an error page. This tradeoff is exactly what CAP theorem formalizes."

Moment 2 — "How do you prevent split-brain?"#

"Quorum — a node only acts as primary if it can reach a majority of nodes. With 3 nodes, quorum is 2. If a partition isolates 1 node from 2, the group of 1 steps down — can't reach quorum. The group of 2 maintains quorum and keeps serving. Only one primary at a time — no conflicting writes, no split-brain."

Moment 3 — "Why odd number of nodes?"#

"Even numbers create deadlock scenarios. With 4 nodes a 2-2 partition means neither group has quorum — both stop, entire system unavailable. With 3 or 5 nodes, one group always has a clear majority. Odd numbers guarantee one group always wins during any partition."

Moment 4 — "What's the difference between quorum and consensus?"#

"Quorum is just a number — the minimum nodes that must agree. R + W > N for read/write safety. Consensus is the full protocol — how leaders are elected, how writes are ordered, how split-brain is prevented. Consensus algorithms like Raft use quorum internally as one of their rules."

Serve vs Refuse Decision Table#

System During Partition Reason
Social feed, recommendations Serve stale Staleness harmless
Shopping cart Serve stale Availability > consistency
Bank balance Refuse Wrong balance = financial loss
Payment processing Refuse Double charge unacceptable
Hotel/seat booking Refuse Double booking = broken trust
Inventory Refuse Overselling = serious problem

Full Checklist#

  • [ ] Defined partition correctly — nodes alive, can't communicate (not a crash)
  • [ ] Mentioned partitions are inevitable — not a design choice
  • [ ] Explained the serve vs refuse decision — system-dependent
  • [ ] Mentioned split-brain — two primaries, conflicting writes
  • [ ] Explained quorum solution — majority must agree, minority steps down
  • [ ] Justified odd number of nodes — even numbers can deadlock
  • [ ] Distinguished quorum (number) from consensus (process)

Quick Reference#

Partition:
  Nodes alive, cannot communicate
  Inevitable in any distributed system
  Each node must: serve stale OR refuse

Split-brain:
  Both nodes think they're primary
  Both accept writes → conflicting data
  Fix: quorum — only majority group continues

Quorum:
  Formula: floor(N/2) + 1
  3 nodes → 2, 5 nodes → 3, 7 nodes → 4
  R + W > N → guaranteed to see latest write
  Always odd numbers → one group always wins

Quorum vs Consensus:
  Quorum    → minimum nodes that must agree (a number)
  Consensus → full protocol — Raft, Paxos (a process)
  Consensus uses quorum internally