Hello Ledger Event

Goal

Append one event to the runtime ledger and read it back as durable truth.

What You Build

A local in-memory runtime proof that writes tutorial.hello.recorded and reads the same event through Ledger.events.

Prerequisites

• Durable truth
• Runtime package
• In-memory backend

Steps

1. Create an in-memory backend state and runtime layer:

   import { Effect } from "effect";
   import { Ledger } from "@agent-os/runtime";
   import { InMemoryBackendState, InMemoryLedgerLive } from "@agent-os/backend-in-memory";
   
   const scope = "tutorial:hello";
   const state = new InMemoryBackendState();
   const ledgerLayer = InMemoryLedgerLive(state);

2. Append one domain event:

   const program = Effect.gen(function* () {
     const ledger = yield* Ledger;
     yield* ledger.commit([
       { kind: "tutorial.hello.recorded", payload: { message: "hello" }, scope },
     ]);
     return yield* ledger.events(scope);
   });

3. Run the proof:

   const events = await Effect.runPromise(program.pipe(Effect.provide(ledgerLayer)));

4. Treat the returned rows as facts. Do not copy them into another mutable “current state” object.

Checkpoint

The readback contains exactly the event you appended:

events.map((event) => event.kind); // ["tutorial.hello.recorded"]
events[0]?.payload; // { message: "hello" }

The event is the source of truth. Any UI state or projection must be derived by folding the event list.

Next

Let an LLM call a tool with weather tool LLM loop.