Backslash Demo Application

Reference web application demonstrating how Backslash components work together to power an event-sourced system in PHP.

The domain is student course subscriptions, inspired by the Dynamic Consistency Boundary by Sara Pellegrini.

Try it live at backslashphp-demo.maximegosselin.com

Getting Started

git clone https://github.com/backslashphp/demo
cd demo
composer install
composer serve

Open http://localhost:8000 in your browser.

Domain Rules

• A course cannot accept more students than its capacity.
• The course capacity can change at any time to any positive integer different from the current one.
• A student cannot subscribe to more than 3 courses.

What You Can Do

• Register students
• Define courses with a capacity
• Change course capacity
• Subscribe and unsubscribe students from courses

The admin panel lets you load demo data, inspect raw events from the event store, rebuild projections from scratch, or replay them up to a specific event. A great way to observe event sourcing in action.

Explore the Database

Events and projections are persisted in a SQLite database at data/demo.sqlite. It is strongly recommended to open it in a database IDE (e.g. DB Browser for SQLite, DBeaver, or the SQLite plugin for PhpStorm/IntelliJ) and inspect its content while interacting with the app.

You will find:

• An event_store table with all persisted domain events: payload, identifiers, and metadata (including the correlation_id added by the StreamEnricher)
• A projection_store table with the current state of all read models

Backslash Components

This app demonstrates the main userland components of Backslash. Each entry below links to a concrete example in the codebase.

Command: A simple readonly DTO carrying the intent of the user.

Command Handler: Loads a model, calls its business methods, and saves changes via the repository.

Event: A readonly class implementing EventInterface. Carries what happened, with identifiers for stream querying.

Model: Extends AbstractModel. Business methods call $this->record(new Event(...)) to emit events. State is rebuilt by apply* methods replaying past events from the event store.

Stream Enricher: Middleware that enriches each recorded event's metadata before it is stored and dispatched, adding a correlation_id to group all events produced by a single command.

Projection: A read model stored in the projection store, tailored for a specific query.

Projector: Implements EventHandlerInterface. Subscribed to specific event types on the event bus. Keeps its projection up to date as events are published.

Test Scenario: Uses Play with given() / when() / then() / thenExpectException(), executed via $this->scenario->play(). See ScenarioShowcaseTest for a tour of all available assertions.

Bootstrapping

All wiring is declared in Container.php: which command handler handles which command, and which projector listens to which event. This is the entry point to understand how a Backslash application is assembled.

This app uses a hand-rolled PSR-11 container to wire everything together, but Backslash does not require one. You can assemble the components however you like.

Architecture

flowchart LR
    CMD([Command]) --> D[Dispatcher]
    D --> H[CommandHandler]
    H --> |load & save| R[Repository]
    R --> ES[(EventStore)]
    R --> EB[EventBus]
    EB --> P[Projectors]
    P --> PS[(ProjectionStore)]

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Testing

The codebase follows a Vertical Slice Architecture: each feature in src/Feature/ is a self-contained slice grouping its commands, events, model, and tests. This is why tests live inside each slice (e.g. src/Feature/CourseSubscription/Test/) rather than in a traditional top-level tests/ directory.

ScenarioShowcaseTest is a standalone test at the root of src/ that showcases all assertions available in the Scenario API, a good starting point to understand what you can express in a test.

composer test