However, we can envision neater, more efficient ways to enable reuse. PEP also affords reasoning about compatible extensions and composite extensions. We are already familiar with many analogues to active proxies as reusable filters. The difference is in the the affordances of the interchange format. Most significantly, since PEP modules are identified by the URI of the protocol they implement, PEP-aware Web tools can negotiate common sets of compatible modules and settings.
PEP enshrines a philosophy of decentralization.
Anyone can publish an extension by maintaining a Web page describing it. Any such module has as much expressive power to rewrite its input as an active page or active proxy. These benefits are not free, though. Selectively applying active proxy extensions requires more logic in the server to select compatible PEP modules and enforce users' and publishers' policies. In truth, it has been easier for extenders to deploy active pages and active proxies than to design for the future.