The
Even-Darker Art
of
Rails Engines

@lazyatom

github.com/lazyatom/engines

Rails 2.3

history

November 2005

it’s distracting!

reuse is overrated!

Evil! Evil!
Evil!
Evil!
Evil!
Evil!
Evil!
Evil! Shit!

eek!

appable_plugins

desert

merb slices?

engines

!\"#$%&!\"#$'#(%&$#)&!\"#$%&*$+,#-.!)
!\"#$%&'()
$$*%&'()$++,$-'%!.%&'()./0#'12%)/'0
\"0!
3$405/0\"6$%1\"$7-(5/06$8/)9$%0$%77:$!/1\";)'1<=
!\"#$\"05/0\">
$$9%6.%77.!/1\";)'1<>
\"0!
3$?\")(106$)1(\"$/#$)9\"$\"05/0\"$69/76$8/)9$%$1'()/05$#/-\"
!\"#$1'()\"!>
$$@/-\"=\"A/6)>B1'()/05.#/-\"C
\"0!
!\"#$D/\"8.7%)9
$$@/-\"=E'/0B!/1\";)'1<F$G%77GF$GD/\"86GC
\"0!

app/controllers/
models/
helpers/
views/
metal/

*$+,#-&\"**&23-'!3$$(!&*$+,#-523-'!3$$(!.!)
;-%66$H-(5/0I'0)1'--\"1/0/1**$#2\"'#3-43-'!3$$(!
$$!\"#$/0!\"A
$$$$1\"0!\"1$J)\"A)$,K$LM\"--'F$H-(5/0NL
$$\"0!
\"0!

9\"-7\"1$J%--
won’t load from your plugin

Ticket #1905

config/routes.rb

*$+,#-&23-6#,&!3+'(%.!)
O;)/'0I'0)1'--\"1JJ?'()/05JJ?'()\"6=!1%8$!'$+2%7+
$$2%7=;'00\";)$G:7%)9GF$J;'0)1'--\"1$,K$L7-(5/0LF$
$$$$$$$$$$$$$$$$$$$$$$$J%;)/'0$,K$L/0!\"AL
\"0!

top precedence

in your plugin:
O;)/'0I'0)1'--\"1JJ?'()/05JJ?'()\"6=!1%8$!'$+2%7+
$$2%7=;'00\";)$G:7%)9GF$J;'0)1'--\"1$,K$L7-(5/0LF$
$$$$$$$$$$$$$$$$$$$$$$$J%;)/'0$,K$L/0!\"AL
\"0!
in your app:
O;)/'0I'0)1'--\"1JJ?'()/05JJ?'()\"6=!1%8$!'$+2%7+
$$3$===$')9\"1$1'()\"6$===
$$2%7=;'00\";)$G:7%)9GF$J;'0)1'--\"1$,K$L%77LF$
$$$$$$$$$$$$$$$$$$$$$$$J%;)/'0$,K$L/0!\"AL
$$3$===$')9\"1$1'()\"6$===
\"0!

in your plugin:
O;)/'0I'0)1'--\"1JJ?'()/05JJ?'()\"6=!1%8$!'$+2%7+
$$2%7=;'00\";)$G:7%)9GF$J;'0)1'--\"1$,K$L7-(5/0LF$
$$$$$$$$$$$$$$$$$$$$$$$J%;)/'0$,K$L/0!\"AL
\"0!
in your app:
O;)/'0I'0)1'--\"1JJ?'()/05JJ?'()\"6=!1%8$!'$+2%7+
$$3$===$')9\"1$1'()\"6$===
$$2%7=;'00\";)$G:7%)9GF$J;'0)1'--\"1$,K$L%77LF$
$$$$$$$$$$$$$$$$$$$$$$$J%;)/'0$,K$L/0!\"AL
$$3$===$')9\"1$1'()\"6$===
\"0!

Ticket #2592
(also Ticket #329)

785*$+,#-&$#)&785*$+,#-.!)
2'!(-\"$P<H-(5/0
$$!\"#$6\"-#=%!!.1'()\"6B7\"*C
$$$$2%7=;'00\";)$L:6)(##LF$J;'0)1'--\"1$,K$L)9/056L
$$\"0!
\"0!
O;)/'0I'0)1'--\"1JJ?'()/05JJ?'()\"6=!1%8$!'$+2%7+
$$3$<'(1$%77$1'()\"6===
$$P<H-(5/0=%!!.1'()\"6B2%7C
$$3$\");===
\"0!

what’s missing?

public assets

copy

file clash

83+!5*$+,#-&#-%'\"$$.!)
2<./2%5\"6$,$H%)90%2\"=0\"8B@/-\"=!/10%2\"B..@QR4..CC$S$
$$$$$$$$$$$$$$L%66\")6:/2%5\"6L
)%15\")$,$?%/-6=1'')$S$L7(&-/;:/2%5\"6:2<.7-(5/0L
@/-\"T)/-6=2U!/1.7B)%15\")C
2<./2%5\"6=;9/-!1\"0=\"%;9$!'$+\"+$
$$$$@/-\"T)/-6=;7.1B\"F$)%15\")$S$\"=&%6\"0%2\"C$
\"0!

83+!5*$+,#-&#-%'\"$$.!)
2<./2%5\"6$,$H%)90%2\"=0\"8B@/-\"=!/10%2\"B..@QR4..CC$S$
$$$$$$$$$$$$$$L%66\")6:/2%5\"6L
)%15\")$,$?%/-6=1'')$S$L7(&-/;:/2%5\"6:2<.7-(5/0L
)%15\")=2%U\".6<2-/0UB2<./2%5\"6C$(0-\"66$)%15\")=\"A/6)>

migrations

simple DSL

rollback

!&:2/51%)\":
version
$$VWWXWYWYYVYVZ[.%-79%=1&
VWWXWYWVY[VZ[\\.&\")%=1&
20090101121234
20090102142345
7-(5/0:!&:2/51%)\":
2008123123456 $$VWW]YVZYZ[\\^.5%22%=1&

1%U\"$!&J1'--&%;U
version
P/51%)'1=1'--&%;U
20090101121234
P/51%)'13;(11\"0).D\"16/'0
20090102142345
P/51%)'132/51%)\"!=-%6)
2008123123456 P/51%)'1=5\").%--.D\"16/'06
BL_4R4I`$D\"16/'0$@?aP$$
$$6;9\"2%.2/51%)/'06LC
=2%7BbJ)'./C=6'1)

it gets worse

!&:2/51%)\":
version
$$WWY.%-79%=1&
WWV.&\")%=1&
001
002
7-(5/0:!&:2/51%)\":
$$WWY.5%22%=1&

http://www.flickr.com/photos/bk2204/475332962/

single timeline

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!&:2/51%)\":
$$WWY.%-79%=1&
WWV.&\")%=1&
WWZ.7-(5/0.)'.WWY=1&
WW[.!\"-)%=1&
7-(5/0:!&:2/51%)\":
$$WWY.5%22%=1&

Symlink?
!&:2/51%)\":
$$WWY.%-79%=1&
WWV.&\")%=1&
Reference?
WWZ.7-(5/0.)'.WWY=1&
WW[.!\"-)%=1&
Copy &
7-(5/0:!&:2/51%)\":
Retimestamp?
$$WWY.5%22%=1&

Ticket #2058
interblah.net/plugin-migrations

my recommendation?
(for the moment, at least)

write a generator
(within your plugin)
c$6;1/7):5\"0\"1%)\"$%;)6.%6.#(.2/51%)/'06
$$$$
$$$\"A/6)6$$!&:2/51%)\"
$$$;1\"%)\"$$!&:2/51%)\":VWWXW\\W\\YYYZZd.&-%9=1&
c$===

techniques

overriding functionality

views just work

controllers

straight override

module

namespaced

playing nice with the
class cache

1/23*8/36/1**$#2\"'#3-43-'!3$$(!/
!
9\"%/)((-/!(73:(;/6!37/'9(/73;+$(/
<
<
'!((/)+'/#%/%'#$$/\"2'#:(<< < <
<<

O;)/D\"_(77'1)JJe\"7\"0!\"0;/\"6=-'%!.'0;\".7%)96

ApplicationController
AppController PluginController

ApplicationController
PluginController

23-6#,&(-:#!3-7(-'.!)
?%/-6JJQ0/)/%-/f\"1=1(0$!'$+;'0#/5+
$$3$\");===
$$;'0#/5=1\"-'%!.7-(5/06$,$)1(\"
$$
\"0!

83+!5*$+,#-&\"**&23-'!3$$(!%&,33;523-'!3$$(!.!)
;-%66$g''!I'0)1'--\"1$h$===
$$(0-'%!%&-\"
$$
$$3$\");$===
\"0!

init.rb woes

785*$+,#-&#-#'.!)
;'0#/5=)'.71\"7%1\"$!'
$$_'2\"O77I-%66=\"A)\"0!BP<i\"9%D/'(1C
$$
\"0!

testing

ApplicationController
AppController PluginController

ApplicationController
ode
ir C
The
Code
Your
PluginController

\"
$
!
#

unit test?

test in your own rails
environment?

plugin_test_helper

I care about testing
my chunk

You care about testing
your whole app

empower the
developer

avoiding fail

;%6\"$;'!\"=;'06(2\"1
$$89\"0$;'!\"=%()9'1
$$$$1\"(6\"=2'6)-<$,j$:\"##\";)/D\":
$$89\"0$1%0!B!\"D\"-'7\"1C
$$$$1\"(6\"=)1/;U/\"1.&().7'66/&-\"B)1(\"C
$$\"-6\"
$$$$1\"(6\"='D\"11%)\"!N
\"0!

consume wisely

Nutrition Facts
Serving Size 14 kloc
Amount Per Serving 1 bad engine
Calories 10,666
% Daily Value*
Total Fat 2674g 98%
34%
Long Methods 1578g
7%
God Objects 326g
Stress 320mg 119%
Hairloss 45g 45.8%
Technical Debt 13g 87%
Untested Code 1578mg 34%
7%
Hidden Bugs 326mg
Benefit 1mg 0.1%

you will not be able to
write some engines.

you will not be able to
use some engines.

extract
specific functionality

establish some
clear integration points

you are customer #1

READ the CODE

james@lazyatom.com
FIN
lets@gofreerange.com