Uses & Abuses of Mocks & Stubs

Manchester
October 2009

Uses & Abuses
of Mocks & Stubs
With examples in Ruby using RSpec

Ashley Moran
Creative Commons Attribution-Noncommercial-Share Alike
2.0 UK: England & Wales License PatchSpace Ltd

Presentation goals
To establish a common language about
mock objects
To show some situations where mocks are
useful
To show some situations where mocks are
inappropriate, and problems they can
cause

Presentation non-goals

The slides do not attempt…
to teach you RSpec
to cover OO design
to use real-world code
So please ask questions if I've glossed over
anything you think I should have explained

Starting Point

A mock is some sort of trivial object that
can be injected into code to make writing
specs easier
A mock can be made to respond to
messages

Traditional Definitions
Gerard Meszaros defines the following
test object roles, among others:
dummy
stub
fake
mock
These are precise and significantly
different from each other

RSpec Deﬁnitions
RSpec provides three (equivalent) factory
methods for creating mock objects

mock("my mock object")
stub("my stub object")
double("my double object")

The RSpec book uses "test double" as the
primary term; I still prefer "mock"

My deﬁnitions
A mock is any trivial object injected into
your code to help describe its behaviour
A stub method is a method that returns a
canned response
This may be on a "real" object, more later
A message expectation is an expectation
that fails if a stub method is not called
appropriately in spec code

RSpec concepts
describe "A behaviour description" do
it "has an example" do
my_mock = mock(Object,
:stub_method => "expected value")
my_mock.stub_method.should == "expected value"
end

it "has an example with a message expectation" do
my_mock = mock(Object,
:expected_method => "value")
my_mock.should_receive(:expected_method)
my_mock.expected_method
end
end

Deﬁning an Interface
Ruby is 99% about objects and interfaces,
and 1% about classes
describe Person do
before(:each) do
@service = mock("service")
@person = Person.new("Fred", "Flintstone")
end

it "can save itself" do
@service.should_receive(:query).with(
"STORE 'Fred', 'Flintstone';"
)
@person.save(@service)
end
end

Deﬁning an Interface
The set of options for message
expectations is too large to go into now,
but is extremely ﬂexible
describe Person do
before(:each) do
@service = mock("service")
end

it "sends relevant details" do
@service.should_receive(:confirm_family).with(
hash_including(:name => "Flintstone")
)
@person.register(@service)
end
end

Observable Behaviour
Sometimes you know how your code
should behave even if you ignore the
details of how it works
describe XmlDoc do
before(:each) do
body = mock("body", :to_xml => "<body />")
@doc = XmlDoc.new(body)
end

it "renders the document and body" do
@doc.to_xml.should == "<doc><body /></doc>"
end
end

Observable Behaviour
This is actually an appropriate situation
for a custom matcher to deliberately
weaken our expectation of the behaviour
describe XmlDoc do
# ...

it "renders the document and body" do
@doc.to_xml.should be_equivalent_xml_to(
"<doc><body /></doc>"
)
end
end

I wrote a matcher for this the other day,
but that's another talk

Dependency Injection
The ability to redeﬁne methods in Ruby
massively reduces the impact of depending on a
named class compared to eg Java

class DatabaseLogger
def initialize(connection_details)
@connection_details = connection_details
end

def log(message)
Database.new(@connection_details).log(message)
end
end

Dependency Injection
A stub can be used to override factory methods
describe DatabaseLogger do
before(:each) do
@connection_details = mock("connection details")
@database = mock(Database, :log => nil)
@database_logger = DatabaseLogger.new(@connection_details)
end

it "creates a new Database" do
Database.should_receive(:new).
with(@connection_details).
and_return(@database)

@database_logger.log("take note")
end
end

Simulating Situations
Mocks let us put our code through situations
that may be hard to recreate for real
describe ObjectSaver do
before(:each) do
@storage = mock("storage")
@object_saver = ObjectSaver.new(@storage)
end

it "should save" do
@storage.should_receive(:save).and_raise(OutOfStorageError)
@object_saver.save(nil)
end
end

Especially useful if there's a large number of
situations (true of some stdlib code)

Degrees of Freedom
describe Shape do
before(:each) do
@triangle = Shape.new(3)
end

it "knows how many sides it has" do
@triangle.sides.should == 3
end
end

This Shape class is one dimensional
Complaints about the examples at the end
please
Only one thing can vary - will one example do?

Degrees of Freedom
class Shape
def initialize(sides)

end

def sides
3
end
end

No - one example is not enough

Degrees of Freedom
We need t wo points to prove that the
implementation isn't simplistic

describe Shape do
before(:each) do
@triangle = Shape.new(3)
@square = Shape.new(4)
end

@triangle.sides.should == 3
@square.sides.should == 4
end
end

Degrees of Freedom
We now have to solve the problem fully,
but at the expense of more spec code
class Shape
def initialize(sides)
@sides = sides
end

def sides
@sides
end
end

This is a trivial example - don't write
getters or use attr_reader in real code!

Degrees of Freedom
We can reduce this to one using a mock
because we can't generate something that
is == the mock from in the code
describe Shape do
before(:each) do
@triangle = Shape.new(@sides = mock("sides"))
end

@triangle.sides.should == @sides
end
end

This technique is not always appropriate - a
getter method is simply the easiest example

Removing Duplication
Delegation introduces duplication
Consider the following class:

class Room
def open_door(key)
key == "sesame" ? :open : :closed
end
end

The spec for this code is fairly complex

describe Room do
before(:each) do
@room = Room.new
end

it "opens with the right key" do
@room.open_door("sesame").should == :open
end

it "won't open with the wrong key" do
@room.open_door("now!!!").should == :closed
end
end

Imagine a decorator object that delegates
to it
class Bedsit
def initialize(room)
@room = room
end

def open_door(key)
@room.open_door(key)
end

def fix_roof
"it's fixed"
end
end

How do you write the spec?

describe Bedsit do
before(:each) do
@bedsit = Bedsit.new(Room.new)
end

it "opens with the right key" do
# WARNING - duplication!
@bedsit.open_door("sesame").should == :open
end

it "won't open with the wrong key" do
# ...
end
end

We actually only care about the delegation
describe Bedsit do
before(:each) do
@open_door_response = mock("open_door_response")
@key = mock("key")
@room = mock(Room, :open_door => @open_door_response)
@bedsit = Bedsit.new(@room)
end

it "delegates #open_door" do
@room.should_receive(:open_door).with(@key)
@bedsit.open_door(@key)
end

it "returns the delegated response" do
@bedsit.open_door(nil).should == @open_door_response
end
end

Hiding Random Behaviour
The following code may be useful to
randomise collections of data

class Randomiser
def initialize(collection)
@collection = collection
end

def to_a
@collection.sort { rand <=> 0.5 }
end
end

This is the client

class Sampler
def initialize(values, logger)
@values, @logger = values, logger
end

def sample
@logger.log(Randomiser.new(@values).to_a[0])
end
end

describe Sampler do
before(:each) do
@first = mock("first")
@randomiser = mock(Randomiser, :to_a => [@first, 2, 3])
Randomiser.stub(:new => @randomiser)

@logger = mock("logger", :log => nil)
@values = mock("values")
@sampler = Sampler.new(@values, @logger)
end
it "makes a new Randomiser" do
Randomiser.should_receive(:new).with(@values)
@sampler.sample
end
it "logs a sample of the data" do
@logger.should_receive(:log).with(@first)
@sampler.sample
end
end

Exploratory Coding
Following the outside in BDD principle leads
you to write client code before provider
code, eg:
A web controller before a domain model
A domain model before a storage system
Data processing before ﬁle system
access
I'm going to defer this until the "Mock
Trainwreck" abuse later

Modifying the Subject
Modifying the subject of a behaviour
description is dangerous, you can:
Introduce false positives
Make the specs brittle
What I call the "subject" is sometimes
known as Object Under Test, Subject Under
Test or System Under Test

This is a stable - albeit incomplete - spec

describe Person do
before(:each) do
end

it "should have a full name" do
@person.name.should == "Fred FLINTSTONE"
end
end

…and one of the possible implementations
class Person
def initialize(first, last)
@first, @last = first, last
end

def name
"#{@first} #{last}"
end

private

def last
@last.upcase
end
end

This spec makes assumptions about the
subject

describe Person do
before(:each) do
@person.stub(:last => "FLINTSTONE")
end

it "should have a full name" do
@person.name.should == "Fred FLINTSTONE"
end
end

This code silently breaks the behaviour
class Person
def initialize(first, last)
@first, @last = first, last
end

def name
"#{@first} #{last}"
end

private

def last
@last.downcase
end
end

False positives are a catastrophe!

Mock Mania
describe ManicObject do
before(:each) do
@db = mock(Database)
@logger = mock(Logger)
@rss_feed = mock(URI)
@kitchen_sink = mock(KitchenSink)
@cleaning_solution = mock(FairyLiquid)
end

# ...
end

Specs like this indicate a Single
Responsibility violation. The ManicObject
class is probably highly unstable

Mock Trainwrecks
Stub methods on non-Class objects should
not (usually) return mocks that you
interact with
This indicates a violation of Law of
Demeter in the code
Mock trainwrecks tie your specs to the
current code structure, so are brittle and
scatter the specs for the other objects

Mock Trainwrecks
The code that passes this spec must be
breaking encapsulation to expose its
internals
describe Train do
before(:each) do
@door = mock(Door)
@cabin = mock(Cabin, :door => @door)
@engine = mock(Engine, :cabin => @cabin)
@train = Train.new(@engine)
end

it "can lock its door" do
@door.should_receive(:lock).once
@train.lock_door
end
end

Mock Trainwrecks
attr_reader shows we're off to a bad start
class Train class Cabin
def initialize(engine) attr_reader :door
@engine = engine
end def initialize(door)
@door = door
def lock_door end
@engine.cabin.door.lock end
end
end

class Engine class Door
attr_reader :cabin def lock
"it's locked"
def initialize(cabin) end
@cabin = cabin end
end
end

Mock Trainwrecks
Let's break this down into single-step
interactions
describe Train do
before(:each) do
@engine = mock(Engine)
end

it "locks its cabin door" do
@engine.should_receive(:lock_door).with(no_args)
@train.lock_door
end
end

Mock Trainwrecks
The interface to engines is now not
exposed to clients of trains

class Train
def initialize(engine)
@engine = engine
end

def lock_door
@engine.lock_door
end
end

Mock Trainwrecks
Apply the pattern recursively…

describe Engine do
before(:each) do
@cabin = mock(Cabin)
@engine = Engine.new(@cabin)
end

@cabin.should_receive(:lock_door).with(no_args)
@engine.lock_door
end
end

Mock Trainwrecks
The cabin interface is now hidden

class Engine
def initialize(cabin)
@cabin = cabin
end

def lock_door
@cabin.lock_door
end
end

Mock Trainwrecks
Apply again…

describe Cabin do
before(:each) do
@door = mock(Door)
@cabin = Cabin.new(@door)
end

@door.should_receive(:lock).with(no_args)
@cabin.lock_door
end
end

Mock Trainwrecks
The door interface is now hidden

class Cabin
def initialize(door)
@door = door
end

def lock_door
@door.lock
end
end

Mock Trainwrecks
Finally we prove that the door actually
locks

describe Door do
it "locks" do
Door.new.lock.should == "it's locked"
end
end

class Door
def lock
"it's locked"
end
end

Mock Trainwrecks
We've hidden the structure of our object
model, which satisﬁes the Law of Demeter
But we've now described three internal
interfaces (on Engine, Cabin and Door)
The code can be broken if any internal
interface changes
In an unstable object model, this can lead
to a lot of rework

Mock Trainwrecks
We could eliminate the use of mocks from
most (or all, if we wanted) of the spec
describe Train do
before(:each) do
@door = mock(Door)
@cabin = Cabin.new(@door)
@engine = Engine.new(@cabin)
end

it "can lock its door" do
@door.should_receive(:lock).once
@train.lock_door
end
end

Be warned that this example is contrived,
and strays far into OO design territory

Mock Trainwrecks
On the subject of trainwrecks

describe "thetrainline.com" do
it "should be loved by all" do
URI.parse("thetrainline.com").should_not be_shit
end
end

Unfortunately, Ruby (as of 1.9.1) does not
deﬁne URI#shit? so a custom matcher would
be required

Beneﬁts of Mocks

Mocks let you explore and drive out new
interfaces needing a full implementation
Mocks let you implement your code
incrementally and get to green faster
Mocks let you prove properties of your
code to the degree of precision and
strength you demand

Trouble from Mocks
Mocks make your specs interface-driven
not behaviour-driven
Fine if using the interface is the
behaviour
Can introduce overhead and rework if
the interfaces are numerous and trival
Better to throw away many of these
Mocked specs are more brittle

References

The RSpec Book - David Chelimsky et al
xUnit Patterns - Gerard Meszaros
Mocks Aren't Stubs - Martin Fowler
Mock Roles, not Objects - Steve Freeman
et al

Contact
Slides prepared by Ashley Moran
ashley.moran@patchspace.co.uk
www.patchspace.co.uk
I'm available for training/consultancy in
Ruby and RSpec on a freelance basis
I code for food

Licence

Creative Commons licensed - contact me
for commercial use

Uses & Abuses of Mocks & Stubs

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