The Singleton Module and Its Pattern In Ruby

The singleton design pattern allows for the creation of an object that is restricted to one single instance. No matter how big or complex your application is, there will only ever be one. This has caused some controversy about the quality and maintainability of code that leverages it. Yet, Ruby’s standard library contains a module that implements this pattern. Let’s take a closer look at this module and the “controversy” around its implemented pattern. Let’s also consider singletons in Ruby and the purpose they serve for the language in general.

Transcript

1. The Singleton Module and Its Pattern In Ruby Ruby Conf 2019 : Nashville, TN : 11.19.2019
2. Hello! I’m Mike Calhoun You can find me at places on the internet where you would find people as @comike011 2
3. 3 We’re Hiring!
4. Overview 2. Singleton.rb 3. Ruby Singleton Class 1. The Singleton Pattern 4
5. How Did We Get Here? Kismet or how curiosity got out of hand 5
9. “ The Singleton module implements the Singleton pattern 9
10. THANKS! Any questions? You can find me at @comike011 10
11. Overview 2. Singleton.rb 3. Ruby Singleton Class 1. The Singleton Pattern 11
12. 1. The Singleton Pattern 12
13. What is The Singleton Pattern? ◦ Restricts to a single instance 13
14. What is The Singleton Pattern? ◦ Restricts to a single instance ◦ Provides single access point to the resource 14
15. What is The Singleton Pattern? ◦ Restricts to a single instance ◦ Provides single access point to the resource ◦ Not a global variable 15
16. What is The Singleton Pattern? ◦ Restricts to a single instance ◦ Provides single access point to the resource ◦ Not a global variable ◦ Problematic misuse 16
17. Are Singletons Bad? Bart Jacobs, January 16, 2018 17
18. “ I agree that the singleton pattern seems convenient and it may occasionally be warranted to use it, but the drawbacks far outweigh the benefits. 18
19. 6 Reasons Why You Should Avoid Singletons David Tanzer, last accessed 9/18/2019 19
20. “ Yes, I am really writing a blog about why the “Singleton Pattern” is bad. In 2016. But it comes up in discussions time and again, so here is a list of reasons why you should avoid singletons. 20
21. Why The Singleton Pattern Sucks and You Should Avoid It WDIS, February 29, 2016 21
22. “ Most of these patterns [from GoF] are straightforward and frankly plain logical...However, there is one specific pattern that gets hate from every coding-related website on the Internet...the infamous Singleton Pattern, probably the most overused and frequently misplaced programming anti-pattern of all time 22
23. Why Singletons are Evil Scott Densmore, May 25, 2004 23
24. Singletons Are Pathological Liars Miško Hevery, April 17, 2008 24
25. Patterns I Hate: #1 Singletons Alex Miller, July 3, 2007 25
26. Singletons I Love You, But You’re Bringing Me Down codingwithoutcomments, October 4, 2008 26
27. Singletons Are Great and You Should Use Them 27
28. Singletons Are Great and You Should Use Them Citation Needed 28
29. Concerns ◦ Concurrent access to a shared resource 29
30. Concerns ◦ Concurrent access to a shared resource ◦ Global state 30
31. Concerns ◦ Concurrent access to a shared resource ◦ Global state ◦ Violation of SRP 31
32. Concerns ◦ Concurrent access to a shared resource ◦ Global state ◦ Violation of SRP ◦ Testing is problematic 32
33. Concerns ◦ Concurrent access to a shared resource ◦ Global state ◦ Violation of SRP ◦ Testing is problematic ◦ Encourages tight coupling 33
34. Common Appropriate Use Cases ◦ Unidirectional flow of data 34
35. Common Appropriate Use Cases ◦ Unidirectional flow of data ◦ Resource requested from multiple and disparate areas 35
36. Common Appropriate Use Cases ◦ Unidirectional flow of data ◦ Resource requested from multiple and disparate areas ◦ Logging 36
37. Common Appropriate Use Cases ◦ Unidirectional flow of data ◦ Resource requested from multiple and disparate areas ◦ Logging ◦ Configurations 37
38. Common Appropriate Use Cases ◦ Unidirectional flow of data ◦ Resource requested from multiple and disparate areas ◦ Logging ◦ Configurations ◦ Nothing modified within the application 38
39. Overview 2. Singleton.rb 3. Ruby Singleton Class 1. The Singleton Pattern 39
40. 2. singleton.rb 40
41. singleton.rb ◦ .new and .allocate are made private 41
42. singleton.rb ◦ .new and .allocate are made private ◦ .instance added 42
43. singleton.rb ◦ .new and .allocate are made private ◦ .instance added ◦ .clone and .dup raise TypeError 43
64. Overview 2. Singleton.rb 3. Ruby Singleton Class 1. The Singleton Pattern 64
65. 3. Ruby Singleton Class 65
66. Singleton class ◦ Also referred to as ‘eigenclasses’ or ‘metaclasses’ 66
67. Singleton class ◦ Also referred to as ‘eigenclasses’ or ‘metaclasses’ ◦ Somewhat hidden 67
68. Singleton class ◦ Also referred to as ‘eigenclasses’ or ‘metaclasses’ ◦ Somewhat hidden ◦ Essential to the Ruby Object Model 68
70. The Ruby Object Model ◦ BasicObject, Object, <Defined> 70
71. The Ruby Object Model ◦ BasicObject, Object, <Defined> ◦ Modules are included above their including object 71
77. An Example What and where is the singleton class? 77
79. 79 Porch Cat "PK"
80. 80 Porch Cat "PK" Pearl
122. Overview 2. Singleton.rb 3. Ruby Singleton Class 1. The Singleton Pattern 122
124. THANKS! 124 Mike Calhoun (@comike011)