What is HCL (Hashicorp Configuration Language), how does it relate to Terraform, and why is it growing in popularity?

HCL (Hashicorp Configuration language), is rapidly growing in popularity. Last year's Octoverse report by GitHub showed it to be the second fastest growing language on the platform, more than doubling in contributors since 2017 (Kotlin was top, with GitHub contributors growing 2.6 times). However, despite its growth, it hasn’t had the level of attention that other programming languages have had.

One of the reasons for this is that HCL is a configuration language. It's also part of a broader ecosystem of tools built by cloud automation company HashiCorp that largely center around Terraform.

What is Terraform?

Terraform is an infrastructure-as-code tool that makes it easier to define and manage your cloud infrastructure. HCL is simply the syntax that allows you to better leverage its capabilities. It gives you a significant degree of control over your infrastructure in a way that’s more ‘human-readable’ than other configuration languages such as YAML and JSON.

HCL and Terraform are both important parts of the DevOps world. They are not only built for a world that has transitioned to infrastructure-as-code, but also one in which this transition demands more from engineers. By making HCL a more readable, higher-level configuration language, the language can better facilitate collaboration and transparency between cross-functional engineering teams.

With all of this in mind, HCL’s growing popularity can be taken to indicate broader shifts in the software development world. HashiCorp clearly understands them very well and is eager to help drive them forward.

But before we go any further, let's dive a bit deeper into why HCL was created, how it works, and how it sits within the Terraform ecosystem.

Why did Hashicorp create HCL?

The development of HCL was borne from of HashiCorp’s experience of trying multiple different options for configuration languages. “What we learned,” the team explains on GitHub, “is that some people wanted human-friendly configuration languages and some people wanted machine-friendly languages.” The HashiCorp team needed a compromise - something that could offer a degree of flexibility and accessibility.

As the team outlines their thinking, it’s clear to see what the drivers behind HCL actually are. JSON, they say, “is fairly verbose and... doesn't support comments” while YAML is viewed as too complex for beginners to properly parse and use effectively.

Traditional programming languages also pose problems. Again, they’re too sophisticated and demand too much background knowledge from users to make them a truly useful configuration language.

Put together, this underlines the fact that with HCL HashiCorp wanted to build something that is accessible to engineers of different abilities and skill sets, while also being clear enough to enable appropriate levels of transparency between teams. It is “designed to be written and modified by humans.”

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How does the Hashicorp Configuration Language work?

HCL is not a replacement for the likes of YAML or JSON. The team’s aim “is not to alienate other configuration languages. It is,” they say, “instead to provide HCL as a specialized language for our tools, and JSON as the interoperability layer.” Effectively, it builds on some of the things you can get with JSON, but reimagines them in the context of infrastructure and application configuration.

According to the documentation, we should see HCL as a “structured configuration language rather than a data structure serialization language.” HCL is “always decoded using an application-defined schema,” which gives you a level of flexibility. It quite means the application is always at the center of the language. You don't have to work around it.

If you want to learn more about the HCL syntax and how it works at a much deeper level, the documentation is a good place to start, as is this page on GitHub.

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The advantages of HCL and Terraform

You can’t really talk about the advantages of HCL without also considering the advantages of Terraform. Indeed, while HCL might well be a well designed configuration language that’s accessible and caters to a wide range of users and use cases, it’s only in the context of Terraform that its growth really makes sense.

Why is Terraform so popular?

To understand the popularity of Terraform, you need to place it in the context of current trends and today’s software marketplace for infrastructure configuration.

Terraform is widely seen as a competitor to configuration management tools like Chef, Ansible and Puppet. However, Terraform isn’t exactly a configuration management - it’s more accurate to call it a provisioning tool (config management tools configure software on servers that already exist - provisioning tools set up new ones).

This is important because thanks to Docker and Kubernetes, the need for configuration has radically changed - you might even say that it’s no longer there. If a Docker container is effectively self-sufficient, with all the configuration files it needs to run, then the need for ‘traditional’ configuration management begins to drop.

Of course, this isn’t to say that one tool is intrinsically better than any other. There are use cases for all of these types of tools. But the fact remains is that Terraform suits use cases that are starting to grow.

Part of this is due to the rise of cloud agnosticism. As multi-cloud and hybrid cloud architectures become prevalent, DevOps teams need tools that let them navigate and manage resources across different platforms.

Although all the major public cloud vendors have native tools for managing resources, these can sometimes be restrictive. The templates they offer can also be difficult to reuse. Take Azure ARM templates, for example - it can only be used to create an Azure resource. In contrast, Terraform allows you to provision and manage resources across different cloud platforms.

Conclusion: Terraform and HCL can make DevOps more accessible

It’s not hard to see why ThoughtWorks sees Terraform as such an important emerging technology. (In the last edition of ThoughtWorks Radar is claimed that now is the time to adopt it.) But it’s also important to understand that HCL is an important element in the success of Terraform. It makes infrastructure-as-code not only something that’s accessible to developers that might have previously only dipped their toes in operations, but also something that can be more collaborative, transparent, and observable for team members.

The DevOps picture will undoubtedly evolve over the next few years, but it would appear that HashiCorp is going to have a big part to play in it.