Practical dependency injection for Ruby

PRACTICAL
DEPENDENCY
INJECTION
(for Ruby)
@thebestie

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WHAT'S COVERED
What is dependency injection?
What problems can it solve?
How do we write code that takes advantage of it?

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WHAT IS IT?
Something, something, inversion of control technique ...

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WHAT PROBLEMS CAN IT SOLVE?
It can be a real help to code that is
Complex to test
Slow to test
Hard to change
Diﬀicult to re-use, even for *very* similar tasks

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The root cause is
COUPLING

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Today we'll explore how our applications o en end up
coupled to global variables
In Ruby constants are global and can be overwritten
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The sky isn't falling.
We still build stuﬀ that works.
We still build stuﬀ that makes money.
How on earth do we manage?

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MONKEY PATCHING
Let's see how we can decouple from globals and reduce our
reliance on monkey patching.

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FRUIT OF THE MONTH CLUB
Subscribers receive a box of fruit
Every month
The fruit changes
We're looking to raise $1.000.000 at a $10.000.000
valuation
We're also hiring senior Rails engineers with 3+ months
experience

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Feature 1
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c
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SO WHAT'S WRONG WITH THAT?
Classic example of an unnecessarily hardcoded
dependency (system time)
They worked around it with monkey patching
The design is rigid and limits functionality, what about
next month?

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LISTEN TO YOUR TESTS
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Feature 2
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Remember what we did?
We just got this feature for free.

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Feature 3
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We may expect our controller to look something like this
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How exactly do we get the authenticated user?

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Google gives us their email address (in a roundabout way)
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We exchange an authentication code for the user's details
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We can test this with our usual monkey patching appproach
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)
e
n
d

View Slide

Seems ok until the next day...
S
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W
h
e
n I s
i
g
n i
n
'

View Slide

It turns out it was more complex than we thought.
JWTs present a few inconveniences:
Time sensitive (configurable)
Opaquely encoded
Optionally encryped
HMAC

View Slide

We have two options now for our integration tests
Monkeypatch time for every test
Remove the time constraint (opening us to replay
attacks)
Furthermore, to sign in a diﬀerent user we need another
opauque awkward fixture!

View Slide

We need to be able to mock out complex components to
reduce complexity.
We need to create 'slots' that we can fit swappable
components into.
We should not rely on the presence of globals.

View Slide

Step 1: Wrap the authication client in an adapter
c
l
a
s
s S
i
g
n
e
t
A
d
a
p
t
e
r
d
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f i
n
i
t
i
a
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i
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e
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c
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t
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d
:
, c
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t
:
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d
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i
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)
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c
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t
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d = c
l
i
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n
t
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c
l
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l
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t
@
r
e
d
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t
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e
d
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t
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r
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d
d
e
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u
t
h
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t
i
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l
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o
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l
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t
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f
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s
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l
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t
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l
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c
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|
= S
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g
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t
:
:
O
A
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h
2
:
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C
l
i
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n
t
.
n
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w
(
# .
.
.
)
e
n
d
# .
.
.
e
n
d

View Slide

We need somewhere to inject it. A service object!
c
l
a
s
s A
u
t
h
e
n
t
i
c
a
t
e
U
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r
d
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t
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(
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h
_
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t = a
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t
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r
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o
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d
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d
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m
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m
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n
d
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m
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f
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t
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h
(
"
e
m
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)
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n
d
d
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f u
s
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f
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a
u
t
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_
c
l
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n
t
.
a
u
t
h
e
n
t
i
c
a
t
e
(
c
o
d
e
)
e
n
d
e
n
d

View Slide

This is an intermediate step
auth_client still has to come from somewhere
The controller still has lots of knowledge
c
l
a
s
s U
s
e
r
S
e
s
s
i
o
n
s
C
o
n
t
r
o
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l
e
r
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f c
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s
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g
n
_
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(
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d
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t
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(
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d
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r
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n
e
w
(
a
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t
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c
l
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n
t
, U
s
e
r
, p
a
r
a
m
s
.
f
e
t
c
h
(
"
c
o
d
e
"
)
)
e
n
d
e
n
d

View Slide

Add another layer to handle dependencies
c
l
a
s
s U
s
e
r
S
e
s
s
i
o
n
s
C
o
n
t
r
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f c
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t
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s
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(
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r
)
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e
d
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t
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(
h
o
m
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d
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r
a
p
p
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a
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t
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c
a
t
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_
u
s
e
r
(
p
a
r
a
m
s
.
f
e
t
c
h
(
"
c
o
d
e
"
)
)
e
n
d
e
n
d

View Slide

An application is an object that oﬀers services
c
l
a
s
s F
r
u
i
t
O
f
T
h
e
M
o
n
t
h
A
p
p
d
e
f i
n
i
t
i
a
l
i
z
e
(
a
u
t
h
_
c
l
i
e
n
t
: .
.
.
)
# I
n
j
e
c
t a
l
l e
x
t
e
r
n
a
l d
e
p
e
n
d
e
n
c
i
e
s h
e
r
e .
.
.
e
n
d
d
e
f a
u
t
h
e
n
t
i
c
a
t
e
_
u
s
e
r
(
c
o
d
e
)
A
u
t
h
e
n
t
i
c
a
t
e
U
s
e
r
.
n
e
w
(
a
u
t
h
_
c
l
i
e
n
t
, U
s
e
r
, c
o
d
e
)
.
c
a
l
l
e
n
d
e
n
d

View Slide

Create a diﬀerently configured app instance for each env
No need for env checks - a pernicious source of ifs
# c
o
n
f
i
g
/
e
n
v
i
r
o
n
m
e
n
t
s
/
t
e
s
t
.
r
b
A
P
P = F
r
u
i
t
O
f
T
h
e
M
o
n
t
h
A
p
p
.
n
e
w
(
a
u
t
h
_
c
l
i
e
n
t
: M
O
C
K
_
A
U
T
H
_
C
L
I
E
N
T
)
c
l
a
s
s A
p
p
l
i
c
a
t
i
o
n
C
o
n
t
r
o
l
l
e
r
p
r
i
v
a
t
e
d
e
f a
p
p
A
P
P
e
n
d
e
n
d

View Slide

Finally we create a mock authentication client.
c
l
a
s
s M
o
c
k
G
o
o
g
l
e
A
u
t
h
e
n
t
i
c
a
t
i
o
n
d
e
f i
n
i
t
i
a
l
i
z
e
(
u
s
e
r
_
i
n
f
o
)
@
u
s
e
r
_
i
n
f
o = u
s
e
r
_
i
n
f
o
e
n
d
d
e
f a
u
t
h
e
n
t
i
c
a
t
e
(
_
c
o
d
e
)
@
u
s
e
r
_
i
n
f
o
e
n
d
e
n
d
M
O
C
K
_
A
U
T
H
_
C
L
I
E
N
T = M
o
c
k
G
o
o
g
l
e
A
u
t
h
e
n
t
i
c
a
t
i
o
n
.
n
e
w
(
"
e
m
a
i
l
" =
> "
b
e
s
t
i
e
@
g
m
a
i
l
.
c
o

View Slide

Now we have control over the implementation of our
authentication mechanism.
Feature tests that are not concerned with authentication can
skip the process vastly reducing setup overhead.

View Slide

DI has helped us
Isolate complex components
Reduce coupling
Reduce if statements
Improve tests
Features for cheap through re-use

View Slide

WHO WAS THAT GUY?
STEPHEN BEST
Freelance consultant
@thebestie
theaudaciouscodeexperiment.com
github.com/bestie

View Slide

Practical dependency injection for Ruby

Practical dependency injection for Ruby

Stephen Best

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