You ain't gonna need to write a GenServer

GenServers plays a central role in Elixir OTP applications. However, when you overuse it, it can become an anti-pattern and introduce bottlenecks in your system.

In this talk you'll learn:

* What's a GenServer
* When GenServer can harm
* GenServer FOMO (fear of missing out)
* How Elixir ecosystem get you covered

After this talk, you'll understand how putting a GenServer in the wrong place can slowdown in your entire system. You'll understand how the Elixir frameworks and libraries enable you to have reliable concurrency applications. You'll feel no shame of being an Elixir developer and never had to write yourself a GenServer.

Transcript

1. You ain't gonna need to write a GenServer by Ulisses Almeida
4. Summary ▪ What is GenServer? ▪ How can you harm your app by putting GenServer in the wrong place? ▪ Why do people think that they need a GenServer all the time? ▪ Considerations before writing a GenServer
5. I’m Ulisses Almeida ▪ Elixir Developer at The Coingaming Group ▪ Brazilian ! ▪ @ulissesalmeida
6. Learn Functional Programming with Elixir ▪ 20% off with Elixir_Brazil_2019 promo code ▪ 3 e-books giveaways
7. Who 
 We Are The Coingaming Group are the crypto gaming pioneers who operate the world’s leading bitcoin gaming and betting brands. https://coingaming.io/
11. Why Tallinn? " ★ Low renting costs ★ Free transportation for residents ★ E-estonia ★ Safe ★ Low taxes ★ Accessible travels ๏ Cold and Dark ☃ ๏ Estonian is the first language " ๏ Russian is the second language % ๏ Eastern/nordic culture is different
12. Ok, GenServers A quick recap
13. https://twitter.com/ulissesalmeida/status/1125455706470088704
18. GenServer ▪ Useful for: Client-Server relation ▪ Keep state ▪ Execute code asynchronously (handle_cast) ▪ Synchronously too (handle_call) ▪ Provide tracing and error reporting features ▪ Can be supervised A process with a powerful set of features
19. Shades of a GenServer ▪ Agent ▪ Task ▪ Task.Supervisor ▪ Supervisor ▪ DynamicSupervisor
20. CalculatorServer Baby 1+1? 1. 2. 3. From: Baby
 1+1? 4. 1+1=2 5.
21. CalculatorServer Baby 7. 6. From: 
 MyCalcGenServer
 2 8.
22. Live demo “…maybe I should have recorded a video”
32. UÉ!?
33. hahahahah GenServer
 bottleneck
 omg what I have 
 done!?
34. Holy %#@! ~ 200 times slower
 ~ 10 times more memory 120km/h 1.6km/h ~75 times slower 10 times bigger
35. What happened!?
36. CalculatorServer AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAA
37. Without GenServer…
38. CalculatorServer
39. CalculatorServer ☠ ta pegando fogo bicho! !
40. Why people want it so much? Can we play just with functions?
41. Erlang OTP ▪ Process hierarchy ▪ Restarting strategies ▪ Concurrency ▪ GenServer plays the central role
42. wait, you said concurrency…
43. GenServer checklist ▪ Model state accessed by multiple processes ▪ Run multiple tasks concurrently ▪ Communicate between servers ▪ Gracefully handle clean startup and exit concerns https://pragprog.com/book/tvmelixir/adopting-elixir
44. CalculatorServer checklist ❌Model state accessed by multiple processes ❌Run multiple tasks concurrently ❌Communicate between servers ❌Gracefully handle clean startup and exit concerns https://pragprog.com/book/tvmelixir/adopting-elixir
45. CalculatorServer has no reason to be a GenServer A GenServer must never be used for code organization purposes https://hexdocs.pm/elixir/GenServer.html GenServer to organize 
 domain, business
 contexts GenServer to model 
 runtime properties
 of your system
46. Fear of missing out? Am I a proper Elixir developer If I'm not doing GenServers?
47. I have been working with Elixir for ~3 years ▪ I never wrote a production GenServer ▪ I wrote GenServers only in recruitment processes and for fun ▪ I deleted some unnecessary production GenServers ▪ I know excellent developers that never wrote one
48. Elixir developers, no shame in never had to create a GenServer.
49. Ecosystem got you covered Or… things that you should consider before writing a GenServer
50. Let's think about a typical web application
51. Phoenix Elixir Database Legacy Legacy of the Legacy Users
53. Users & App
54. Users -> App checklist ✔ We need to handle multiple users connections concurrently ✔ Other processes might send messages to that connection ✔ We have to handle connections openings and closings gracefully.
55. GenServers!
56. However… ▪ Each user’s request has a process that runs concurrently ▪ You can focus on the functional part ▪ (functional part? functional programming ) Phoenix and Cowboy handles that for you
57. App & Database
58. App & Database checklist ✔Databases are an external service communication ✔We have to provide a limited number of connections to a lot of processes ✔These connections have to run ✔We need to handle connection openings and closings gracefully
59. GenServers!!!!
60. However… ▪ Ecto supports a pool of connections ▪ Ecto handle that concurrently ▪ Work here is also done ▪ You can focus on the business Ecto handles that for you
61. App &Other Services
62. App & Other Services checklist ✔ Usually, we communicate through a network ✔ We might need to offer a pool as a backpressure mechanism to not bring down weak services ✔ These pools have to run multiple requests concurrently ✔ Gracefully handle connections opening and closings
63. GenSeerveerss!!!
64. However… ▪ You can start simple, for example, no pool ▪ Watch out for big timeouts ▪ Circuit breakers can help you out ▪ Choose the libraries, and you can focus on features Hackney and other libraries handles that for you
65. NoSQL storage
66. Not everything is stored in SQL databases ▪ Examples: caching, session, users presence ▪ Transient or Persistent? ▪ How big will it be? ▪ Write vs Read ✔ This data will be accessible by multiple processes
67. GenSeeerveerS!!!!!
68. WITH ETS!!!!!!!
69. Phoenix Elixir Elixir ETS table Phoenix Elixir ETS table Phoenix Elixir ETS table User
70. However… ▪ A better abstraction reveals intention, for example, Cachex vs GenServer + ETS ▪ How does your deploy work? ▪ Shutdown and booting the app will lose GenServer/ ETS data ▪ Redis for the rescue ▪ Discuss the right tool for the job, compare the tradeoffs You might want to use a specific abstraction.
71. Serializable operations
72. Serializable operations Some resources can’t be changed at same time 1 1 5 I want to withdraw 2 I want to withdraw 6 Company Wallet ✔ Manage state accessed by multiple processes
73. …GenServer?
74. However… ▪ You would need an Erlang cluster to run a global GenServer ▪ What happens with network partition? ▪ Use your database power ▪ Transactions or locks in the database ▪ Discuss with your team, compare the tradeoffs You environment might not allow. a.k.a Heroku
75. Background tasks
76. Tasks outside of a request/response cycle ▪ Welcome e-mail ▪ Push notifications ▪ Scheduled tasks ✔ We need to spawn or schedule these background tasks ✔ We need to run the background tasks concurrently
77. Dynamic Supervisors 
 & Task!!!
78. However… ▪ GenServers aren’t persistent ▪ Shutting downs might lose data ▪ RabbitMQ has persistent queues ▪ Libraries that use Redis to store the jobs and their arguments ▪ Discuss with your team, compare the tradeoffs How to handle the failures? Do you need some persistence?
79. We covered a lot 
 of considerations before 
 “let's write a GenServer” decision And they are still not over
80. GenServers are fundamental
81. Elixir frameworks and libraries cover most of the use cases that you would need to write one
82. But it doesn’t mean you should not learn about GenServer
83. You need to know GenServer to tune their configurations
84. When stuff goes wrong, you’ll need to debug and fix it.
85. You won’t might need to write a GenServer 95% of time…
86. But that 5%…
87. Then, keep learning!
88. Remember
89. Putting a GenServer in the wrong place brings more harm than good
90. It’s good to compare and to discuss the tradeoffs with your team.
91. There’s no shame in never need to write a GenServer
92. Thank you
93. References ▪ Adopting Elixir - https://pragprog.com/book/tvmelixir/adopting- elixir ▪ Spawn or not spawn - https://www.theerlangelist.com/article/ spawn_or_not ▪ You may not need a GenServer - https://pragtob.wordpress.com/ 2019/04/24/you-may-not-need-genservers-and-supervision-trees ▪ GenServer (Elixir Docs) - https://hexdocs.pm/elixir/GenServer.html ▪ Elixir discussion forum - https://elixirforum.com/t/you-may-not- need-genservers-and-supervision-trees/12947