Defensive Programming vs. Let It Crash

Shared at the second meetup of Elixir Shanghai.

A talk highly inspired by Ferd's "The Zen of Erlang" post, but from a different angle.

What does Erlang bring to the table that could enable us to be less defensive and more confident about our code?

Transcript

1. Defensive(Programming
   vs.$Let$It$Crash
   Lou$Xun$aquarhead@ela.build$/$Ela$Workshop
   

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2. A"cri&cal"bug"happened...
   • Resources)exhausted
   • Users)disconnect,)system)becomes)laggy
   • Cascading)failure
   • Loads)of)log,)errors)everywhere
   • "What)the)hell)is)the)root$cause?"
   Lou$Xun$aquarhead@ela.build$/$Ela$Workshop
   

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3. You$started$debugging...
   • Follow&the&stacktrace
   • Looks&like&it's&some&cleanup&code
   • Something&is&wrong&in&the&try&block&(without&log!)
   • And&you&failed&to&recover&it&in&the&catch&block
   Lou$Xun$aquarhead@ela.build$/$Ela$Workshop
   

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4. What%if%you%had...
   • The%exact%loca,on%where%things%start%to%fail
   • The%exact%"context"%at%there
   • local%variables,%global%variables
   • object%state
   • etc..
   • Essen,al%informa,ons%are%very%easy%to%lose
   Lou$Xun$aquarhead@ela.build$/$Ela$Workshop
   

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5. You're'defensive,'because...
   • If$you$don't$catch,$a$raise$will$propagate$to$the$
   top...
   • and$crash$everything
   • You$write$code$that$you$wish$never$get$executed$in$
   produc;on
   • You're$hiding$a$broken$state$in$the$system
   Lou$Xun$aquarhead@ela.build$/$Ela$Workshop
   

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6. You$program$defensively
   because'you'cannot'simply
   Let$It$Crash
   Lou$Xun$aquarhead@ela.build$/$Ela$Workshop
   

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7. Erlang's)Process
   • Isolated
   • one+does+not+kill+all
   • Share+nothing
   • less+context+to+be+concerned
   • Lightweight
   • use+as+many+as+you+need
   • not+as+many+as+you+can
   Lou$Xun$aquarhead@ela.build$/$Ela$Workshop
   

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8. Other&ingredients
   • A#func(onal#language
   • much#less#state
   • Message#passing
   • async#opera(on
   • Monitor#Link
   • handle#process#down,#fail#as#a#unit
   • Preemp(ve#scheduling,#Distribu(on
   Lou$Xun$aquarhead@ela.build$/$Ela$Workshop
   

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9. Erlang/OTP
   • A#set#of#Erlang#libraries#and#design'principles
   • gen_server#etc..
   • maintain#state
   • unified#interface
   • supervisor
   • sole#job#is#supervising#children
   • start,#stop,#monitor,#restart
   Lou$Xun$aquarhead@ela.build$/$Ela$Workshop
   

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10. State%&%Restart
    • State
    • Why%restart%works?
    • What%can%restart%fix?
    • Cannot%fix%everything%(of%course)
    • Clean%state
    • Isn't%it%the%same%as%restar?ng%my%daemon?
    Lou$Xun$aquarhead@ela.build$/$Ela$Workshop
    

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11. Supervision+Tree
    Lou$Xun$aquarhead@ela.build$/$Ela$Workshop
    

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12. Structured(supervision
    • Processes(close(to(root(are(robust,(solid,(rarely(
    change
    • Processes(down(to(leaves(can(be(fragile,(change(a(
    lot,(fail(regularly
    • When(a(process(fail,(supervisor(restarts(according(
    to(the(structure
    • Use(structure(to(recover(from(failure
    • Not(code
    Lou$Xun$aquarhead@ela.build$/$Ela$Workshop
    

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13. Let$It$Crash
    • Don't'be'afraid'of'failures,'excep6ons'or'crashes
    • They'are'tools'we'can'use
    • No'more'defensive'programming,'only'code'as'if'
    everything'goes'right
    • Or'if'you'absolutely'know'how'to'deal'with'a'
    certain'failure
    • Otherwise'just'let$it$crash
    Lou$Xun$aquarhead@ela.build$/$Ela$Workshop
    

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14. vs.$Defensive$Programming
    • Supervision,Tree,vs.,Nested,try..catch
    • Process,vs.,?
    • Self7healing,in,produc;on
    • Fail,early,,fail,fast,aids,debugging
    • Avoid,wri;ng,unused,error,handling,code
    • Produc;vity,,readability
    Lou$Xun$aquarhead@ela.build$/$Ela$Workshop
    

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15. Thanks
    h"p:/
    /ferd.ca/the.zen.of.erlang.html
    Lou$Xun$aquarhead@ela.build$/$Ela$Workshop
    

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