Distributed Systems · advanced
Consensus
Raft, Paxos, leader election.
Mental model
Consensus is how a cluster agrees on one value despite crashes and network delays. Raft makes it understandable: elect a leader, replicate an append-only log, and a write commits once a majority (quorum) has it. A majority guarantees any two quorums overlap.
How to study Consensus
Begin by restating the mental model in your own words, then connect it to a concrete system you have built or operated. Name the mechanism, the constraint it addresses, and the trade-off it introduces. Use The Raft Consensus Algorithm to check details, but close the source before writing your explanation. Retrieval is the learning step; rereading is only preparation.
Next, compare Consensus with CAP & Consistency Models. Ask what changes in correctness, latency, resource use, operability, and failure recovery. Complete Leader election walkthrough and preserve the command, input, output, and one failed attempt as evidence. Finish by explaining the idea without jargon to someone who has not studied the track.
Proof of understanding
- Explain the mechanism from first principles and identify the state it reads or changes.
- Give one situation where the concept is the right choice and one where it is not.
- Predict a realistic failure mode before running the drill, then compare the prediction with evidence.
- Connect the result to a roadmap or build artifact instead of treating the concept as isolated trivia.
Common mistakes
- Assuming consensus survives a network partition without a majority
- Confusing leader election with data replication — Raft does both
- Thinking more replicas always means more durability (it costs latency)
Learn from primary sources
Practice and explain it back
Leader election walkthrough
Walk through Raft leader election: timeouts, candidacy, vote granting, split votes. Trace what happens when the current leader is partitioned from a majority.
Expected evidence: A 3-node trace showing one election cycle, including a split-vote retry.
Open the interactive drill →Review prompts
- Why does a majority quorum give consensus its safety property?
Build evidence
Synthesize: Distributed Systems
Reason about coordination, data placement, logs, durable workflows, consistency, and recovery under partial failure. Produce one working system, benchmark, or evidence-backed design that integrates the path.
- Implements or precisely models the core mechanisms from all three milestones
- Includes at least one injected failure or adversarial case and demonstrates recovery
- Reports quality, latency, resource, reliability, or usability measurements relevant to the domain
- Ships a concise architecture note explaining decisions, trade-offs, and remaining risks