Real Life Concurrency in Go

The structure of a programming language reflects the challenges and solutions the designers decided to address. Each designer coming with his/her own background decides to tackle some specific issues in a novel way and/or often decides to borrow existing paradigms from other languages. We can’t, then, fairly judge a language without understanding what problem the language designer was trying to address.

Today we are going to look at Google’s Go language. Go approaches concurrency from an interesting view point. But instead of digging into the history and reasoning which led to this approach, I’d like to show you the language constructs by actually writing real life code.

Fetching web resources concurrently

The following example is taken from my recent presentation Ruby vs. the World. I explored a few programming languages and showed how they changed my Ruby.

To show how Go addresses concurrency, I decided to build a program which would concurrently fetch various web resources, wait for all of them to be fetched, then process them all at once. In other programming languages, we could have used threads and a semaphore, actors or callbacks. Go’s approach is slightly different, let’s walk through the code together.

The first part of our code gets us setup:

package main

import (
	"fmt"
	"net/http"
	"time"
)

var urls = []string{
	"https://www.rubyconf.org/",
	"https://golang.org/",
	"https://matt.aimonetti.net/",
}

The code above names our package then imports a few standard libraries that we are going to need. It then defines an array/slice of strings representing the urls we are going to fetch.

Next we define a type we will use a bit later:

type HttpResponse struct {
	url      string
	response *http.Response
	err      error
}

You can think of a struct type as a simple representation of a class. Technically, we are defining a structure with some typed attributes. We will later on, create instances of this defined type.

Go implements OOP slightly differently than other languages.

Methods in Go are more general than in C++, Java: they can be defined for any sort of data, even built-in types such as plain, “unboxed” integers. They are not restricted to structs (classes).

We can therefore define methods/functions for any type of data, including “any/all” types. This approach to types is called structural typing.

Here is the code:

func asyncHttpGets(urls []string) []*HttpResponse {
	ch := make(chan *HttpResponse)
	responses := []*HttpResponse{}
	client := http.Client{}
	for _, url := range urls {
		go func(url string) {
			fmt.Printf("Fetching %s \n", url)
			resp, err := client.Get(url)
			ch <- &HttpResponse{url, resp, err}
			if err != nil && resp != nil && resp.StatusCode == http.StatusOK {
				resp.Body.Close()
			}
		}(url)
	}

	for {
		select {
		case r := <-ch:
			fmt.Printf("%s was fetched\n", r.url)
			if r.err != nil {
				fmt.Println("with an error", r.err)
			}
			responses = append(responses, r)
			if len(responses) == len(urls) {
				return responses
			}
		case <-time.After(50 * time.Millisecond):
			fmt.Printf(".")
		}
	}
	return responses
}

This is the meat of our application. And there is quite a lot of going on in just a few lines. Assuming you aren’t familiar with Go, I’ll walk though the code.

Let’s start with the signature:

• the function is named asyncHttpGets
• it takes an argument namedurls which is an “array” of strings (I used quotes around the word array because it’s technically what Go calls a slice)
• it returns an “array” of HttpResponse pointers

Then in the function body:

We start by creating an instance of a channel and assigning it to the ch variable name. Think of a channel as a pipe like in unix. We can write to and read from that channel.

In the next line we create an empty instance of a slice containing pointers to HttpResponse objects.

Then, using the for range language construct, we iterate through our urls, storing the current value being used into the scoped variable url. The url is then available within the block/lambda/closure marked by the curly braces.

Now this is where the async construct comes in. Using the go keyword, we define an anonymous function that takes a string argument representing a url.

The function prints this string, then uses the net/http library to fetch the web resource. We use the returned data to create an instance of our HttpResponse type and send it to the channel.

This part gets a bit confusing because I reused the name url. We call this anonymous function right away passing it the url variable set by the loop.

You might wonder why we bother to create an anonymous function and call it right away instead of just executing the code directly. The go keyword executes the code that is passed in as a goroutine which is well explained here

A goroutine is a function executing concurrently with other goroutines in the same address space. It is lightweight, costing little more than the allocation of stack space. And the stacks start small, so they are cheap, and grow by allocating (and freeing) heap storage as required.

In other words, you start a goroutine and you let the “system” handle how it wants to deal with the low level details. Technically, goroutines might run in one or multiple threads, but you don’t need to know. We trigger each http fetch in a separate goroutine and then each response is pushed down the channel.

The second block of code begins with another for loop containing a switch/case statement. The case statement checks if something is in the channel. If there is something, we…

• allocate the data to the r variable
• print the resource’s url
• append the resource to the slice we created at the beginning of the function.

If the length of the array is the same as the length of all urls we want to fetch, we are done fetching all our resources and can return. While still waiting for responses, we print a dot every 50ms.

Update: In the first version of this blog post I had used a ‘default’ case statement to print the dot and sleep for 50ms so the loop wouldn’t be too tight and the concurrency effect was more obvious. But some HN comments pointed out that it wasn’t needed and I shouldn’t block. For reference here is what I had before (don’t use this code):

	for {
		select {
		case r := <-ch:
			fmt.Printf("%s was fetched\n", r.url)
			responses = append(responses, r)
			if len(responses) == len(urls) {
				return responses
			}
		default:
			fmt.Printf(".")
			time.Sleep(50 * time.Millisecond)
		}
	}

Thank you HackerNews.

With that code constructed, our main can make use of it like this:

func main() {
	results := asyncHttpGets(urls)
	for _, result := range results {
		if result != nil && result.response != nil {
			fmt.Printf("%s
                 result.response.Status)
		}
	}
}

Running the code looks like this:

$ go run concurrency_example.go
.Fetching https://www.rubyconf.org/
Fetching https://golang.org/
Fetching https://matt.aimonetti.net/
.....https://golang.org/ was fetched
.......https://www.rubyconf.org/ was fetched
.https://matt.aimonetti.net/ was fetched
https://golang.org/
https://www.rubyconf.org/
https://matt.aimonetti.net/

As you can see from the print statements, the 3 urls are triggered in a sequential way, but the responses come back in different orders due to different server latencies and response transfer time.

Conclusion

Go was designed to make concurrency easy for developers. It is a very well documented language and you will find on this page a lot of information about its concurrency philosophy and details about each available constructs works.

I like that the language is very simple and the constructs explicit. If you want to write concurrent code, Go pushes you to do it in a specific style. That style is clear and comfortable for me. My code stays simple, I don’t go crazy with callbacks, and the conventions make it simple for everyone else to understand my code.

Whether or not Go appeals to you stylistically, clearly the designers stayed close to the goal of developing to a 21st century C with a special focus on concurrency with a unix approach.