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

Alchemy Domain is the @alchemy.run/domain package, shipped with Alchemy 2.0.0-beta.85. The Domain docs call it a domain-driven design framework for Effect³⁵. 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 build a bank, and its first example deposits money into an account.¶

The DepositMoney command 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 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 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 runs the real model in memory, so a story describes what the model does. On Cloudflare, every aggregate instance, view key, and policy gets its own Durable Object⁶⁰, 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 adapter³⁰ connects a Domain Api to a Foldkit²⁷ app, as the Foldkit chapter describes.¶

Last changed 2026-11-06 in a23ab72. Source on GitHub. Change log.