Tag: HTTP

Definition by Tucker et al.¹ and Maier et al.²:

”A punctuation is a pattern p inserted into the data stream with the meaning that no data item i matching p will occur further on in the stream.”

For event processing systems, events are the fundamental unit of information³. This means each event is processed atomically, i.e. completely or not at all. For RDF stream processing systems this can cause problems if events are modelled as graphs consisting of multiple quadruples: How can a receiver of an event know that all quadruples pertaining to the event are transmitted in order to start processing the event?

For streams of RDF graphs punctuation can be used like this: A punctuation is a pattern ”p” inserted into the quadruple stream with the meaning that no quadruples i from graph p will occur further on in the stream.

Punctuation could be implemented using special ("magic") quadruples but when using the Web stack(!) we can do punctuation out-of-band, i.e. implement punctuation on a lower layer of the stack. For example, we can communicate through ”chunked transfer encoding” (Fielding et al. 1999, Section 3.6.1)⁴ from HTTP 1.1. Each chunk contains a complete graph and the receiver will know that after a chunk is received the event is completely received and can be processed further in an atomic fashion. There is a guarantee that no quads for this graph will arrive later. Using HTTP chunked connections no special (or magic) quads are needed.

”Chunked transfer encoding” is also used by the RDF publish/subscribe middleware Ztreamy⁵ to provide long-lived connections using pure HTTP with the goal of disseminating events to subscribers. Further related work⁶ investigates the exchange of RDF over different protocols such as XMPP on top of HTTP (and thus TCP) but even UDP. However, none of these protocols provides pure HTTP stream URIs which are easily referenced in Linked Data.