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But ensuring that we are back at tabular rasa is difficult with global objects and running tests in a single process space. Many of the test cases are dependant on second order effects of function calls to configure internal objects, instead of being true unit tests that only call on a single object.Also, these global objects force us to do a bunch of hacks in unit tests. We need to do tricks to ensure the object is initialized as we want. We also need to save and restore its state between runs.I don't agree with the statement that they force "a bunch of hacks," clearing state is a perfectly normal thing to do, it is done for any servers that get started, any mocks that are made, and every test database. Making sure that modifications to a configuration object are cleaned up is no different: there is no "save and restore" just always start from a blank slate and set thingsas required, same as in the non-global model.
This kind of refactoring is the norm in large projects, and leads to better tested code paths, reusuable objects, and objects that are easier to understand and track.
One way that these types of things have been described is via the SOLID acronym. THis is a collection of best practices:
http://en.wikipedia.org/wiki/SOLID_%28object-oriented_design%29The Global config violates quite a few of these principals. By reading from a configuration object, you implicitly violate Single responsibility. Now it has at least two, one of which is figuring out how to construct and initialize itself. It also doesn't really follow the Open Closed principal: Once you get state from a Config object, you can no longer easily extend it, unless that extended object also gets its state from the same config. The big one is D: Dependency. By using a global config, you make dependencies on implementations, not abstractions. Dependency injection is pretty much impossible with a global config.
Keystone really stands to benefit from reusability. An example: right now, we can only have one SQL Datasource, but it is likely that some users would have one data source for Identity and a different one for Tokens. That same set up has been describved for LDAP: Using a centralized LDAP server for Authentication, but a local one for Authorization. To do that, we have to split the LDAP config (and allow multiple) from the Identity config.
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