On design patterns: When should I use the singleton?

Question

The glorified global variable - becomes a gloried global class. Some say breaking object-oriented design.

Give me scenarios, other than the good old logger where it

Answer 1

A practical example of a singleton can be found in Test::Builder, the class which backs just about every modern Perl testing module. The Test::Builder singleton stores and brokers the state and history of the test process (historical test results, counts the number of tests run) as well as things like where the test output is going. These are all necessary to coordinate multiple testing modules, written by different authors, to work together in a single test script.

The history of Test::Builder's singleton is educational. Calling new() always gives you the same object. First, all the data was stored as class variables with nothing in the object itself. This worked until I wanted to test Test::Builder with itself. Then I needed two Test::Builder objects, one setup as a dummy, to capture and test its behavior and output, and one to be the real test object. At that point Test::Builder was refactored into a real object. The singleton object was stored as class data, and new() would always return it. create() was added to make a fresh object and enable testing.

Currently, users are wanting to change some behaviors of Test::Builder in their own module, but leave others alone, while the test history remains in common across all testing modules. What's happening now is the monolithic Test::Builder object is being broken down into smaller pieces (history, output, format...) with a Test::Builder instance collecting them together. Now Test::Builder no longer has to be a singleton. Its components, like history, can be. This pushes the inflexible necessity of a singleton down a level. It gives more flexibility to the user to mix-and-match pieces. The smaller singleton objects can now just store data, with their containing objects deciding how to use it. It even allows a non-Test::Builder class to play along by using the Test::Builder history and output singletons.

Seems to be there's a push and pull between coordination of data and flexibility of behavior which can be mitigated by putting the singleton around just shared data with the smallest amount of behavior as possible to ensure data integrity.