The Rails Engines presentation given by James Adam at RailsConf 2009.

1. The
Even-Darker Art
of
Rails Engines

2. @lazyatom

3. github.com/lazyatom/engines

4. Rails 2.3

5. history

6. November 2005

7. it’s distracting!

8. reuse is overrated!

9. Evil! Evil!
Evil!
Evil!
Evil! Evil!
Evil!
Evil! Shit!

10. eek!

11. appable_plugins

12. desert

13. merb slices?

15. engines

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

17. app/controllers/
models/
helpers/
views/
metal/

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

19. 9quot;-7quot;1$J%--
won’t load from your plugin

20. Ticket #1905

21. config/routes.rb

22. *$+,#-&23-6#,&!3+'(%.!)
O;)/'0I'0)1'--quot;1JJ?'()/05JJ?'()quot;6=!1%8$!'$+2%7+
$$2%7=;'00quot;;)$G:7%)9GF$J;'0)1'--quot;1$,K$L7-(5/0LF$
$$$$$$$$$$$$$$$$$$$$$$$J%;)/'0$,K$L/0!quot;AL
quot;0!

23. top precedence

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

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

26. Ticket #2592
(also Ticket #329)

27. 785*$+,#-&$#)&785*$+,#-.!)
2'!(-quot;$P<H-(5/0
$$!quot;#$6quot;-#=%!!.1'()quot;6B7quot;*C
$$$$2%7=;'00quot;;)$L:6)(##LF$J;'0)1'--quot;1$,K$L)9/056L
$$quot;0!
quot;0!
O;)/'0I'0)1'--quot;1JJ?'()/05JJ?'()quot;6=!1%8$!'$+2%7+
$$3$<'(1$%77$1'()quot;6===
$$P<H-(5/0=%!!.1'()quot;6B2%7C
$$3$quot;);===
quot;0!

28. what’s missing?

29. public assets

30. copy

31. file clash

32. 83+!5*$+,#-&#-%'quot;$$.!)
2<./2%5quot;6$,$H%)90%2quot;=0quot;8B@/-quot;=!/10%2quot;B..@QR4..CC$S$
$$$$$$$$$$$$$$L%66quot;)6:/2%5quot;6L
)%15quot;)$,$?%/-6=1'')$S$L7(&-/;:/2%5quot;6:2<.7-(5/0L
@/-quot;T)/-6=2U!/1.7B)%15quot;)C
2<./2%5quot;6=;9/-!1quot;0=quot;%;9$!'$+quot;+$
$$$$@/-quot;T)/-6=;7.1Bquot;F$)%15quot;)$S$quot;=&%6quot;0%2quot;C$
quot;0!

33. 83+!5*$+,#-&#-%'quot;$$.!)
2<./2%5quot;6$,$H%)90%2quot;=0quot;8B@/-quot;=!/10%2quot;B..@QR4..CC$S$
$$$$$$$$$$$$$$L%66quot;)6:/2%5quot;6L
)%15quot;)$,$?%/-6=1'')$S$L7(&-/;:/2%5quot;6:2<.7-(5/0L
)%15quot;)=2%Uquot;.6<2-/0UB2<./2%5quot;6C$(0-quot;66$)%15quot;)=quot;A/6)>

34. migrations

35. simple DSL

36. rollback

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

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

39. it gets worse

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

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

42. single timeline

43. You 1
tio n
ig ra tio n1
app
m
mi gra
lug in tio n2
p i gra 3
Me pp
m at ion
a igr
nst all ap pm ig ra
i nm
plu gi
ra de
u pg

44. tio n1
You i gra n1
m tio
app mi gra
gin tio n2
plu ra
pm
ig
tio n3
ap ig ra n2
pm tio
ap mi gra
plu gin

45. !&:2/51%)quot;:
$$WWY.%-79%=1&
WWV.&quot;)%=1&
WWZ.7-(5/0.)'.WWY=1&
WW[.!quot;-)%=1&
7-(5/0:!&:2/51%)quot;:
$$WWY.5%22%=1&

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

47. Ticket #2058
interblah.net/plugin-migrations

48. my recommendation?
(for the moment, at least)

49. write a generator
(within your plugin)
c$6;1/7):5quot;0quot;1%)quot;$%;)6.%6.#(.2/51%)/'06
$$$$
$$$quot;A/6)6$$!&:2/51%)quot;
$$$;1quot;%)quot;$$!&:2/51%)quot;:VWWXWWYYYZZd.&-%9=1&
c$===

50. techniques

51. overriding functionality

52. views just work

53. controllers

54. straight override

57. module

61. namespaced

65. playing nice with the
class cache

66. 1/23*8/36/1**$#2quot;'#3-43-'!3$$(!/
<
'!((/)+'/#%/%'#$$/quot;2'#:(<< < <
<< !
9quot;%/)((-/!(73:(;/6!37/'9(/73;+$(/
<

67. O;)/Dquot;_(77'1)JJequot;7quot;0!quot;0;/quot;6=-'%!.'0;quot;.7%)96

68. ApplicationController
AppController PluginController

69. ApplicationController
PluginController

70. 23-6#,&(-:#!3-7(-'.!)
?%/-6JJQ0/)/%-/fquot;1=1(0$!'$+;'0#/5+
$$3$quot;);===
$$;'0#/5=1quot;-'%!.7-(5/06$,$)1(quot;
$$
quot;0!

71. 83+!5*$+,#-&quot;**&23-'!3$$(!%&,33;523-'!3$$(!.!)
;-%66$g''!I'0)1'--quot;1$h$===
$$(0-'%!%&-quot;
$$
$$3$quot;);$===
quot;0!

72. init.rb woes

73. 785*$+,#-&#-#'.!)
;'0#/5=)'.71quot;7%1quot;$!'
$$_'2quot;O77I-%66=quot;A)quot;0!BP<iquot;9%D/'(1C
$$
quot;0!

74. testing

75. ApplicationController
AppController PluginController

76. ApplicationController
ir C ode
The
Your Code
PluginController

78. quot;
$
!
#

80. unit test?

81. test in your own rails
environment?

82. plugin_test_helper

83. I care about testing
my chunk

84. You care about testing
your whole app

85. empower the
developer

86. avoiding fail

87. ;%6quot;$;'!quot;=;'06(2quot;1
$$89quot;0$;'!quot;=%()9'1
$$$$1quot;(6quot;=2'6)-<$,j$:quot;##quot;;)/Dquot;:
$$89quot;0$1%0!B!quot;Dquot;-'7quot;1C
$$$$1quot;(6quot;=)1/;U/quot;1.&().7'66/&-quot;B)1(quot;C
$$quot;-6quot;
$$$$1quot;(6quot;='Dquot;11%)quot;!N
quot;0!

88. consume wisely

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

90. you will not be able to
write some engines.

91. you will not be able to
use some engines.

92. extract
specific functionality

93. establish some
clear integration points

94. you are customer #1

95. READ the CODE

96. james@lazyatom.com
FIN
lets@gofreerange.com