Modular Design in Elixir (ElixirConf EU 2019)

A talk given at ElixirConf EU 2019 in Prague. It talks about splitting big systems into smaller, simpler, more manageable modules that are easier to understand, test and work with.

Transcript

1. mkaszubowski94 Maciej Kaszubowski
2. mkaszubowski94 Maciej Kaszubowski THE BIG BALL OF NOUNS
3. mkaszubowski94 time complexity & size
4. mkaszubowski94 915
5. mkaszubowski94 1556
6. mkaszubowski94 527
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8. mkaszubowski94
9. mkaszubowski94 4 -7
10. mkaszubowski94 Maciej Kaszubowski Dealing with complexity
11. mkaszubowski94 https://www.confluent.io/blog/event-sourcing-cqrs-stream-processing-apache-kafka-whats-connection/ Event Sourcing / CQRS
12. mkaszubowski94 Domain Driven Design
13. mkaszubowski94 Microservices
14. mkaszubowski94 They are all about system decomposition
15. mkaszubowski94 ... and we can do the same in a monolith with a single PostgreSQL instance
16. mkaszubowski94 Maciej Kaszubowski Modular Software Design
17. mkaszubowski94 1972
18. mkaszubowski94 one big problem
19. mkaszubowski94
20. mkaszubowski94 Modular design smaller, simpler modules strong boundaries low coupling
21. mkaszubowski94 Each module can be: understood in isolation modified in isolation tested in isolation replaced or removed
22. mkaszubowski94 ok, but how?
23. mkaszubowski94 Maciej Kaszubowski Example Project
24. mkaszubowski94 create a new job name image url price publish John Alchemist create new job manage jobs find a job logout description date
25. mkaszubowski94 your jobs John Alchemist create new job manage jobs find a job logout my awesome job accept my other job cancel accept cancel yet another one and one more accepted this didn’t go well canceled accept cancel
26. mkaszubowski94 browse jobs John Alchemist create new job manage jobs find a job logout my awesome job my other job yet another one and one more filters 100$ 50$ 3500$ 99$ xxxx yyy zzz search
27. mkaszubowski94 start this job John Alchemist create new job manage jobs find a job logout Job Title I need something done start a job https://example.com/jobs/1234 100$
28. mkaszubowski94 defmodule MyApp.Job do use Ecto.Schema schema "jobs" do field :name, :string field :description, :string field :image_url, :string field :price, :integer field :datetime, :utc_datetime belongs_to :creator, ModularElixir.User belongs_to :contractor, ModularElixir.User end end
29. mkaszubowski94 JobContext - fetch_by_id() - search() - find_by_user_id() - start() - accept() - cancel() - publish()
30. mkaszubowski94 new feature: slugs
31. mkaszubowski94 start this job John Alchemist create new job manage jobs find a job logout my awesome job Don’t ask, you’ll get paid start a job https://example.com/jobs/my-awesome-job
32. mkaszubowski94 defmodule MyApp.Job do use Ecto.Schema schema "jobs" do field :name, :string field :description, :string field :image_url, :string field :paid, :boolean field :price, :integer field :slug, :string field :datetime, :utc_datetime belongs_to :creator, ModularElixir.User belongs_to :contractor, ModularElixir.User end end
33. mkaszubowski94 JobContext - fetch_by_id() - search() - find_by_user_id() - start() - accept() - cancel() - publish() - create() - find_by_slug()
34. mkaszubowski94 new feature: pay before publishing
35. mkaszubowski94 create a new job name image url price continue John Alchemist create new job manage jobs find a job logout description date
36. mkaszubowski94 pay for job pay 110$ John Alchemist create new job manage jobs find a job logout my awesome job job price………….………..100$ fee (10%)…………………….10$ total……………………………..110$ Don’t ask, you’ll get paid Your job will be published after the payment
37. mkaszubowski94 defmodule MyApp.Job do use Ecto.Schema schema "jobs" do field :name, :string field :description, :string field :image_url, :string field :slug, :string field :paid, :boolean field :price, :integer field :datetime, :utc_datetime belongs_to :creator, ModularElixir.User belongs_to :contractor, ModularElixir.User end end
38. mkaszubowski94 JobContext - fetch_by_id() - search() - find_by_user_id() - start() — accept() - cancel() - publish() - create()
39. mkaszubowski94 JobContext - fetch_by_id() - search() - find_by_user_id() - start() — accept() - cancel() - publish() - create()
40. mkaszubowski94 JobContext - fetch_by_id() - search() - find_by_user_id() - start() - accept() - cancel() - publish() - create()
41. mkaszubowski94 open-closed principle
42. mkaszubowski94 JobContext - fetch_by_id() - search() - find_by_user_id() - start() - accept() - cancel() - publish() - create()
43. mkaszubowski94 new feature: job reminders
44. mkaszubowski94 browse jobs John Alchemist create new job manage jobs find a job logout my awesome job my other job yet another one and one more filters 100$ 50$ 3500$ 99$ xxxx yyy zzz search Upcoming job Job starting in 54 minutes ! x
45. mkaszubowski94 defmodule MyApp.Job do use Ecto.Schema schema "jobs" do field :name, :string field :description, :string field :image_url, :string field :paid, :boolean field :slug, :string field :started, :boolean field :notification_sent, :boolean field :price, :integer field :datetime, :utc_datetime belongs_to :creator, ModularElixir.User belongs_to :contractor, ModularElixir.User end end
46. mkaszubowski94 JobContext - fetch_by_id() - search() - find_by_user_id() - start() - accept() - cancel() - publish() - create() - find_by_slug() - send_reminder()
47. mkaszubowski94 JobContext jobs
48. mkaszubowski94 jobs JobContext Payments preload jobs
49. mkaszubowski94 jobs JobContext User Context users join (jobs_count) Payments preload jobs
50. mkaszubowski94 jobs JobContext User Context users join (jobs_count) Payments preload jobs Chat Context
51. mkaszubowski94 jobs JobContext User Context users join (jobs_count) Payments preload jobs Chat Context JobSubtask Context
52. mkaszubowski94 jobs JobContext User Context users join (jobs_count) Payments JobSubtask Context preload jobs Chat Context JOB USER CONVERSATION MESSAGE PAYMENT SUBTASK
53. mkaszubowski94 In most projects: • few nouns / entities • a lot of interactions between them • logic is organised around the nouns • most actions represented by updates • a lot of coupling as a result
54. mkaszubowski94 We’re used to nouns • Object Oriented Programming • Databases • REST • CRUD
55. mkaszubowski94 nouns are not universal
56. mkaszubowski94 defmodule MyApp.Job do use Ecto.Schema schema "jobs" do field :name, :string field :description, :string field :image_url, :string field :paid, :boolean field :slug, :string field :started, :boolean field :notification_sent, :boolean field :price, :integer field :datetime, :utc_datetime belongs_to :creator, ModularElixir.User belongs_to :contractor, ModularElixir.User end end
57. mkaszubowski94 a “job” means many different things
58. mkaszubowski94 Maciej Kaszubowski Extracting behaviour
59. mkaszubowski94 what is the desired outcome?
60. mkaszubowski94 1. what is the desired outcome?
61. mkaszubowski94 1. what is the desired outcome? 2. what data do I need to do this?
62. mkaszubowski94 1. what is the desired outcome? 2. what data do I need to do this? 3. how can I get this data?
63. mkaszubowski94 1. what is the desired outcome? 2. what data do I need to do this? 3. how can I get this data? 4. what should be exposed in the interface?
64. mkaszubowski94 Maciej Kaszubowski Extracting Slugs
65. mkaszubowski94 start this job John Alchemist create new job manage jobs find a job logout my awesome job Don’t ask, you’ll get paid start a job https://example.com/jobs/my-awesome-job
66. mkaszubowski94 desired outcome: jobs should be accessible via human-friendly slugs
67. mkaszubowski94 required data: id-slug mapping for jobs
68. mkaszubowski94 SEO Context
69. mkaszubowski94 SEO Context slugs
70. mkaszubowski94 SEO Context slugs - register_slug() - get_id_by_slug() - get_slugs_by_ids() Interface:
71. mkaszubowski94 def show(conn, %{"job_id"  slug}) do job = slug  SeoContext.fetch_id_by_slug("job")  JobContext.fetch_by_id() #  end
72. mkaszubowski94 defp put_slugs(jobs) do ids = Enum.map(jobs, & &1.id) slugs = SeoContext.fetch_slugs_by_ids(ids, "job") Enum.map(jobs, fn job  Map.put(job, :slug, slugs[job.id]) end) end
73. mkaszubowski94 defmodule MyApp.Job do use Ecto.Schema schema "jobs" do field :name, :string field :description, :string field :image_url, :string field :paid, :boolean field :slug, :string field :started, :boolean field :notification_sent, :boolean field :price, :integer field :datetime, :utc_datetime belongs_to :creator, ModularElixir.User belongs_to :contractor, ModularElixir.User end end
74. mkaszubowski94 JobContext - fetch_by_id() - search() - find_by_user_id() - start() - accept() - cancel() - publish() - create() - find_by_slug() - send_reminder()
75. mkaszubowski94 SEO Context • slugs for other resources - easy to do
76. mkaszubowski94 SEO Context • slugs for other resources - easy to do • multiple slugs - easy to do
77. mkaszubowski94 SEO Context • slugs for other resources - easy to do • multiple slugs - easy to do • slugs are resolved at the controller level • no slugs in domain code
78. mkaszubowski94 no slugs in domain logic /api/jobs/my-awesome-job/subtasks from( s in Subtask, join: j in Job, on: s.job_id  j.id where: j.id  ^job_id or j.slug  ^job_id, # … ) /api/jobs/1/subtasks
79. mkaszubowski94 Maciej Kaszubowski Extracting Notifications
80. mkaszubowski94 browse jobs John Alchemist create new job manage jobs find a job logout my awesome job my other job yet another one and one more filters 100$ 50$ 3500$ 99$ xxxx yyy zzz search Upcoming job Job starting in 54 minutes ! x
81. mkaszubowski94 desired outcome: notification is sent before the start of a job
82. mkaszubowski94 required data: job information
83. mkaszubowski94 Job Reminders upcoming_jobs
84. mkaszubowski94 Job Reminders upcoming_jobs - put_job() - remove_job() Interface:
85. mkaszubowski94 Job Reminders upcoming_jobs - put_job() - remove_job() Interface: It's hard to use
86. mkaszubowski94 Interface: Job Context search Job Reminders sent_reminders
87. mkaszubowski94 defmodule Notifications.Worker do use GenServer def handle_info(:work, state) do Notifications.send_notifications(now()) Process.send_after(self(), :work, @interval) {:noreply, state) end end
88. mkaszubowski94 defmodule Notifications do def send_notifications(datetime) do jobs = JobContext.search(…) already_sent = fetch_already_sent() sender = FirebaseAdapter :ok = Logic.send( jobs, datetime, already_sent, sender ) end end
89. mkaszubowski94 defmodule Notifications.Logic do def send(jobs, datetime, already_sent, sender) do Enum.each(jobs, fn job -> if starting_soon?(job) && not_sent_yet?(job, already_sent) do sender.send_notification(job) end end) end end
90. mkaszubowski94 logic is isolated from the data source
91. mkaszubowski94 Logic desired outcome Notifications Job Context required data data source
92. mkaszubowski94 Logic desired outcome Notifications required data events stream
93. mkaszubowski94 Logic desired outcome Notifications required data
94. mkaszubowski94 Logic desired outcome Notifications required data TESTS
95. mkaszubowski94 defmodule MyApp.Job do use Ecto.Schema schema "jobs" do field :name, :string field :description, :string field :image_url, :string field :paid, :boolean field :slug, :string field :started, :boolean field :notification_sent, :boolean field :price, :integer field :datetime, :utc_datetime belongs_to :creator, ModularElixir.User belongs_to :contractor, ModularElixir.User end end
96. mkaszubowski94 JobContext - fetch_by_id() - search() - find_by_user_id() - start() - accept() - cancel() - publish() - create() - find_by_slug() - send_reminder()
97. mkaszubowski94 Maciej Kaszubowski Extracting Publishing
98. mkaszubowski94 desired outcome: job should be published only after the payment is made
99. mkaszubowski94 problem: the same model is used for multiple behaviours
100. mkaszubowski94 JobContext • adding / publishing jobs • searching for available jobs • managing ongoing jobs • archive / history • …
101. mkaszubowski94 JobContext - fetch_by_id() - search() - find_by_user_id() - start() - accept() - cancel() - publish() - create() - find_by_slug() - send_reminder()
102. mkaszubowski94 Job Publishing drafts Job Context jobs publish
103. mkaszubowski94 Job Publishing drafts Job Context jobs publish
104. mkaszubowski94 Job Publishing drafts Job Board jobs publish
105. mkaszubowski94 Job Publishing drafts Job Board jobs publish enforces job structure in params
106. mkaszubowski94 defmodule JobBoard do defmodule Job do @keys [:name, :description, ] @enforce_keys @keys defstruct @keys end def publish(%Job{} = job) do #  end end
107. mkaszubowski94 Job Publishing drafts Job Board jobs publish enforces job structure in params controls the rules for publishing
108. mkaszubowski94 Job Publishing - save_new_draft() - get_drafts() - pay()
109. mkaszubowski94 defmodule MyApp.JobBoard.Job do use Ecto.Schema @schema_prefix “job_board” schema "jobs" do field :name, :string field :description, :string field :image_url, :string field :paid, :boolean field :slug, :string field :started, :boolean field :notification_sent, :boolean field :price, :integer field :datetime, :utc_datetime belongs_to :creator, ModularElixir.User belongs_to :contractor, ModularElixir.User end end
110. mkaszubowski94 JobBoard - fetch_by_id() - search() - find_by_user_id() - start() - accept() - cancel() - publish() - create() - find_by_slug() - send_reminder()
111. mkaszubowski94 JobBoard - fetch_by_id() - search() - find_by_user_id() - start() - accept() - cancel() - publish() - create() - find_by_slug() - send_reminder()
112. mkaszubowski94 JobBoard • can be tested in isolation (without mocking payments) • logic without conditionals • no risk of showing unpublished jobs
113. mkaszubowski94 JobPublishing • rules for publishing can be changed in isolation • can be used alongside other modules
114. mkaszubowski94 Job Board JobBoard params are enforced by the interface Job Publishing
115. mkaszubowski94 Job Board JobBoard params are enforced by the interface Admin
116. mkaszubowski94 Job Board JobBoard params are enforced by the interface TESTS
117. mkaszubowski94 JobBoard jobs Ongoing Jobs jobs
118. mkaszubowski94 defmodule MyApp.JobBoard.Job do use Ecto.Schema @schema_prefix “job_board” schema "jobs" do field :name, :string field :description, :string field :image_url, :string field :paid, :boolean field :slug, :string field :started, :boolean field :notification_sent, :boolean field :price, :integer field :datetime, :utc_datetime belongs_to :creator, ModularElixir.User belongs_to :contractor, ModularElixir.User end end
119. mkaszubowski94 JobBoard - fetch_by_id() - search() - find_by_user_id() - start() - accept() - cancel() - publish() - create() - find_by_slug() - send_reminder()
120. mkaszubowski94 Maciej Kaszubowski Isolate important decisions
121. mkaszubowski94 start with a list of important decisions
122. mkaszubowski94 Important decisions • what are the rules for publishing jobs? • when/what notifications are sent? • which jobs are visible on the job board? • how is a contractor chosen for each job? • what happens after a job is started? when the job ends? • how payments work? • …
123. mkaszubowski94 then, create a module for each decision
124. mkaszubowski94 Maciej Kaszubowski Summary
125. mkaszubowski94 JobContext jobs
126. mkaszubowski94 JobContext • no clear responsibility • hard to change and understand • hard to test in isolation • grows with each new feature • hard to delete old features
127. mkaszubowski94 Job Publishing drafts Job Board jobs Reminders devices SEO slugs Ongoing Jobs jobs Job Archive jobs
128. mkaszubowski94 focused on behaviour, not nouns
129. mkaszubowski94 Smaller modules • clear responsibilities for each module • easy to change, refactor, understand • easy to test in isolation • tend to stay small • trivial to delete if no longer necessary • written once and then forgotten
130. mkaszubowski94 this is not a silver bullet
131. mkaszubowski94 Downsides • statistics / reports / admin panels • UI has to change sometimes • More initial work • Performance (?) • ...?
132. mkaszubowski94 it’s just a mindset
133. mkaszubowski94 focus on behaviour, not data
134. mkaszubowski94 organise code around behaviour
135. mkaszubowski94 encapsulate important decisions
136. mkaszubowski94 Maciej Kaszubowski THANKS!