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

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

4.
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

5.
Learn Functional Programming with Elixir
■ 20% off with
Elixir_Ukraine_2019
promo code

6.
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/

7.
Ok, GenServers
A quick recap

8.
https://twitter.com/ulissesalmeida/status/1125455706470088704

9.
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

10.
Shades of a GenServer
■ Agent
■ Task
■ Task.Supervisor
■ Supervisor
■ DynamicSupervisor
■ GenStage
■ …

11.
CalculatorServer
Baby
1+1?
1.
2.
3.
From: Baby
1+1?
4.
1+1=2
5.

12.
CalculatorServer
Baby
7.
6.
From:
MyCalcGenServer
2
8.

13.
Live demo
“…maybe I should have
recorded a video”

14.
Wut!?

15.
hahahahah GenServer
bottleneck
omg what I have
done!?

16.
Holy %#@!
~ 200 times slower
~ 10 times more
memory
120km/h
1.6km/h
~75 times slower
10 times bigger

17.
What happened!?
🤔

18.
CalculatorServer
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAA

19.
Without GenServer…
🤔

20.
CalculatorServer

21.
CalculatorServer
☠🔥
🔥 🔥
🔥
Omg the server is on fire!
!

22.
Why people want it so
much?
Can we play just with
functions?

23.
Erlang OTP
■ Process hierarchy
■ Restarting strategies
■ Concurrency
■ GenServer plays the
central role

24.
wait, you said
concurrency…🤔

25.
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

26.
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

27.
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

28.
Fear of missing
out?
Am I a proper Elixir
developer If I'm not
doing GenServers?

29.
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

30.
Elixir developers, no
shame in never had to
create a GenServer.

31.
Ecosystem got you
covered
Or… things that you should consider
before writing a GenServer

32.
Let's think about a typical
web application

33.
Phoenix
Elixir
Database
Legacy
Legacy of the Legacy
Users

34.
Users & App

35.
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.

36.
GenServers!

37.
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

38.
App &
Database

39.
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

40.
GenServers!!!!

41.
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

42.
App &Other
Services

43.
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

44.
GenSeerveerss!!!

45.
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

46.
NoSQL storage

47.
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

48.
GenSeeerveerS!!!!!

49.
WITH ETS!!!!!!!

50.
Phoenix
ElixirElixir
ETS table
Phoenix
Elixir
ETS table
Phoenix
Elixir
ETS table
User

51.
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.

52.
Serializable
operations

53.
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

54.
…GenServer?

55.
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

56.
Background
tasks

57.
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

58.
Dynamic Supervisors
& Task!!!

59.
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?

60.
We covered a lot
of considerations before
“let's write a GenServer” decision
And they are still not
over

61.
GenServers are
fundamental

62.
Elixir frameworks and
libraries cover most of
the use cases that you
would need to write one

63.
But it doesn’t mean
you should not learn
about GenServer

64.
You need to know
GenServer to tune
their configurations

65.
When stuff goes
wrong, you’ll need
to debug and fix it.

66.
You won’t might need
to write a GenServer
95% of time…

67.
But that 5%…

68.
Then, keep
learning!

69.
Remember

70.
Putting a GenServer in
the wrong place brings
more harm than good

71.
It’s good to compare
and to discuss the
tradeoffs with your
team.

72.
There’s no shame
in never need to
write a GenServer

73.
Thank you

74.
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