Obtaining a proxy for another Service Component

One SCA component can call the service provided by another SCA component. The service a component provides is made up of all of its public methods. SCA for PHP currently provides two ways for one component to call another: either locally (i.e. within the same PHP run-time, and on the same call stack) or remotely if the called component exposes a Web service binding.

In order for one component to call another, the calling component needs a proxy for the called component. This proxy is usually provided as an instance variable in the calling component, though proxies can also be obtained with the SCA::getService() call, as we shall see later. When a component is constructed, proxies are constructed for any instance variable which refer to another component, and these proxies are "injected" into the instance variables. Proxies are always used, whether the component is local or remote, in order to provide identical calling behavior between remote and local calls (for example, local calls are made to always pass data by-value). The proxies know how to locate the required component and to pass the calls made on to them.

Instance variables which are intended to hold proxies for services are indicated by the two PHPDocumentor-style annotations, @reference and @binding. Both annotations are placed in the documentation section for a class instance variable, as shown by the code below.

The @reference annotation before an instance variable indicates that the instance variable is to be initialized with a proxy to a component.

The @binding annotation has two forms @binding.php and @binding.soap, and indicates that the proxy is either for a local component or for a Web service respectively. For both @binding.php and @binding.soap, the annotation gives a target URI.

At the moment, with the annotation-based method of specifying dependencies, the only way to alter the intended target of a reference is to alter the annotation within the component.

In our ConvertedStockQuote example, the $exchange_rate instance variable will be initialized with a proxy to the local ExchangeRate component whenever an instance of the ConvertedStockQuote is constructed.

For @binding.php, the URI identifies the location of the script containing the implementation of the component. The component will be called locally. The service provided is the set of public methods of the component. The URI must be a simple pathname, either absolute or relative. The component will be loaded with the PHP include directive, after testing to see if it is already loaded with class_exists(). If the URI is a relative path, it is resolved relative to the component containing the annotation. Note that this is different from the normal PHP behaviour where scripts would be looked for along the PHP include_path, This is intended to provide some location-independence for cross-component references.

If this ExchangeRate service were remote and to be called as a Web service, only the @binding line changes. Instead of giving the location of a PHP class, it gives the location of the WSDL describing the web service. In our example component, this is illustrated by the second reference:

The StockQuote component will be called via a Web service request. In this case the URI for the WSDL can be a simple pathname, or may contain a PHP wrapper and begin, for example, with file:// or http://. In the event that it is a simple pathname, it can be absolute or relative, and if relative will be resolved relative to the component containing the annotation. Note that this is like the behaviour for @binding.php, and different from the normal PHP behaviour where the file would be looked for relative to the PHP current working directory, which would usually be the location of the first script to be called. This behaviour is intended to give consistency across the different bindings and to provide some location-independence for references between components.