ArchitectureKit

Two places where event-sourced applications rely on luck.

Two concurrent writes to the same thing on the way in, and a read right after a write on the way out. ArchitectureKit handles both, and it covers both sides of an application – commands, events, and state for writing, projections, views, and queries for reading.

• Every command writes with its preconditions. The database rejects duplicates and concurrent changes, not your luck.
• A read can wait for your own write. Views track the revision they have seen, and a query waits until it is reached.
• Tests need no database. A test package checks deciders, projections, and queries without a database.

var acquireBook = architecturekit.Decider[AcquireBook, Book]{
  State: bookState,
  Decide: func(ctx context.Context, cmd AcquireBook, book Book) ([]architecturekit.Event, error) {
    if book.IsAcquired {
      return nil, architecturekit.NewDomainError("book %s has already been acquired", cmd.BookID)
    }

    return []architecturekit.Event{
      BookAcquired{Title: cmd.Title, Author: cmd.Author, ISBN: cmd.ISBN},
    }, nil
  },
}

A decider receives a command and the current state, and returns the events to write. Execute reads the events, evolves the state, decides, and writes.