Site Map
Vision
Schematic
Concept of Operation
Distribution System
Home
Concept - Delivery System
  • The purpose of this page is to explain how assertions are distributed from a publisher site to the various subscriber sites in accordance with current subscription contracts.
  • It is here that the principle mentioned in the page called "vision" has its greatest impact. Because of this principle, every piece of information on the dynamic web can be regarded as "an indivisible and immutable version which is semantically complete and has its own provenance".
  • This assurance greatly simplifies the delivery system as compared with that of a distributed relational data base. Each assertion is an individual statement of fact or opinion by a known source that deems it to be valid for a stated period of time and within a stated context. For this reason any such assertion can be copied without limit and sent anywhere without compromising its integrity. This is exactly what we need on the web since every site is independent and there is nothing to stop a subscribing site re-publishing information that it receives.

The Distribution Mechanism

The mechanism through which new versions of an assertion type are distributed to subscribing sites is executed entirely by generic software that relies on their conformance with a standard format. For this reason sites can participate without the exercise of programming skills and free of the attendant risks. The distribution is performed by just one process which is triggered whenever a new assertion is added to the local site knowledge store . This process contains the following steps:

Contract Check. The new assertion is checked against the set of current subscription contracts to indentify the subscribing sites that should be sent a copy.
Outward Filter. For each subscription contract the provenance of the new assertion is checked against the specified criteria. These criteria comprise conditions that must be satisfied by the values of each provenance factor.
Despatch. If the filter stage is successful then the assertion, complete with subject, predicate, object, context and provenance, is copied into a "receive and store" soap message together with the identity (URI) of the applicable subscription contract. The message is then sent to the subscribing site where it invokes an "AssertionReceiveAndStore" function.
Receipt. On receipt of an inwara "receive and store" soap message under a subscription contract that is current, the subject, predicate, context and provenance of the assertion are again checked against the specified criteria. This check is a safety procedure to ensure that the integrity of the subscribing site cannot be compromised by any error at the publishing site. If the check is successful the assertion is added to the local site knowledge store by the subscribing site's "AssertionReceiveAndStore" function.

Forwarding and Re-Publishing

From a technical viewpoint there is nothing to prevent the owner of a subscribing site from using some of the dynamic knowledge data it receives to feed a service to its own subscribers. This can be achieved either by acting as a transparent forwarding agent retaining the provenance as found on receipt or by re-publishing the content but with a new provenance citing its own URI as the ultimate source.

However any action that exploits information that has been placed on the dynamic web may be limited by contractual, ethical or practical considerations. For example:

  • The subscription contract under which original updates are received may specify that forwarding is not permitted;
  • The subscriber to original updates may not want the responsibility of forwarding;
  • The potential subscriber to a forwarding service may be better served by contracting with the original source.

For these reasons I believe that the possibility of forwarding is covered adequately by simply requiring all subscription contracts to specify whether it is permitted or not. The enforcement of such a condition is easy because, in the event of non-compliance, the service is easily withdrawn.

Integrity of the Dynamic Web as a Global Entity

On the dynamic web there is no attempt to maintain a single variable state for anything and so there is nothing to synchronize. This means that there is no need for the publisher to track where copies have been sent or whether they arrive. If a copy gets lost - no matter - this is just a minor degradation at the subscribing site concerned. It cannot threaten the integrity or operation of the rest of the dynamic web as a whole.