Snicco-Kernel: A bootstrapper for applications with a plugin architecture

The Kernel component of the Snicco project helps to bootstrap an application that uses a plugin architecture.

Table of contents

1. Installation
2. Definitions
   1. Kernel
   2. Bootstrappers
   3. Bundles
   4. Environment
   5. Directories
   6. Configuration files
3. Usage
   1. Creating kernel
   2. Booting the kernel
   3. Lifecycle hooks
   4. Using the booted kernel
4. Contributing
5. Issues and PR's
6. Security

Installation

composer require snicco/kernel

Definitions

Kernel

The Kernel class helps to load and cache configuration files, define services in a dependency-injection container and bootstrap an application in a controlled manner using any number of bootstrappers and bundles.

---

Bootstrappers

A bootstrapper can be any class that implements the Bootstrapper interface:

A bootstrapper is a class responsible for "bootstrapping" one cohesive part of an application.

"Bootstrapping" could mean, for example: registering definitions in a dependency injection container or creating event listeners.

Bootstrappers are the central place to configure the application.

interface Bootstrapper
{
    public function shouldRun(Environment $env): bool;

    public function configure(WritableConfig $config, Kernel $kernel): void;

    public function register(Kernel $kernel): void;

    public function bootstrap(Kernel $kernel): void;
}

---

Bundles

A bundle can be any class that implements the Bundle interface. The Bundle interface extends the Bootstrapper interface.

interface Bundle extends Bootstrapper
{
    public function alias(): string;
}

The difference between a bundle and a bootstrapper is that a bundle is meant to be publicly distributed, while a bootstrapper is internal to a specific application.

Bundles are aware of other bundles that are used by the same Kernel instance.

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Environment

A Kernel always needs an environment to run in.

The following environments are possible:

• production
• staging
• development
• testing
• debug (in combination with any non production env.)

use Snicco\Component\Kernel\ValueObject\Environment;

$environment = Environment::prod()
$environment = Environment::testing()
$environment = Environment::staging()
$environment = Environment::dev()
$environment = Environment::fromString(getenv('APP_ENV'));

---

Directories

A Kernel always needs a Directories value object that defines the location of:

• the base directory of the application.
• the config directory of the application.
• the log directory of the application.
• the cache directory of the application.

use Snicco\Component\Kernel\ValueObject\Directories;

$directories = new Directories(
    __DIR__, // base directory,
    __DIR__ .'/config', // config directory
    __DIR__. '/var/cache', // cache directory
    __DIR__ . '/var/log' // log directory
)

// This is equivalent to the above:
$directories = Directories::fromDefaults(__DIR__);

---

Dependency injection container

The Kernel needs an instance of DIContainer, which is an abstract class that extends the PSR-11 container interface.

There are currently two implementations of this interface:

• snicco/pimple-bridge, using pimple/pimple
• snicco/illuminate-container-bridge, using illuminate/container

You can also provide your own implementation and test it using the test cases in snicco/kernel-testing.

The DIContainer class is an abstraction meant to be used inside bundles.

Since bundles are distributed packages, they can't rely on a specific dependency-injection container. However, the PSR-11 container interface only defines how to fetch services from the container, not how to define them, which is why the DIContainer abstraction is used.

---

Configuration files

Every .php file inside the config directory will be used to create a Config instance once the kernel is booted.

The following configuration inside your config directory:

// config/routing.php

return [

    'route_directories' => [
        /* */    
    ]       
        
    'features' => [
        'feature-a' => true,
    ]       
]

would be loaded into the config instance like so:

$config->get('routing');

$config->get('routing.route_directories');

$config->get('routing.features.feature-a');

The kernel.php configuration file is reserved since this is where bundles and bootstrappers are defined:

// config/kernel.php

use Snicco\Component\Kernel\ValueObject\Environment;

return [

    'bundles' => [
        
        // These bundles will be used in all environments
        Environment::ALL => [
            RoutingBundle::class
        ],
        // These bundles will only be used if the kernel environment is dev.
        Environment::DEV => [
            ProfilingBundle::class
        ],      
        
    ],
    
    // Bootstrappers are always used in all environments.
    'bootstrappers' => [
        BootstrapperA::class
    ]   

]

Usage

Creating a kernel

use Snicco\Component\Kernel\Kernel;

$container = /* */
$env = /* */
$directories = /* */

$kernel = new Kernel(
    $container,
    $directories,
    $env
);

---

Booting a kernel

There is a difference in what happens when the kernel is booted based on the current environment and whether the configuration is already cached.

$kernel = /* */

$kernel->boot();

Booting an "uncached" kernel:

1. All configuration files inside the config directory will be loaded from disk to create an instance of WritableConfig.
2. The bundles and bootstrappers are read from the kernel.php configuration file.
3. The shouldRun() method is called for all bundles.
4. The shouldRun() method is called for all bootstrappers.
5. The configure() method is called for all bundles.
6. The configure() method is called for all bootstrappers.
7. The WritableConfig is combined into one file and written to the cache directory (if the current environment is production/staging).
8. The register() method is called for all bundles.
9. The register() method is called for all bootstrappers.
10. The boot() method is called for all bundles that are defined in the kernel.php configuration file.
11. The boot() method is called for all bootstrappers that are defined in the kernel.php configuration file.
12. The DIContainer is locked and no further modifications can be made.

Booting a "cached" kernel:

1. The cached configuration file is loaded from disk and a ReadOnlyConfig is created.
2. The bundles and bootstrappers are read from the ReadOnlyConfig.
3. The shouldRun() method is called for all bundles.
4. The shouldRun() method is called for all bootstrappers.
5. The register() method is called for all bundles.
6. The register() method is called for all bootstrappers.
7. The boot() method is called for all bundles.
8. The boot() method is called for all bootstrappers.
9. The DIContainer is locked and no further modifications can be made.

---

• The configure() method should be used to extend the loaded configuration with default values and to validate the configuration for the specific bundle. The configure() method is only called if the configuration is not cached yet.

• The register() method should only be used to bind service definitions into the DIContainer.

• The boot() method should be used to fetch services from the DIContainer and to configure them (if necessary). The container is already locked at this point and further modifications of service definitions are not possible. Attempting to modify the container from inside the boot() method of a bundle or bootstrapper will throw a ContainerIsLocked exception.

Each of these methods is always called first on all bundles, then on all bootstrappers.

This allows bootstrappers to customize behaviour of bundles (if desired).

---

Lifecycle hooks

There are two extension points in the booting process of the kernel.

• After the configuration was loaded from disk (only if the configuration is not cached already). This is the last opportunity to modify the configuration before its cached.
• After all bundles and bootstrappers have been registered, but before they are booted. This is the last opportunity to change service definitions before the container is locked.

use Snicco\Component\Kernel\Configuration\WritableConfig;use Snicco\Component\Kernel\Kernel;

$kernel = /* */

$kernel->afterConfigurationLoaded(function (WritableConfig $config) {
    if( $some_condition ) {
        $config->set('routing.features.feature-a', true);    
    }
});

$kernel->afterRegister(function (Kernel $kernel) {
    if($some_condition) {
        $kernel->container()->instance(LoggerInterface::class, new TestLogger());
    }
});

$kernel->boot();

---

Using the booted kernel

After the container is booted, services provided by all bundles can be safely fetched.

An example:

use Nyholm\Psr7Server\ServerRequestCreator;

$kernel->boot();

$server_request_creator = $kernel->container()->make(ServerRequestCreator::class);
$http_kernel = $kernel->container()->make(HttpKernel::class);

$response = $http_kernel->handle($server_request_creator->fromGlobals());

Contributing

This repository is a read-only split of the development repo of the Snicco project.

This is how you can contribute.

Reporting issues and sending pull requests

Please report issues in the Snicco monorepo.

Security

If you discover a security vulnerability, please follow our disclosure procedure.