Only lazy people haven’t written already about how global variables are evil. It started in 1973 when W. Wulf et al. claimed that “the non-local variable is a major contributing factor in programs which are difficult to understand.”
In a nutshell, global variables make code difficult or impossible to compose in ways which are different from what its original author expected.
start! is a static method of the
App class, which you have to declare as a child of their default parent
Sinatra::Base. To tell the app which TCP port to listen to you have to preconfigure it:
require 'sinatra/base' class App < Sinatra::Base get '/' do 'Hello, world!' end end App.set(:port, 8080) App.start!
What do you do if you want to start two web servers? For the purpose of testing this may be a pretty logical requirement. For example, since Zold is a distributed network, it is necessary to test how a number of servers communicate to each other. I can’t do that! There is absolutely no way. Because Sinatra is designed with the assumption that only one server may exist in the entire application scope.
Can this really be fixed? Let’s take a look at their code. Class
Sinatra::Base is essentially a Singleton, which is not supposed to have more than one instance. When we call
App.set(:port, 8080), the value
8080 is saved into an attribute of a single instance. The number
8080 becomes available for all methods of
Sinatra::Base, no matter what instance they are called from.
Technically speaking, their design is “globally scoped.”
Sinatra::Base treats the entire application as its scope of visibility. No matter who calls it, everything is visible, including what was created in previous calls and in previously instantiated objects. This “class” is a giant bag of global variables.
Every global variable is a troublemaker of that kind. While the application is small and its test coverage is low, global variables may not hurt. But the bigger the app and the more sophisticated its automated testing scenarios, the more difficult it will be to compose objects which depend on global variables, singletons, or class variables.
My recommendation? Under no circumstances even think about any global variables.