Table of Contents

Sample 2 — Event Sourced

Concept: Event-sourced aggregate + projection (read/write separation) — what changes when you replace the in-memory repository with an event store.

What you'll see

  1. Domain events — AccountOpened, AmountDeposited, AmountWithdrawn as immutable sealed records.
  2. Account aggregate with public set on properties (snapshot-deserialize-friendly) and Apply(…) methods that apply each event type.
  3. InMemoryEventStore — the sample's stand-in for Stratara.EventSourcing.EntityFrameworkCore. Same semantics, simpler dependencies.
  4. AccountBalanceProjection — a read-model that re-builds itself from the event stream.
  5. Write-side invariant check — Withdraw $999 rejection is computed by rebuilding the aggregate from events to compute the current balance, not by reading the projection.

Running

dotnet run --project samples/Stratara.Sample.EventSourced

Expected output (abridged):

=== Stratara Event-Sourced ===

--- Open account ---
  Opened {guid} with $100

--- Deposit $50 + Deposit $25 + Withdraw $40 ---

--- Read-side via projection ---
  Alice's balance: $135.00

--- Withdraw $999 (should fail — write-side rebuilds aggregate from events to check invariant) ---
  Rejected: Account {guid} has balance $135.00; cannot withdraw $999.00.

--- Underlying event stream (this is what's persisted) ---
  AccountOpened { … }
  AmountDeposited { … Amount = 50 … }
  AmountDeposited { … Amount = 25 … }
  AmountWithdrawn { … Amount = 40 … }

What changed vs. Sample 1

Sample 1 (CRUD) Sample 2 (Event Sourced)
account.Deposit(50) mutates in memory Appends AmountDeposited(50) to the stream
repo.Get(id).Balance reads current state directly projection.Get(id).Balance reads a derived read-model
Invariant check reads account.Balance Invariant check rebuilds the aggregate from all events first
1 source of truth (the Account object) 1 source of truth (the event stream); aggregate + projection both derive from it

What's missing (covered by later samples)

  • No async dispatch — the event store is consumed synchronously inside the command handler. Async fan-out to projections via the outbox is Sample 3.
  • No saga — there's no second command issued in response to the first. Sample 4 shows fan-out via a saga.