Wednesday, March 31, 2010

Meetings kill Loose Coupling

There is a brilliant, yet underutilized, sociological observation called the Conway’s Law. It was stated by Mel Conway in his 1968 paper. It states that:

Any organization that designs a system will inevitably produce a design whose structure is a copy of the organization's communication structure.

The key phrase is “communication structure”, that is roughly, but not always, equal to organization structure.
You may have not heard of or read his paper, but I am sure you have seen the effect of his observation:

If you have two teams called Application and Kernel, then chances are you end up with two deployment units called applications and kernel (jars or folders or zip files or any other form of bundling code), on the other hand if you engineering organization has two teams called Books and Games, then you end up with a book and a game application, if there is no other team/group, it is unlikely that you can see a kernel or core subsystem that encapsulate the common construct between the two.

Now, we all know about the principal of loose coupling, and how it enables flexibility, efficiency, increased productivity etc. One very effective way of creating loose coupling between two system (let’s call then consumer and producer) is to make sure that members of the producer team do not meet with members of consumer team! You may convey requirements to them, but forbid meetings. If there is not communication, it is hard to build hard coupling. It is a bit strange and counter-intuitive, but we have used this with success in building key infrastructure services.

I have found out that a good early predictor of the level of coupling (or quality of interfaces in general) of a system is the list of invitees to their early design meetings. Whenever one of few engineers from a future consumer is part of the meeting, there is a good indication that they systems will be somehow coupled (either thru the types or domain values of parameters passed or the way errors are handled, the marshalling of the result, or even the terminology used) – This of course is a generalization, the team somehow has to collect requirements, or share result s and get early feedback, this is all OK, but if your goal is to create “loosely coupled” system, you should make sure your communication structure,  are loosely coupled as well.
Take a service that verifies a credit card number to ensure validity and that a provided name indeed matches credit card issuer record. This service may have a method/operation in its interface:

Result VerifyCreditCard(UserId id)

It assumes that somehow the service can obtain a credit card number from a supplied UserID. This is a very tightly coupled interface that shows the service provider has had too much knowledge about its consumer.
Here is a less tightly coupled (better) example:

Result VerifyCreditCard(CreditCard card, BuyerName buyerName)

This is not too bad, but the choice of “BuyerName” indicates that the provider has the knowledge that consumer (the one she probably met with) happens to deal with principals that are buyer.
Consider a loosely coupled version that one would probably write if there is no additional knowledge of potential consumers

Result VerifyCreditCard(CreditCard card, Name name)

Communication channels are very important interface design, this means people in each design meetings should be selected carefully, if system A should not be dependent on system B, the best way to ensure it, is to reduce communication between developers of system A and system B.


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