Mock almost any web service with these 100 lines of ruby code

Many applications are now dependent upon web services to provide much of their functionality.  Testing applications and their interactions with these web services can be pretty hard to automate without a mock service.

I recently needed to write some unit tests for a bit of code that communicated with a web service, and so I started reading up on Sinatra.

It turns out that Sinatra makes it very simple to generate just about any web service.  It is especially trivial to mock data for a given web service end point.  Take this example right out of the Sinatra README:

get '/hello/:name' do
  # matches "GET /hello/foo" and "GET /hello/bar"
  # params['name'] is 'foo' or 'bar'
  "Hello #{params['name']}!"
end

This seemed awesome, but I realized that I had maybe 50 endpoints that I wanted to mock.  I guess this still isn't too bad, but I really wanted to generalize my ruby code in such a way that it could mock any endpoint without code modification.

So I wrote a Sinatra service that has only 4 end points using splat captures (*), which is basically a wildcard.  I made one route for each of the main HTTP verbs (POST,PUT,GET,DELETE), and had each route match ANY endpoint that matched its verb, meaning:

get '*' do     ...
put '*' do     ...
post '*' do    ...
delete '*' do  ...

I then wrote each route to mirror a "fixtures" directory on the filesystem.

For example, say I had an endpoint to some API for getting user information:

GET /companies/company1/users/jdoe

The get '*' route handles this and browses the file on the filesystem at ./fixtures/companies/company1/users/jdoe.json, and returns the contents of this file in the body of the response:

get '*' do
    path = Pathname.new("#{File.dirname(__FILE__)}/#{FIXTURESDIR}#{params[:splat].first()}")

    pathPlusJson = Pathname.new("#{path}.json")
    if path.directory?
        response = get_directory_contents_array(path.to_path)
        return response.to_json
    elsif pathPlusJson.exist? and pathPlusJson.file?
        response = JSON.parse(IO.read(pathPlusJson.to_path))
        return response.to_json
    else
        return create_response( {"error" => "Not Found" }.to_json, 404 )
    end
end

If I wanted to iterate all users, I could instead request a get on a directory instead of a file:

 GET /companies/company1/users

Now the get '*' route browses to the directory and iterates through all the ".json" files returning an array of JSON encoded user objects in the body.

If I want to add a new user I can simply do a POST:

POST /companies/company1/users/dsmith

with some JSON in the body, and this JSON will be written to ./fixtures/companies/company1/dsmith.json

Delete also works as you might expect, deleting the file in the fixtures directory.

All of this can be done with very little code thanks to the domain specific nature of Sinatra.  The full ruby code can be found in one of my GitHub Gists's:

https://gist.github.com/jhnbwrs/6e940679a3dd1a23cbc0

Now, if you are unit testing a ruby application you can stub any requests to your api endpoint using webmock:

# spec/spec_helper.rb RSpec.configure 
do |config| 
  config.before(:each) do 
    stub_request(:any, /api.company1.com/).to_rack(JSONFileServer) 
  end 
end

If you aren't unit testing a ruby application, you should still be able to fire up this service during the unit test phase of your build, and redirect your API requests to localhost:4567.