Exception handling and testing it with Minitest

Posted by Ilija Eftimov on July 29, 2015

When testing our code, we usually go for the happy path (TM). We are awesome developers, we test our code, we are careful and there’s no way our code might crash. Or not really? I often try to think of software as a live being. It thinks, it does stuff and sometimes it gets some things wrong. Just like us. We sometimes trip up while walking, we drop our keys or forget them on our desk at the office. It’s normal. So, how can we test our code for these rare occurances?

What is what?

Errors, exceptions and failures. It’s really hard to tell them apart. More so, it’s harder to know when to use what. Avdi Grimm in his book “Exceptional Ruby” uses Bertrand Meyer’s definition of these words:

  • An exception is the occurrence of an abnormal condition during the execution of a software element.
  • A failure is the inability of a software element to satisfy its purpose.
  • An error is the presence in the software of some element not satisfying its specification.

Also, he mentions:

Note that failures cause exceptions… and are in general due to errors.

Give yourself time for this to sink in. I think that these definitions very much explain the difference between errors, exceptions and failures.

Why testing them?

Let’s start this with a hypotetical example. Say you are building a gem, that’s basically an API wrapper. The code has nice coverage percentage and you are confident that it works as it should. I guess you know where I am heading with this example. The obvious question is - what happens if for whatever reason, the API is not responsive? Maybe a switch in the datacentre died and they need couple of minutes to re-route the network. Or, maybe the API server crashed for whatever reason. What do we do than?

The problem with errors (which cause failures, whose product is exceptions) is that they are very often hard to think about. But they are real. Just like us, we never think about forgetting our keys on our desk, but, it happens. While in real life we can pretty much always go back and get the keys from our desk, software isn’t that intelligent by default. It’s our duty to make it intelligent, or in other words, we have to handle exceptions in our code.

So, why test them? Well, if you have error handling code, you should have tests. You should always aim for 100% code coverage. Simple as that.

Add error handling

I am the author of this tiny gem called Forecastr. Given that the gem doesn’t know how to handle errors, let’s add the code so it can handle errors and test it.

require "net/http"
require 'json'

module Forecastr
  class Client
    class << self
      def search_by_city(city_name)
        uri = UriBuilder.by_city(city_name)
        response = Net::HTTP.get(uri)
        json = JSON.parse(response)
        Forecastr::DataContainer.new(json)
      end

      def search_by_coordinates(latitude, longitude)
        uri = UriBuilder.by_coordinates(latitude, longitude)
        response = Net::HTTP.get(uri)
        json = JSON.parse(response)
        Forecastr::DataContainer.new(json)
      end
    end

  end
end

This is the code that actually does the API calls. The first method, search_by_city accepts a city name as argument and issues the API call with the city name. The second method, search_by_coordinates accepts the coordinates of the location that we want to get the weather for. Just like the first method, it issues the API call and parses the JSON response.

(As I sidenote - yes, I am aware that these methods are not as DRY as they should be, but lets stick to this code, at least for the purpose of this post.)

Now, after seeing this code, there’s an obvious question - when it issues the GET request to the API, what would happen if the API is down? Or, if our internet connection dies?

I guess you can notice that there’s a clear gap here. There’s no code that can handle a HTTP timeout, or any other type of failure.

There are couple of ways that we can do this.

Make some noise

When an exception happens, one option is to let it make noise. What I mean by that is just to let it explode.

client.rb:12:in `search_by_coordinates': execution expired (Timeout::Error)

Boom! And this is fine, but you want to have this covered by tests.

  def test_raises_timeout_error
    stub_get("http://api.openweathermap.org/data/2.5/weather?lat=42.0&lon=21.4333").to_timeout
    assert_raises TimeoutError do
      Forecastr::Client.search_by_coordinates(42.0, 21.4333)
    end
  end

As you can see it the test above, the syntax to do this in Minitest is to use the assert_raises method. It accepts an exception class as it’s first parameter. Also, it expects that it’s block will raise the exception specified as argument.

Make some noise, your way

Another option is to create your own exception classes. You can do this by subclassing an stdlib error, like TimeoutError.

module Forecastr
  class RequestTimeout < ::TimeoutError; end

  class Client
    class << self
      def search_by_coordinates(lat, lon)
        uri = UriBuilder.by_coordinates(lat, lon)
        begin
          response = Net::HTTP.get(uri)
        rescue TimeoutError
          raise Forecastr::RequestTimeout.new("Request timeout.")
        end
        json = JSON.parse(response)
        Forecastr::DataContainer.new(json)
      end
    end

  end
end

Here, we rescue the TimeoutError that the GET request returns and we raise our own custom error.

Again, we can test this out by using the assert_raises assertion.

  def test_raises_timeout_error
    stub_get("http://api.openweathermap.org/data/2.5/weather?lat=42.0&lon=21.4333").to_timeout
    assert_raises Forecastr::RequestTimeout do
      Forecastr::Client.search_by_coordinates(42.0, 21.4333)
    end
  end

This is particularly useful because when you know how the API works, you can customize your errors to mimick the errors that the API can produce.

Benign values

This is also a very interesting approach to exception handling. Benign values mean that the program won’t bomb out, but, it will return a meaningless value of some sort.

What do I mean? Let’s see an example:

def search_by_city(city_name)
  uri = UriBuilder.by_city(city_name)
  begin
    response = Net::HTTP.get(uri)
  rescue TimeoutError
    response = ""
  end
  json = JSON.parse(response)
  Forecastr::DataContainer.new(json)
end

You see, instead of bombing out, we set the response to blank string. This will not make the program stop, but, it will be obvious that the values we got back are not right.

Testing this is pretty trivial:

class Forecastr::ClientTest < Minitest::Test
  def test_handling_exceptions_with_benign_values
    stub_get("http://api.openweathermap.org/data/2.5/weather?lat=42.0&lon=21.4333").to_timeout
    result = Forecastr::Client.search_by_coordinates(42.0, 21.4333)
    assert_equal nil, result.temeprature
  end
end

Basically, our program didn’t bomb out, but the forecast object got created without any real data in it. Usually, when doing this, it’s best to log these events so there’s a written record that this occurred.

Conclusion

While there are more tactics to handling exceptions in Ruby, it’s very crucial we test them. If we are not testing how our applications respond to errors, eventually, we will find out. The problem is that the experience will not be a pleasant one. Whichever way you prefer, it’s best to know how our application will handle the exceptions and (possibly) recover from them.


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