Backslash Demo Application

This is a simple console-based application designed to manage students, courses and enrollments.

Its goal is to demonstrate how Backslash components work together in order to power event-sourced systems in PHP.

The application is interacted with through a few scripts located in the bin folder. It relies solely on Backslash for simplicity in learning, without using any additional libraries.

Featured Backslash components are:

• Domain events and states
• Commands and command handlers
• Projections
• Test scenarios

Domain rules

Rules are inspired by the Course Subscriptions example of the Dynamic Consistency Boundary website.

• 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.

Getting started

Start by installing dependencies with Composer:

composer install

Move to the bin folder and run the demo.php script:

cd bin
php demo.php

It initializes the SQLite database in data/demo.sqlite where events and projections are persisted. It also runs commands to create some demo data.

The script outputs projections of students, courses and current subscriptions.

----- STUDENTS -----
[1] John (Biology)
[2] Mary (Arts, Physics)
[3] Bill (Physics)
[4] James
[5] Lucy
[6] Brad
[7] Kelly
[8] Alice

----- COURSES -----
[1] Algebra (0/5)
[2] Biology (1/4)
    - John
[3] Arts (1/3)
    - Mary
[4] Physics (2/3)
    - Mary
    - Bill
[5] Grammar (0/4)

As you play with the scripts, you may open the SQLite database in your favourite IDE to inspect its content.

To understand how all parts of the application are connected, take a look at src/Infrastructure/Container.php.

Usage

Commands in these examples must be run from the bin folder.

Register a student

php register-student.php --id=123 --name=Max

Create a course

php define-course.php --id=1000 --name=Geology --capacity=10

Change course capacity

php change-course-capacity.php --id=1000 --capacity=15

Subscribe a student to a course

php subscribe.php --student=123 --course=1000

Unsubscribe a student from a course

php unsubscribe.php --student=123 --course=1000

Output the current state of the system

php show.php

Management scripts

List events persisted in the event store

php events.php

| #  | UID                                  | CLASS                                            | PAYLOAD                                        | IDENTIFIERS                      | METADATA                                                  | TIMESTAMP                        |
| 1  | 9ffd43a3-dee5-4caa-9d91-6e29c8ae3fd9 | Demo\Domain\Event\StudentRegisteredEvent         | {"studentId":"1","name":"John"}                | {"studentId":"1"}                | {"correlation_id":"b74e7829-2810-4a23-8a44-6e9d9a4f59c3"} | 2025-06-20T02:35:31.638777+00:00 |
| 2  | 33a197d5-a8cc-4ef6-b9be-f63608af5fd0 | Demo\Domain\Event\StudentRegisteredEvent         | {"studentId":"2","name":"Mary"}                | {"studentId":"2"}                | {"correlation_id":"b74e7829-2810-4a23-8a44-6e9d9a4f59c3"} | 2025-06-20T02:35:31.654526+00:00 |
| 3  | 8784f39f-f1bd-472f-a543-426ba4f3ac2e | Demo\Domain\Event\StudentRegisteredEvent         | {"studentId":"3","name":"Bill"}                | {"studentId":"3"}                | {"correlation_id":"b74e7829-2810-4a23-8a44-6e9d9a4f59c3"} | 2025-06-20T02:35:31.667294+00:00 |
| 4  | 231aa070-03bb-43aa-9482-e00052ae1780 | Demo\Domain\Event\StudentRegisteredEvent         | {"studentId":"4","name":"James"}               | {"studentId":"4"}                | {"correlation_id":"b74e7829-2810-4a23-8a44-6e9d9a4f59c3"} | 2025-06-20T02:35:31.687732+00:00 |
| 5  | 9a1fe35d-afe7-4fe9-ae61-213da79326e2 | Demo\Domain\Event\StudentRegisteredEvent         | {"studentId":"5","name":"Lucy"}                | {"studentId":"5"}                | {"correlation_id":"b74e7829-2810-4a23-8a44-6e9d9a4f59c3"} | 2025-06-20T02:35:31.702003+00:00 |
| 6  | c99af1e6-4d9d-4042-955c-567ddae78219 | Demo\Domain\Event\StudentRegisteredEvent         | {"studentId":"6","name":"Brad"}                | {"studentId":"6"}                | {"correlation_id":"b74e7829-2810-4a23-8a44-6e9d9a4f59c3"} | 2025-06-20T02:35:31.718004+00:00 |
| 7  | 3404d47e-25f0-48db-bab2-199feb13880e | Demo\Domain\Event\StudentRegisteredEvent         | {"studentId":"7","name":"Kelly"}               | {"studentId":"7"}                | {"correlation_id":"b74e7829-2810-4a23-8a44-6e9d9a4f59c3"} | 2025-06-20T02:35:31.734666+00:00 |
| 8  | 4770359c-0a0f-4872-971e-134ea2fdc4cb | Demo\Domain\Event\StudentRegisteredEvent         | {"studentId":"8","name":"Alice"}               | {"studentId":"8"}                | {"correlation_id":"b74e7829-2810-4a23-8a44-6e9d9a4f59c3"} | 2025-06-20T02:35:31.750996+00:00 |
| 9  | aa16a7c9-8902-4e91-92da-ef9803e00c63 | Demo\Domain\Event\CourseDefinedEvent             | {"courseId":"1","name":"Algebra","capacity":5} | {"courseId":"1"}                 | {"correlation_id":"b74e7829-2810-4a23-8a44-6e9d9a4f59c3"} | 2025-06-20T02:35:31.767753+00:00 |
| 10 | 776f7f2d-591b-4c2f-86af-e595b8e86a9e | Demo\Domain\Event\CourseDefinedEvent             | {"courseId":"2","name":"Biology","capacity":4} | {"courseId":"2"}                 | {"correlation_id":"b74e7829-2810-4a23-8a44-6e9d9a4f59c3"} | 2025-06-20T02:35:31.783081+00:00 |

...

Rebuild projections

This script deletes all stored projections and rebuilds them by replaying events.

php rebuild-projections.php

No:                 1
Event:              Demo\Domain\Event\StudentRegisteredEvent
Timestamp:          2025-06-20T02:38:32.082613+00:00

No:                 2
Event:              Demo\Domain\Event\StudentRegisteredEvent
Timestamp:          2025-06-20T02:38:32.098321+00:00

No:                 3
Event:              Demo\Domain\Event\StudentRegisteredEvent
Timestamp:          2025-06-20T02:38:32.110229+00:00

...

Testing

Test scenarios can be found in the tests folder.

vendor/bin/phpunit