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Aspects

Introduction

Aspect oriented programming is the idea that some logic abstractions can be applied across various type hierarchies "after the fact", altering the behaviour 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.

The wiki article provides a much more in-depth explanation!

In the context of this dependency injector, AOP is achieved thanks to PHP's __call magic method combined with the Proxy design pattern.

In practice

  • 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

A worked example

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 - note that this doesn't cover all cases used by SilverStripe so is not a complete solution, more just a guide to how it would be used.

<?php

/**
 * Redirects write queries to a specific database configuration
 *
 * @author <marcus@silverstripe.com.au>
 * @license BSD License http://www.silverstripe.org/bsd-license
 */
class MySQLWriteDbAspect implements BeforeCallAspect {
    /**
     *
     * @var MySQLDatabase
     */
    public $writeDb;
    
    public $writeQueries = array('insert','update','delete','replace');

    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

  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

  MySQLWriteDbAspect:
    properties:
      writeDb: %$WriteMySQLDatabase

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

  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 in DB.php

  MySQLDatabase:
    class: AopProxyService
    properties:
      proxied: %$ReadMySQLDatabase
      beforeCall:
        query: 
          - %$MySQLWriteDbAspect

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

  • proxied : This is the 'read' database connectino 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


Injector:
  ReadMySQLDatabase:
    class: MySQLDatabase
    constructor:
      - type: MySQLDatabase
        server: slavecluster.hostname.db
        username: user
        password: pass
        database: read_database
  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: 
          - %$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 byref &$alternateReturn variable, and returns false after doing so.

    $alternateReturn = $this->writeDb->query($sql, $code);
    return false;

By returning false from the beforeCall() method, the wrapping proxy class will not 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.

Comments

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