What are your expectation as a consumer from a possible service telling you waht time it is?
Availability: Service must be available when consumer needs it.
If a consumer relies only on mobile phone and battery runs out. The service is not available when consumer may need it.
Reliability: Service response must be reliable and should not require double checking.
A consumer is not sure of the time shown by a clock at public place, he needs to double check it with another reliable source anyway.
Accessibility: Consumer can access the service whenever he needs it.
A clock in laptop in backpack is not easily accessible in a crowded commuter train.
Trust: Consumer trusts that service is backed up by accountability on part of a service provider. When it is a question of life and death, one can trust service provided by one's own piece of equipment to be accurate, reliable and available than a general purpose service. As a consumer one tend to trust no one but oneself as most accountable service provider.
This is a very important insight which helps one prepare a proper versioning policy.
Without that trust versioning schemes can be mis-used for creating specialized services.
In enterprises, since SOA is mandated, project owners will use services. But they will make sure that they get their own private version of a service. This is quite easy by getting a veto power over life cycle of a service and mis-using governance for this sake. Assume a service version 1 is in use. Second consumer wants to create another version, because it has additional needs. This version 2 is derived from version 1. Now first consumer wants an upgrade to his existing service. But he is not willing to accept service version 2 as base for its next version. He will find any execuse to make sure he gets a service version 1.1 rather than 3. This pattern will keep repeating. And soon there will be a lot of versions, changing only in second qualifier. So you will get, what I call parversion (parallel version) anti-pattern.
SOA governance must make sure it guards against this anti-pattern and creates appropriate policies and controls to minimize and eliminate its occurences. Morover governance must recognise that this is a symptom and root cause lies somewhere else.
Moral of the story: Consumer is willing to bend rules to get an acceptable, reliable, accessible and trustworthy service. Watch out for such rules violation and fix root cause.
