FREE Systems LESSON · Systems
Coordinate without shared certainty
Replication, consensus, time, and consistency
Distributed coordination exists because messages can be delayed, duplicated, reordered, or lost while machines fail independently.
Replication creates multiple copies but also disagreement. Consensus protocols establish one ordered history among a changing set of participants under a stated fault model; quorums connect reads and writes through intersecting voters; logical clocks capture causal information when wall clocks cannot. The user-facing consistency contract determines which coordination cost is necessary.
Start with the anomaly the product must prevent, then buy only the coordination needed to prevent it.
Consensus does not make an unavailable dependency available.
A healthy leader can still wait on storage, exhaust connections, or serve an operation whose downstream effect is not transactional with the log. Membership changes and split-brain recovery add their own hazards. Trace the complete acknowledgement boundary and distinguish agreement on intent from successful external execution.
Agreement is one mechanism inside a larger state transition.