# Alchemy Domain models a business as commands, aggregates, and events

> For agents: start with the [agent guide](https://ratstack.sh/llms.txt). Every page is Markdown by default; add `Accept: text/html` for HTML.

New on /news: [effect-discord-perms](/news).

Alchemy Domain is the `@alchemy.run/domain` package, shipped with [Alchemy](/skills/learn-alchemy) `2.0.0-beta.85`.
The [Domain docs](https://alchemy.run/domain/) call it a domain-driven design framework for [Effect](/lore/effect-basics).
It models a business in its own terms: commands, aggregates, events, views, projections, and policies.

## Commands, aggregates, and events

A command asks for a change.
An aggregate decides whether the change happens, using its current state.
An event records what happened.
Views, projections, and policies then answer reads, feed other stores, and react to events.

## The example: deposit money into an account

The [Domain docs](https://alchemy.run/domain/) build a bank, and its first example deposits money into an account.

The [`DepositMoney` command](https://github.com/alchemy-run/alchemy/blob/d0b4a3326718309793459160e94b35abee517b23/website/src/content/docs/domain/index.mdx#L32-L45) names its target, an account id.
It takes an amount as input.

It declares the one event it can record, `MoneyDeposited`, with the amount and the balance after.
It declares the one way it can be refused, `NotOpen`.
It replies with the new balance.

The [account aggregate](https://github.com/alchemy-run/alchemy/blob/d0b4a3326718309793459160e94b35abee517b23/website/src/content/docs/domain/index.mdx#L49-L54) decides the command against the account's state.
A closed account refuses it with `NotOpen`.
An open account records `MoneyDeposited` with the balance plus the amount.

A [story](https://github.com/alchemy-run/alchemy/blob/d0b4a3326718309793459160e94b35abee517b23/website/src/content/docs/domain/index.mdx#L58-L67) tests it in the business's own words.
Given an account opened for an owner, when 100 is deposited, then `MoneyDeposited` records 100 with a balance of 100.
The reply is a balance of 100.

The links point at the docs page at the commit tagged `v2.0.0-beta.85`, so the example matches the version rat-stack pins.

## One model, two places to run it

The same model runs in memory for tests and on Cloudflare Durable Objects in production, with one ordered partition per key.
[Story](https://alchemy.run/domain/story/) runs the real model in memory, so a story describes what the model does.
On [Cloudflare](https://alchemy.run/domain/cloudflare/), every aggregate instance, view key, and policy gets its own Durable Object, and the app declares none of them.
Swapping `DomainCloudflare.layerDurableObjects()` for `Domain.InMemory` changes nothing else in the program.

## Where it meets rat-stack

rat-stack pins Alchemy `2.0.0-beta.85`, the release that ships this package.
rat-stack's own Workers do not run on Alchemy Domain yet.
An optional `@alchemy.run/domain/Foldkit` [adapter](/lore/hexagonal-architecture) connects a Domain Api to a Foldkit app, as the [Foldkit chapter](https://github.com/alchemy-run/alchemy/blob/d0b4a3326718309793459160e94b35abee517b23/website/src/content/docs/domain/foldkit.mdx) describes.

## Sources

1. [Domain](https://alchemy.run/domain/)
   Alchemy. What the package models, where it runs, and the bank account example. Accessed 2026-11-06.

2. [Cloudflare Durable Objects](https://alchemy.run/domain/cloudflare/)
   Alchemy. Hosting a model on Durable Objects behind a Worker. Accessed 2026-11-06.

3. [Story](https://alchemy.run/domain/story/)
   Alchemy. Given, when, then tests that run the real model in memory. Accessed 2026-11-06.

4. [packages/domain at v2.0.0-beta.85](https://github.com/alchemy-run/alchemy/tree/d0b4a3326718309793459160e94b35abee517b23/packages/domain)
   alchemy-run/alchemy. Source of @alchemy.run/domain at the Alchemy version rat-stack pins. Accessed 2026-11-06.

## Lore on this page

- [CAFE names the Cloudflare, Alchemy, Foldkit and Effect stack](/lore/cafe-stack)

## Linked from

- Agent guide → An Effect stack so pure (aspirational) Kit Langton will blush. → [Read page](https://ratstack.sh/llms.txt)

- Change log → What changed in the files served here, newest first. → [Read page](https://ratstack.sh/log)

- Full agent guide → An Effect stack so pure (aspirational) Kit Langton will blush. → [Read page](https://ratstack.sh/llms-full.txt)

- Rat Stack lore | rat-stack → Short, source-grounded notes on the ideas and decisions behind rat-stack. → [Read page](https://ratstack.sh/lore)

- Source change history (log.md) → The generated change log as Markdown. → [Read page](https://ratstack.sh/log.md)