# Course subscriptions

A runnable Gleam and Factos simulation of the
[DCB course subscriptions example](https://dcb.events/examples/course-subscriptions/).

## Challenge

Students subscribe to courses under constraints that cross traditional aggregate
boundaries:

- course identifiers must be unique;
- a course cannot exceed its current capacity;
- a student cannot subscribe to the same course twice;
- a student cannot subscribe to more than five courses.

Course capacity can also change after the course is defined.

The source article uses a ten-course student limit. This compact runnable
package sets the same configurable constraint to five.

## DCB approach

`StudentSubscribedToCourse` is tagged with both `course:<course_id>` and
`student:<student_id>`. A subscription command builds one decision context from
two items: the course history needed to calculate capacity and occupancy, and
the student's history needed to calculate their subscription count.

Factos folds matching events into command-specific state. The tests use
`factos/simulate` for deterministic scenarios, while the development app
dispatches the same commands through `factos_sqlight` into in-memory SQLite.

The package also demonstrates:

- unique course definition;
- course-capacity changes;
- domain errors for missing, full, unchanged, and duplicate cases;
- synchronized concurrent subscription attempts against the same course;
- a JSON encoder returning `factos.Event(json.Json)` and a schema decoder
  selected by event type and version.

The dispatch builder receives the encoder and decoder factory directly.
Malformed payloads become `factos.DecodeError`; unsupported event types or
versions become `factos.InvalidSchema`.

The server component uses `lustre.application`; its `init` and `update`
functions return managed Lustre effects instead of performing database work
directly. Effects dispatch projection results back through messages. The
command message wraps `course_subscription.Command`, so the UI exercises the
domain model directly. Strong subscriptions update both projections in the
same transaction as the event append.

The command-to-state flow is diagrammed in the module documentation in
[`src/course_subscription.gleam`](src/course_subscription.gleam).

## Run it

Requirements: [Gleam](https://gleam.run/).

Run the tests:

```sh
gleam test
```

Run the Ewe development server, then open <http://127.0.0.1:8080>:

```sh
gleam run -m course_subscriptions_dev
```

No database or container runtime is required.

## Package layout

- [`src/course_subscription.gleam`](src/course_subscription.gleam) contains the
  commands, events, decision functions, DCB contexts, JSON codec, and model.
- [`test/course_subscriptions_test.gleam`](test/course_subscriptions_test.gleam)
  verifies capacity, duplicate definition, and duplicate student scenarios.
- [`dev/course_subscriptions_dev.gleam`](dev/course_subscriptions_dev.gleam)
  serves the document, Lustre runtime, and server-component WebSocket with Ewe.
- [`dev/course_subscription_component.gleam`](dev/course_subscription_component.gleam)
  wraps domain commands in Lustre messages and renders the unstyled semantic UI.
- [`dev/course_subscription_projection.gleam`](dev/course_subscription_projection.gleam)
  maintains and queries the in-memory SQLite projections.