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

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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

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I’m Ulisses Almeida ■ Elixir Developer at The Coingaming Group ■ Brazilian ! ■ @ulissesalmeida

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Learn Functional Programming with Elixir ■ 20% off with Elixir_Brazil_2019 promo code ■ 3 e-books giveaways

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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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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

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Ok, GenServers A quick recap

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

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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

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Shades of a GenServer ■ Agent ■ Task ■ Task.Supervisor ■ Supervisor ■ DynamicSupervisor

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CalculatorServer Baby 1+1? 1. 2. 3. From: Baby
 1+1? 4. 1+1=2 5.

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CalculatorServer Baby 7. 6. From: 
 MyCalcGenServer
 2 8.

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

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UÉ!?

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hahahahah GenServer
 bottleneck
 omg what I have 
 done!?

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Holy %#@! ~ 200 times slower
 ~ 10 times more memory 120km/h 1.6km/h ~75 times slower 10 times bigger

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What happened!?

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CalculatorServer AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAA

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Without GenServer…

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CalculatorServer

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CalculatorServer ☠ ta pegando fogo bicho! !

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Why people want it so much? Can we play just with functions?

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Erlang OTP ■ Process hierarchy ■ Restarting strategies ■ Concurrency ■ GenServer plays the central role

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wait, you said concurrency…

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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

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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

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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

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Fear of missing out? Am I a proper Elixir developer If I'm not doing GenServers?

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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

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Elixir developers, no shame in never had to create a GenServer.

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Ecosystem got you covered Or… things that you should consider before writing a GenServer

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Let's think about a typical web application

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Phoenix Elixir Database Legacy Legacy of the Legacy Users

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Users & App

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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.

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GenServers!

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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

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App & Database

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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

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GenServers!!!!

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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

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App &Other Services

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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

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GenSeerveerss!!!

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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

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NoSQL storage

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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

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GenSeeerveerS!!!!!

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WITH ETS!!!!!!!

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Phoenix Elixir Elixir ETS table Phoenix Elixir ETS table Phoenix Elixir ETS table User

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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.

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Serializable operations

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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

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…GenServer?

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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

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Background tasks

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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

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Dynamic Supervisors 
 & Task!!!

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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?

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We covered a lot 
 of considerations before 
 “let's write a GenServer” decision And they are still not over

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GenServers are fundamental

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Elixir frameworks and libraries cover most of the use cases that you would need to write one

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But it doesn’t mean you should not learn about GenServer

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You need to know GenServer to tune their configurations

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

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You won’t might need to write a GenServer 95% of time…

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But that 5%…

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Then, keep learning!

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Remember

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Putting a GenServer in the wrong place brings more harm than good

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It’s good to compare and to discuss the tradeoffs with your team.

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There’s no shame in never need to write a GenServer

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Thank you

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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