Ian Robinson on Coupling

In my opinion, coupling is the most fundamental attribute of a system architecture and tight coupling is probably the most common architectural problem I see in distributed systems. The manner in which system components interact can be a chief determinant of the scalability and reliability of the final system.

So I really like Ian Robinson’s post on Temporal and Behavioural Coupling where he uses two coupling dimensions and the inevitable magic quadrant to classify systems based on their degree of temporal and behavioural coupling.

See Ian’s post for the slick professional graphics, but to summarise – event-oriented systems with low coupling  occupy the “virtuous” third quadrant of the matrix. Conversely the brittle “3-tier” applications that many of us struggle with, occupy the “evil” first quadrant where coupling in both dimensions is high.

However I’m a little miffed to see no mention of my favourite “document-oriented message” in Ian’s diagram. As Bill Poole writes; document messages have lower behavioural coupling than command messages, but more than event messages. So would you put document-oriented messages near the middle top of the matrix between command-oriented and event-oriented messages? Unfortunately that would break the symmetry. But it also highlights another problem.

Any type of message – document, command or event-oriented could temporally be tightly or loosely coupled. Temporal coupling is more a property of the message transport than of the message type. So I suggest that the two coupling dimensions are characterised as follows:

• Temporal coupling – characterised by message transport from RPC (tight coupling) through to MOM (loose coupling).
• Behavioural coupling – characterised by the message type from event-oriented (tight) through document-oriented to event-oriented (loose).

It so happens that distributed 3-tier systems generally employ both command-oriented messages and RPC transports – hence making them inherently “evil”. Whereas events (being asynchronous)  are naturally virtuous by typically being carried over MOM transports (it’s difficult to request an event notification).

Between heaven and hell, it is in the murky mortal realms of SOA where we need to be constantly mindful of the interactions between message type and transport – lest our system ends up in limbo.