One of the major problems in developing long running applications (and applications in general really) is cleanly handling shutdown. Among them a particular subproblem of phoenix singletons - what happens sometimes if one entity references the other (typically a global singleton), which has already been unloaded.

For instance, consider the following shutdown code in moduleA (using simplified Python syntax):

moduleA:
...
def shutdown():
    moduleB.callme()

there is no guarantee that moduleB has not been shut down yet. Now, if moduleB is also written in a delayed initialization fashion, ex:

moduleB:
...
impl = Impl()
...
def callme():
    if not impl.initialized():
        impl.initialize()
    return impl.callme()
...
def shutdown():
     impl.cleanup()

then what happens upon moduleA's shutdown is a reinitialization of the impl - one sort of a phoenix. It just went on me that instead of building a complex synchronization schemes to handle this cleanly, all I need to do to prevent this is just

moduleB:
 ...
def shutdown():
    impl.cleanup()
    del impl

and now as impl is just not there, the moduleA's attempt to reference it at shutdown will fail and throw - a much more appropriate behaviour in a given situation. This is less clean a solution, but how simple it is !