Distributed systems

When the tasks are on different machines: partial failure, no shared clock, and the protocols that reach agreement anyway.

All categories · How they connect

• Distributed computing — Tasks on separate machines that communicate over a network: their events are only partially ordered, and one part can fail while the rest keeps running.
• Partial failure — In a distributed system one component can fail while the others continue, and a caller often cannot tell a slow peer from a dead one.
• Clock skew and drift — Skew is how far apart two machines' clocks are at one moment, drift how fast they move apart — the reason timestamps from different machines cannot reliably order events.
• Logical clocks — Counters that order events by cause instead of by wall-clock time: a Lamport clock gives every event a consistent order, a vector clock can also tell that two events were concurrent.
• Consensus — Getting a group of machines to agree on one value, or one log of commands, even though some of them fail — the problem Paxos and Raft solve.
• Idempotency — An operation that has the same effect whether it runs once or several times, which is what makes retrying after a timeout safe.
• Consistency models — The promise a distributed store makes about what a reader will see — from strong consistency, where every read sees the latest write, to eventual consistency, where replicas agree only in the end.
• Multi-version concurrency control — Keeping several versions of each record so that readers see a consistent snapshot while writers add new versions, instead of readers and writers locking each other out.
• Remote procedure call — Calling a function that runs on another machine as though it were local — convenient until the network fails in a way no local call can.
• Durable execution — Recording a long-running workflow's progress — checkpoints, an event log — so that after a crash it resumes where it stopped instead of starting over.

Inside this category

flowchart LR
  n_clock_skew["Clock skew and drift"]
  n_durable_execution["Durable execution"]
  n_idempotency["Idempotency"]
  n_logical_clocks["Logical clocks"]
  n_clock_skew ---|or| n_logical_clocks
  n_durable_execution -->|uses| n_idempotency