Directionality

One key factors to differentiate between protocols is their directionality:

• Unidirectional = a client requests from a server and the server responds. The server cannot initiate the communication.

• Bi-directional = the client and the server can send messages without being requested.

The advantage of bidirectionality is that one can also adopt a push mode, where the server sends data to the client. For example regarding inventory: a product on platform A is purchased, but this product is also sold on platform B, platform A could "push" the information that the available stock has changed to platform B. The need to have uni- or bi-directionality is therefore related to the need to have data synchronization and notifications in real time. An alternative in the example above using unidirectionality could be that servers send pull requests, every minute for example, to know if the stock has changed. Is it sufficient ? Do we want real time or near real time to 1 or 5 minutes?

So far the protocol used in the semantic web is rather unidirectional, HTTP or LDP. It can be a simple request about a resource (file: LDP or base of triplestore: HTTP) or more complete queries with Sparql.

There is today very little R&D on bidirectionality in the semantic world, it is still very experimental (AMQP + SPARQL, XMPP + SPARQL, HTTP2 + SPARQL, WebSocket). The terrain is slippery, not yet mature. The theoretical ideal would of course be bi-directional, but the rational and practical solution of the consortium is rather to use unidirectionality. We prefer to focus our vision towards new and proven standards and technologies. We will see later when technologies will mature if it makes sense to switch to bidirectional.

Conclusion: implement a unidirectional solution