Flow is an event-sourcing library for Elixir. Instead of storing state directly, a Flow-based service stores the sequence of events that produced it. Any value you need — the current capacity of a course, whether it is full, whether a subscription is allowed — is derived by reading the relevant events and folding them into a result.
Flow is built on the concept of Dynamic Consistency Boundaries (DCB). Rather than committing to static aggregates that fix the transactional scope up front, each read or write declares which events it depends on, and that declaration becomes its consistency boundary. Two operations touching disjoint sets of events never conflict; two touching the same events are ordered by optimistic concurrency. The boundary is shaped by the code that needs it, not by a pre-committed data model.
Flow is built from a small number of pieces, each covered on its own page.