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https://github.com/github/dat-science

Replaced by https://github.com/github/scientist
https://github.com/github/dat-science

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Replaced by https://github.com/github/scientist

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README 

        
# Science is happening elsewhere!



*This repository is historical. Up-to-date bits are over in [`github/scientist`](https://github.com/github/scientist).*



A Ruby library for carefully refactoring critical paths. Science isn't a feature

flipper or an A/B testing tool, it's a pattern that helps measure and validate

large code changes without altering behavior.



## How do I do science?



Let's pretend you're changing the way you handle permissions in a large web app.

Tests can help guide your refactoring, but you really want to compare the

current and new behaviors live, under load.



```ruby

require "dat/science"



class MyApp::Widget

  def allows?(user)

    experiment = Dat::Science::Experiment.new "widget-permissions" do |e|

      e.control   { model.check_user(user).valid? } # old way

      e.candidate { user.can? :read, model } # new way

    end



    experiment.run

  end

end

```



Wrap a `control` block around the code's original behavior, and wrap `candidate`

around the new behavior. `experiment.run` will always return whatever the

`control` block returns, but it does a bunch of stuff behind the scenes:



* Decides whether or not to run `candidate`,

* Runs `candidate` before `control` 50% of the time,

* Measures the duration of both behaviors,

* Compares the results of both behaviors,

* Swallows any exceptions raised by the candidate behavior, and

* Publishes all this information for tracking and reporting.



If you'd like a bit less verbosity, the `Dat::Science#science` helper

instantiates an experiment and calls `run`:



```ruby

require "dat/science"



class MyApp::Widget

  include Dat::Science



  def allows?(user)

    science "widget-permissions" do |e|

      e.control   { model.check_user(user).valid? } # old way

      e.candidate { user.can? :read, model } # new way

    end

  end

end

```



## Making science useful



The examples above will run, but they're not particularly helpful. The

`candidate` block runs every time, and none of the results get

published. Let's fix that by creating an app-specific sublass of

`Dat::Science::Experiment`. This makes it easy to add custom behavior

for enabling/disabling/throttling experiments and publishing results.



```ruby

require "dat/science"



module MyApp

  class Experiment < Dat::Science::Experiment

    def enabled?

      # See "Ramping up experiments" below.

    end



    def publish(name, payload)

      # See "Publishing results" below.

    end

  end

end

```



After creating a subclass, tell `Dat::Science` to instantiate it any time the

`science` helper is called:



```ruby

Dat::Science.experiment = MyApp::Experiment

```



### Controlling comparison



By default the results of the `candidate` and `control` blocks are compared

with `==`. Use `comparator` to do something more fancy:



```ruby

science "loose-comparison" do |e|

  e.control    { "vmg" }

  e.candidate  { "VMG" }

  e.comparator { |a, b| a.downcase == b.downcase }

end

```



### Ramping up experiments



By default the `candidate` block of an experiment will run 100% of the time.

This is often a really bad idea when testing live. `Experiment#enabled?` can be

overridden to run all candidates, say, 10% of the time:



```ruby

def enabled?

  rand(100) < 10

end

```



Or, even better, use a feature flag library like [Flipper][]. Delegating the

decision makes it easy to define different rules for each experiment, and can

help keep all your entropy concerns in one place.



[Flipper]: https://github.com/jnunemaker/flipper



```ruby

def enabled?

  MyApp.flipper[name].enabled?

end

```



### Publishing results



By default the results of an experiment are discarded. This isn't very useful.

`Experiment#publish` can be overridden to publish results via any

instrumentation mechanism, which makes it easy to graph durations or

matches/mismatches and store results. The only two events published by an

experiment are `:match` when the result of the control and candidate behaviors

are the same, and `:mismatch` when they aren't.



```ruby

def publish(event, payload)

  MyApp.instrument "science.#{event}", payload

end

```



The published `payload` is a Symbol-keyed Hash:



```ruby

{

  :experiment => "widget-permissions",

  :first      => :control,

  :timestamp  => ,



  :candidate => {

    :duration  => 2.5,

    :exception => nil,

    :value     => 42

  },



  :control => {

    :duration  => 25.0,

    :exception => nil,

    :value     => 24

  }

}

```



`:experiment` is the name of the experiment. `:first` is either `:candidate` or

`:control`, depending on which block was run first during the experiment.

`:timestamp` is the Time when the experiment started.



The `:candidate` and `:control` Hashes have the same keys:



* `:duration` is the execution in ms, expressed as a float.

* `:exception` is a reference to any raised exception or `nil`.

* `:value` is the result of the block.



#### Adding context



It's often useful to add more information to your results, and

`Experiment#context` makes it easy:



```ruby

science "widget-permissions" do |e|

  e.context :user => user



  e.control   { model.check_user(user).valid? } # old way

  e.candidate { user.can? :read, model } # new way

end

```



`context` takes a Symbol-keyed Hash of additional information to publish and

merges it with the default payload.



#### Keeping it clean



Sometimes the things you're comparing can be huge, and there's no good way

to do science against something simpler. Use a `cleaner` to publish a

simple version of a big nasty object graph:



```ruby

science "huge-results" do |e|

  e.control   { OldAndBusted.huge_results_for query }

  e.candidate { NewHotness.huge_results_for query }

  e.cleaner   { |result| result.count }

end

```



The results of the `control` and `candidate` blocks will be run through the

`cleaner`. You could get the same behavior by calling `count` in the blocks,

but the `cleaner` makes it easier to keep things in sync. The original

`control` result is still returned.



## What do I do with all these results?



Once you've started an experiment and published some results, you'll want to

analyze the mismatches from your experiment.  Check out

[`dat-analysis`](https://github.com/github/dat-analysis) where you'll find an

analysis toolkit to help you understand your experiment results.



## Hacking on science



Be on a Unixy box. Make sure a modern Bundler is available. `script/test` runs

the unit tests. All development dependencies will be installed automatically if

they're not available. Dat science happens primarily on Ruby 1.9.3 and 1.8.7,

but science should be universal.



## Maintainers



[@jbarnette](https://github.com/jbarnette) and [@rick](https://github.com/rick)