It is sometimes desirable to defer the materialization of an object until it is absolutely required, usually for performance reasons. For example, suppose that ProductDescription objects reference a Manufacturer object, but only very rarely does it need to be materialized from the database. Only rare scenarios cause a request for manufacturer information, such as manufacturer rebate scenarios in which the company name and address are required.
The deferred materialization of "children" objects is known as lazy materialization. Lazy materialization can be implemented using the Virtual Proxy GoF pattern - one of many variations of Proxy.
Figure 37.15 Commands for database operations
A Virtual Proxy is a proxy for another object (the real subject) that materializes the real subject when it is first referenced; therefore, it implements lazy materialization. It is a lightweight object that stands for a "real" object that may or may not be materialized.
A concrete example of the Virtual Proxy pattern with ProductDescription and Manufacturer is shown in Figure. This design is based on the assumption that proxies know the OID of their real subject, and when materialization is required, the OID is used to help identify and retrieve the real subject.
Note that the ProductDescription has attribute visibility to an IManufacturer instance. The Manufacturer for this ProductDescription may not yet be materialized in memory. When the ProductDescription sends a getAddress message to the ManufacturerProxy (as though it were the materialized manufacturer object), the proxy materializes the real Manufacturer, using the OID of the Manufacturer to retrieve and materialize it.
Who Creates the Virtual Proxy?
Observe in Figure that the ManufacturerProxy collaborates with the PersistenceFacade in order to materialize its real subject. But who creates the ManufacturerProxy? Answer: The database mapper class for ProductDescription. The mapper class is responsible for deciding, when it materializes an object, which of its "child" objects should also be eagerly materialized, and which should be lazily materialized with a proxy.
Consider these alternative solutions: one uses eager materialization, the other lazy materialization.// EAGER MATERIALIZATION OF MANUFACTURER
Here is the lazy materialization solution:// LAZY MATERIALIZATION OF MANUFACTURER
Implementation of a Virtual Proxy
The implementation of a Virtual Proxy varies by language. The details are outside the scope of this chapter, but here is a synopsis:
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