Aspects#

Aspect oriented programming is the idea that some logic abstractions can be applied across various type hierarchies "after the fact", altering the behavior of the system without altering the code structures that are already in place.

In computing, aspect-oriented programming (AOP) is a programming paradigm which isolates secondary or supporting functions from the main program's business logic. It aims to increase modularity by allowing the separation of cross-cutting concerns, forming a basis for aspect-oriented software development.

In the context of the Silverstripe CMS Dependency Injector, Aspects are achieved thanks to PHP's __call magic method combined with the Proxy Design Pattern.

Assume an existing service declaration exists called MyService. An AopProxyService class instance is created, and the existing MyService object is bound in as a member variable of the AopProxyService class.

Objects are added to the AopProxyService instance's "beforeCall" and "afterCall" lists; each of these implements either the beforeCall or afterCall method.

When client code declares a $dependency on MyService, it is actually passed in the AopProxyService instance.

Client code calls a method MyMethod that it knows exists on MyService - this doesn't exist on AopProxyService, so __call is triggered.

All classes bound to the beforeCall list are executed; if any explicitly returns 'false', myMethod is not executed. Otherwise, myMethod is executed.

All classes bound to the afterCall list are executed.

To provide some context, imagine a situation where we want to direct all write queries made in the system to a specific database server, whereas all read queries can be handled by slave servers.

A simplified implementation might look like the following.

// app/src/Aspect/MySQLWriteDbAspect.php
namespace App\Aspect;

use SilverStripe\Core\Injector\BeforeCallAspect;
use SilverStripe\ORM\Connect\MySQLDatabase;

class MySQLWriteDbAspect implements BeforeCallAspect
{
    private MySQLDatabase $writeDb;

    private array $writeQueries = [
        'insert',
        'update',
        'delete',
        'replace',
    ];

    public function setWriteDB(MySQLDatabase $db): static
    {
        $this->writeDb = $db;
        return $this;
    }

    public function beforeCall($proxied, $method, $args, &$alternateReturn)
    {
        if (isset($args[0])) {
            $sql = $args[0];
            $code = isset($args[1]) ? $args[1] : E_USER_ERROR;

            if (in_array(strtolower(substr($sql, 0, strpos($sql, ' '))), $this->writeQueries)) {
                $alternateReturn = $this->writeDb->query($sql, $code);
                return false;
            }
        }
    }
}

To actually make use of this class, a few different objects need to be configured. First up, define the writeDb object that's made use of above.

# app/_config/aspects.yml
SilverStripe\Core\Injector\Injector:
  WriteMySQLDatabase:
    class: MySQLDatabase
    constructor:
      - type: MySQLDatabase
        server: write.hostname.db
        username: user
        password: pass
        database: write_database

This means that whenever something asks the Injector for the WriteMySQLDatabase object, it'll receive an object of type MySQLDatabase, configured to point at the 'write_database'.

Next, this should be bound into an instance of the Aspect class

# app/_config/aspects.yml
SilverStripe\Core\Injector\Injector:
  App\Aspect\MySQLWriteDbAspect:
    properties:
      writeDb: '%$WriteMySQLDatabase'

Next, we need to define the database connection that will be used for all non-write queries

# app/_config/aspects.yml
SilverStripe\Core\Injector\Injector:
  ReadMySQLDatabase:
    class: MySQLDatabase
    constructor:
      - type: MySQLDatabase
        server: slavecluster.hostname.db
        username: user
        password: pass
        database: read_database

The final piece that ties everything together is the AopProxyService instance that will be used as the replacement object when the framework creates the database connection.

# app/_config/aspects.yml
SilverStripe\Core\Injector\Injector:
  MySQLDatabase:
    class: AopProxyService
    properties:
      proxied: '%$ReadMySQLDatabase'
      beforeCall:
        query:
          - '%$App\Aspect\MySQLWriteDbAspect'

The two important parts here are in the properties declared for the object.

• proxied : This is the 'read' database connection that all queries should be initially directed through.
• beforeCall : A hash of method_name => array containing objects that are to be evaluated before a call to the defined method_name

Overall configuration for this would look as follows

# app/_config/aspects.yml
SilverStripe\Core\Injector\Injector:
  ReadMySQLDatabase:
    class: MySQLDatabase
    constructor:
      - type: MySQLDatabase
        server: slavecluster.hostname.db
        username: user
        password: pass
        database: read_database
  App\Aspect\MySQLWriteDbAspect:
    properties:
      writeDb: '%$WriteMySQLDatabase'
  WriteMySQLDatabase:
    class: MySQLDatabase
    constructor:
      - type: MySQLDatabase
        server: write.hostname.db
        username: user
        password: pass
        database: write_database
  MySQLDatabase:
    class: AopProxyService
    properties:
      proxied: '%$ReadMySQLDatabase'
      beforeCall:
        query:
          - '%$App\Aspect\MySQLWriteDbAspect'

Changing what a method returns#

One major feature of an Aspect is the ability to modify what is returned from the client's call to the proxied method. As seen in the above example, the beforeCall method modifies the &$alternateReturn variable, and returns false after doing so.

namespace App\Aspect;

use SilverStripe\Core\Injector\BeforeCallAspect;
use SilverStripe\ORM\Connect\MySQLDatabase;

class MySQLWriteDbAspect implements BeforeCallAspect
{
    private MySQLDatabase $writeDb;
    // ...

    public function beforeCall($proxied, $method, $args, &$alternateReturn)
    {
        // ...
        $alternateReturn = $this->writeDb->query($sql, $code);
        return false;
        // ...
    }
}

By returning false from the beforeCall() method, the wrapping proxy class willnot call any additional beforeCall handlers defined for the called method. Assigning the $alternateReturn variable also indicates to return that value to the caller of the method.

Similarly the afterCall() aspect can be used to manipulate the value to be returned to the calling code. All the afterCall() method needs to do is return a non-null value, and that value will be returned to the original calling code instead of the actual return value of the called method.

API documentation#

• AopProxyService
• BeforeCallAspect
• AfterCallAspect
• Injector