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You ain't gonna need to write a GenServer v1.1

You ain't gonna need to write a GenServer v1.1

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.

It was presented in Elixir Club Ukraine


Ulisses Almeida

September 28, 2019

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  1. You ain't gonna need to write a GenServer by Ulisses

  2. 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
  3. I’m Ulisses Almeida ▪ Elixir Developer at The Coingaming Group

    ▪ @ulissesalmeida
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  9. 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
  10. Learn Functional Programming with Elixir ▪ 20% off with Kyiv_Ukraine_2019

    promo code
  11. 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/
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  14. Ok, GenServers A quick recap

  15. https://twitter.com/ulissesalmeida/status/1125455706470088704

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  20. GenServer ▪ Useful for: Client-Server relationship ▪ 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
  21. Shades of a GenServer ▪ Agent ▪ Task ▪ Task.Supervisor

    ▪ Supervisor ▪ DynamicSupervisor ▪ GenStage ▪ …
  22. CalculatorServer Baby 1+1? 1. 2. 3. From: Baby
 1+1? 4.

    1+1=2 5.
  23. CalculatorServer Baby 7. 6. From: 
 2 8.

  24. Live demo “…maybe I should have recorded a video”

  25. Wut!?

  26. hahahahah GenServer
 omg what I have 

  27. Holy %#@! ~ 200 times slower
 ~ 10 times more

    memory 120km/h 1.6km/h ~75 times slower 10 times bigger
  28. What happened!?


  30. Without GenServer…

  31. CalculatorServer

  32. CalculatorServer ☠ Omg the server is on fire! !

  33. Why people want it so much? Can we play just

    with functions?
  34. Erlang OTP ▪ Process hierarchy ▪ Restarting strategies ▪ Concurrency

    ▪ GenServer plays the central role
  35. wait, you said concurrency…

  36. 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
  37. 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
  38. 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
  39. Fear of missing out? Am I a proper Elixir developer

    If I'm not doing GenServers?
  40. I have been working with Elixir for ~3 years ▪

    I hardly often write a GenServer ▪ I mostly wrote GenServers in recruitment processes and for fun ▪ I deleted some unnecessary production GenServers ▪ I know excellent developers that never wrote one
  41. Elixir developers, no shame in never had to create a

  42. Ecosystem got you covered Or… things that you should consider

    before writing a GenServer
  43. Let's think about a typical web application

  44. Phoenix Elixir Database Legacy Legacy of the Legacy Users

  45. None
  46. Users & App

  47. 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.
  48. GenServers!

  49. 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
  50. App & Database

  51. 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 concurrently ✔We need to handle connection openings and closings gracefully
  52. GenServers!!!!

  53. 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
  54. App &Other Services

  55. 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
  56. GenSeerveerss!!!

  57. 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
  58. NoSQL storage

  59. 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
  60. GenSeeerveerS!!!!!

  61. WITH ETS!!!!!!!

  62. Phoenix Elixir Elixir ETS table Phoenix Elixir ETS table Phoenix

    Elixir ETS table User
  63. 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.
  64. Serializable operations

  65. 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
  66. …GenServer?

  67. 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
  68. Background tasks

  69. 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
  70. Dynamic Supervisors 
 & Task!!!

  71. 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?
  72. We covered a lot 
 of considerations before 

    write a GenServer” decision And they are still not over
  73. GenServers are fundamental

  74. Elixir frameworks and libraries cover most of the use cases

    that you would need to write one
  75. But it doesn’t mean you should not learn about GenServer

  76. You need to know GenServer to tune their configurations

  77. When stuff goes wrong, you’ll need to debug and fix

  78. You won’t might need to write a GenServer 95% of

  79. But that 5%…

  80. Then, keep learning!

  81. Remember

  82. Putting a GenServer in the wrong place brings more harm

    than good
  83. It’s good to compare and to discuss the tradeoffs with

    your team.
  84. There’s no shame in never need to write a GenServer

  85. Thank you

  86. 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