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Karl Matthias

DevOps, sysadmin, coder. Principal Systems Engineer at Nitro. Dublin, Ireland.

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Writing Testable Code in Go (golang)

If you are coming from Ruby or another dynamic language you are used to mocking and stubbing extensively for testing. This works fabulously well in Ruby, for example, where you can easily modify a class or object at runtime when testing it. Because Go is strongly typed and statically compiled, mocking and stubbing is harder to achieve and far less flexible. You can do it to an extent, but you’re really working against the grain of the language. But that doesn’t mean that you can’t test your code just as well.

The key to testable code in Go is using interfaces. When I started to write production Go code about 9 months ago, I was given that exact piece of advice and well, it was less than enlightening. It was not immediately obvious how interfaces solve this problem. My first attempts worked fine, but required writing a lot of needless code implementing other people’s interfaces for test stubbing. It took me awhile to realize how to properly leverage Go’s powerful interface mechanism. This is what I learned and it has drastically improved the testability of my Go code.

Dependency Injection

You really must be injecting all of your dependencies.

Dependency injection is good coding practice in most languages, but in dynamic languages you can get away with isolating dependencies with runtime magic and with stubbing in your test code. In Go you need to always inject your dependencies since you can’t (mostly) patch things at runtime in a safe way.

Here’s some sample Go code showing the injection of a dependency. Let’s say we need to be able to get some JSON data from the web and then do something with it. In the real code I want to get this from the web with an HTTP request, but in testing, I want to test my logic not whether I can make an HTTP request. To remove the dependency from my code, I pass in an HttpResponseFetcher that knows how to get things from the web.

Yes, there are libraries that let you handle HTTP testing without doing this, but this is an example that I think is easy to understand. And in many ways this is better, more flexible code. Here we’ll require in a dependency in the method signature.

func populateInfo(fetcher HttpResponseFetcher, parsedInfo *Info) error {
	response, err := fetcher.Fetch("http://example.com/info")

	if err == nil {
		err = json.Unmarshal(response, parsedInfo)

		if err == nil {
			return nil
		}
	}

	return err
}

There isn’t that much involved in this code. We have a dependency on an HttpResponseFetcher which will be passed in and then we simply call a method on it (fetcher) to retrieve a []byte of data, then unmarshal the JSON into a struct we were passed.

Other than the fact that fetcher implements an interface, there are a couple of things that are worth noting here. One is that we only call a single method on fetcher and another is that it returns a simple []byte. A []byte is trivial to mock statically in test code.

Using Interfaces

The dependency injection gets us a handle on the data that will be manipulated in the function. We can pass in an HttpResponseFetcher implemented however we like and the code will be happy with it. Here’s the whole definition for our interface:

type HttpResponseFetcher interface {
	Fetch(url string) ([]byte, error)
}

Our real production implementation of the HttpResponseFetcher looks something like this:

func (Fetcher) Fetch(url string) ([]byte, error) {
	response, err := http.Get(url)
	if err != nil {
		return nil, err
	}

	defer response.Body.Close()

	contents, err := ioutil.ReadAll(response.Body)
	if err != nil {
		return nil, err
	}

	return contents, nil
}

That talks to the network, reads the response and packages it up into a []byte. But when we test this we just write a StubFetcher that implements the Fetch method and we’re good to go. It’s easy to implement:

var infoOutput []byte = []byte(
	`{ "Environment": "production" }`
)

var statusOutput []byte = []byte(
	`{ "Status": "up" }`
)

type stubFetcher struct{}

func (fetcher stubFetcher) Fetch(url string) ([]byte, error) {
	if strings.Contains(url, "/info") {
		return infoOutput, nil
	}

	if strings.Contains(url, "/status") {
		return statusOutput, nil
	}

	return nil, errors.New("Don't recognize URL: " + url)
}

Here we just look at the URL that was requested and return a static []byte that has the contents we want to test with.

So our call inside our test method would pass in the stubbed Fetcher like this:

var info *Info
var stub stubFetcher

// We would make some assertions around this:
populateInfo(stubFetcher, info)

Use Your Own Interfaces Most of the Time

Here’s the thing that took the longest to figure out, and which I think will be the biggest help to those on the same learning path: you should usually inject your own interfaces, not stdlib or library-defined interfaces. That was not apparent at first, but here’s why that’s powerful.

In the code above, by using our own interface, we can actually handle more than one URL in the same stubbed method. This is useful for things where you make more than one request in a function. Because we started with our own interface from the beginning, when we add a second request in the same function, we don’t have to go back and rewrite our tests to support it.

Here’s what we might have done: we could have passed in a Reader and stubbed it in testing and passed in an HTTP response body in real usage. But when we needed to make more than one request in the same function, we couldn’t then do that easily because our stub Reader wouldn’t know which request had been made.

There is a more powerful reason, though. Implementing many interfaces requires that you have many methods. So to make a stub you would need to implement all of them.

But remember that in Go anything that implements the correct method signatures is accepted as an implementation of the interface. So if you don’t need all of those other methods in your code, don’t use that interface. Define one of your own that only includes the methods you actually need. That lets your stub be one or two methods in many cases. And that’s easy to implement.

A perfect example are the various Socket interfaces. Many of them are 15 or more methods. If you made a stub Socket to pass to your function in testing, you would have a lot of work on your hands. If you define your own interface and pass that instead, you can easily test it. On top of that, your code is now even more flexible because your requirements for your dependency are very narrowly scoped to only the methods you really care about.

Conclusion

These are some of the tools I have found for writing testable code in Go. As is usually the case, testable code is also better written. I hope this article helps direct some other people down a path it took me some time to discover.


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