Glossary

The Raft RPC that carries new log entries from the leader to followers.

A transaction either commits entirely or has no effect at all — there is no partial result.

Services publish and subscribe to events; each participant knows only its own next step, no central coordinator.

Bertrand Meyer's 1988 principle: a method either changes state or returns a result, never both.

The business operation that undoes the effect of an earlier saga step (e.g., refund a charge).

A group of nodes agreeing on a single value despite failures and dropped messages.

Command Query Responsibility Segregation — separating the model that handles writes from the model that handles reads.

Create / Read / Update / Delete — the basic operations of most simple data-driven applications.

A queue for messages or operations that cannot be processed and require human intervention.

A read that returns the intermediate state of an in-flight transaction, which may be rolled back.

Storing state as a log of immutable events instead of a current snapshot; current state is computed from the log.

The read model converges to match the write model over time, never instantly.

A periodic message from the leader to followers signalling that the leader is still alive.

An operation that has the same effect whether called once or many times.

A value or record that cannot be changed after it is created; updates create new records instead.

The transactional property that in-flight transactions don't see each other's intermediate state.

Discarding old entries that have been captured in a snapshot, to keep log size bounded.

Adding or removing nodes from a Raft cluster safely, without violating consensus guarantees.

A change that cannot be undone (e.g., sending an email, shipping a package).

A single coordinator service drives the saga, calling each step and walking compensations on failure.

The original distributed consensus algorithm (Lamport, 1989). Correct but notoriously hard to implement — every production system using Paxos has its own subtle variant. Named after a Greek island where Lamport set a fictional parliamentary procedure as a teaching device.

A function that walks every event in order and accumulates a derived state used for reads.

The minimum number of nodes (usually a majority) that must agree before a decision is committed.

A distributed consensus algorithm (Ongaro & Ousterhout, Stanford, 2014), designed to be more understandable than Paxos. Not an acronym — the name was chosen as a humble visual metaphor (a simple floating craft that survives storms), deliberately contrasting Paxos's Greek-island academic vibe. The popular backronym "Reliable, Replicated, Redundant, And Fault-Tolerant" came after the name was chosen.

The guarantee that a user reads back the state they just wrote, even if the broader system is eventually consistent.

A sequence of local transactions, each paired with a compensating action that semantically reverses it on failure.

A flag on a record indicating it is participating in an in-flight saga, blocking other transactions from reading or modifying it.

A committed checkpoint of derived state used to avoid replaying the full event log on every read.

A partition in which two halves of a cluster both elect leaders and accept divergent writes.

A logical period that acts like Raft's clock; each term has at most one leader.

A distributed transaction protocol where a coordinator asks all participants to prepare, then commit (or abort) atomically.

Code that translates older versions of stored events into the current schema shape during replay.