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Consistency and coordination

Module 8 of the System Design Masterclass. Every lesson here is free to read, with no account.

  • 7 lessons
  • 2 h 24 min of reading
  1. 47CAP and consistency levels: what you give up when the network breaksThe levels of consistency from strong to eventual, what a network partition is, what the CAP theorem really says, and the cost in milliseconds that PACELC adds for normal times.24 minCore Fast track
  2. 48Agreeing with the majority (quorums)How a write and a read each ask some of the copies, why R + W > N makes a read meet the newest write, and how to choose R and W for fast reads, fast writes or failures.12 minDepth
  3. 49Whose clock is right? Putting events in orderWhy two servers never agree on the time, why sorting by timestamp can put a reply before its question, and how per-key sequence numbers and logical clocks give a safe order.24 minCore
  4. 50When many places accept writes: conflictsWhat happens when more than one server accepts writes for the same data: how a conflict is found, and four ways to settle it, from last write wins to merge rules that always work.24 minSenior
  5. 51Only one worker at a time: locks, leases, fencing tokensWhy a lock shared by many servers is hard to make safe: the lease that ends while its holder is paused, clocks and failed lock servers, and the fencing token that lets the data reject a stale holder.22 minSenior
  6. 52Leader election and consensus in plain wordsHow a group of servers chooses one leader for each term: the Raft idea of terms, votes and a majority, why an old leader cannot save changes, what it costs, and when to use a ZooKeeper-style tool instead of writing it yourself.12 minSenior
  7. 53Case study: Distributed cache and key-value storeDesign a store that keeps keys on many servers: place keys on a ring, keep three copies, use quorums, store data on each server so that writes are fast, and see what changes when it is only a cache.26 minDepth · Case study

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