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You ain't gonna need to write a GenServer

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.

Ulisses Almeida

May 25, 2019

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  1. 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
  2. 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/
  3. 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
  4. 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
  5. Holy %#@! ~ 200 times slower
 ~ 10 times more

    memory 120km/h 1.6km/h ~75 times slower 10 times bigger
  6. 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
  7. 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
  8. 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
  9. 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
  10. 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.
  11. 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
  12. 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
  13. 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
  14. 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
  15. 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
  16. 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
  17. 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.
  18. 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
  19. 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
  20. 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
  21. 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?
  22. We covered a lot 
 of considerations before 

    write a GenServer” decision And they are still not over
  23. 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