Handwaving about Dependencies

One of the things I think about a lot is Dependencies, or specifically, how to draw them out in layers and learn to spot (and avoid) cyclic dependencies.

I originally gave this talk at the Melbourne Ruby meetup on October 26, 2016.

Transcript

1. Handwaving about Dependencies Sebastian von Conrad @vonconrad
4. I care about Dependencies. A lot.
6. H2 0
7. H2 0
8. H2 0
9. What are Dependencies?
10. “Need to Know.”
11. It’s about knowledge.
12. If I need to know about something to do my job, I depend on it.
13. Dependencies are unavoidable.
14. Otherwise, nothing would be connected.
15. Dependencies have directions.
16. A B
17. A B
19. A B
20. This means that we can describe things in layers. H2 0
21. Most things in the computing world is layered.
22. Internet.
23. MAC (Media Access Control) IP TCP HTTP TLS/SSL Browser
24. Application “Stack.”
25. Infrastructure Database Backend Frontend
26. Systems.
27. Application
28. Application
29. A B C D E F G H
30. Code.
31. Controller Model View
32. Controller Model View
33. Controller Model View Presenter Use Case
34. More granular code.
35. Order Product Customer
36. Layers are good. H2 0
37. Layers allow us to reason about one thing at a time.
38. Infrastructure Database Backend Frontend
39. MAC (Media Access Control) IP TCP HTTP TLS/SSL Browser
40. Controller Model View Presenter Use Case
41. Layers allow us to change one thing at a time.
42. MAC (Media Access Control) IP TCP HTTP TLS/SSL Browser Browser
43. Application Application
44. Infrastructure Database Backend Frontend in last week’s JS framework Frontend in this week’s JS framework
45. Cyclic dependencies. H2 0
46. A.k.a. Circular dependencies.
47. A B
48. A B C
49. A B C D E F
51. Everything always depends. See what I did there?
52. Is lasagna the best food in the world?
53. Well, it depends.
54. Are Scandinavians the best people in the world?
55. Well, it depends. !
56. Should we refactor this bit of code?
57. Well, it depends.
58. Do we need to write tests for this?
59. Well, it depends.
60. Is Ruby the right tool for this problem?
61. Well, it depends.
62. Can we have a cyclic dependency?
63. Not while I live and breathe.
64. You’re fired!
65. Okay, why is it so bad?
66. Because it’s no longer layered.
67. A B C D E F
68. MAC (Media Access Control) IP TCP HTTP TLS/SSL Browser
69. If TCP needed knowledge of browsers to do its job, we could no longer clearly separate them.
70. One cannot be reasoned about without the other.
71. And one cannot change without the other.
72. They become tightly coupled to each other.
73. But this, of course, is an absurd notion.
74. Our communication layers aren’t about to become cyclic.
75. Probably not our stacks either.
76. But we see it all the time in our system diagrams.
77. And we sure as hell see them in our code.
78. It looks something like this.
79. Order Product Customer
83. Order Product Customer
84. Order Product Customer
89. Order Product Customer
90. And applications aren’t this simple.
91. Order Product Customer Invoice Address Coupon Offer Category
93. This is why I spend so much time thinking about dependencies.
94. This makes our jobs more difficult.
95. More difficult to learn systems.
96. More difficult to change systems.
97. More difficult to lower the barrier of entry.
98. Unfortunately, Rails teaches us to do this.
99. So what can we do?
100. Step away from the keyboard.
101. Read up on the Acyclic Dependencies Principle.
102. Draw out your dependencies.
103. Get more clarity.
104. Learn to spot the cyclic ones.
105. And kill them with fire.
106. Tack så mycket! Sebastian von Conrad @vonconrad