Distributed Systems · advanced
Replication
Leader-follower, multi-leader, quorum.
Mental model
Replication keeps copies of data on multiple nodes for durability and read scaling. Single-leader is simple but the leader is a write bottleneck; multi-leader and leaderless trade simplicity for availability. The core tension is replication lag — followers are always slightly behind.
How to study Replication
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 Designing Data-Intensive Applications (Kleppmann) — book site, In Search of an Understandable Consensus Algorithm (Raft, extended), Chain Replication for Supporting High Throughput and Availability (OSDI 04) to check details, but close the source before writing your explanation. Retrieval is the learning step; rereading is only preparation.
Next, compare Replication with CAP & Consistency Models, Consensus. Ask what changes in correctness, latency, resource use, operability, and failure recovery. Complete Read-your-writes under lag 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
- Reading from a follower and seeing stale data ("read your own writes" violation)
- Assuming failover is instant and lossless
- Ignoring replication lag in capacity and correctness planning
Learn from primary sources
Practice and explain it back
Read-your-writes under lag
Write to primary, read replica 200ms later, lag 500ms. User sees stale read. Two fixes?
Expected evidence: Read-after-write from primary; session stickiness; sync quorum for critical reads.
Open the interactive drill →Review prompts
- A user updates their profile and the next page shows the old value. Name the guarantee that is missing and two ways to provide it.
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
Prerequisites
None assigned yet.