Declarative vs Imperative Programming for Infrastructure as Code (IaC)

Infrastructure as Code, or IaC, is a DevOps practice that involves turning your infrastructure configurations into software scripts. This allows you to automatically provision and patch new resources, which saves time and leads to fewer configuration mistakes. IaC tools use either a declarative or imperative programming language to execute configuration scripts. Let’s define these two programming paradigms before comparing declarative vs imperative programming for DevOps infrastructure as code so you can see the advantages and disadvantages of each.

Declarative Programming for IaC: Defining What

In declarative programming, you specify the name and properties of the infrastructure resources you wish to provision, and then the IaC tool figures out how to achieve that end result on its own. You declare to your IaC tool what you want, but not how to get there. Some examples of popular IaC tools that use the declarative programming paradigm include Terraform, Puppet, Ansible, Salt, and CloudFormation.

Imperative Programming for IaC: Defining How

In imperative programming, you specify a list of steps the IaC tool should follow to provision a new resource. You tell your IaC tool how to create each environment using a sequence of command imperatives. Chef is a popular imperative IaC tool, and both Ansible and Salt have some support for imperative programming as well.

Declarative vs Imperative Programming

When you compare declarative vs imperative programming, you’ll find that each paradigm comes with advantages and disadvantages that you need to consider before choosing your approach to IaC.

Pros and Cons of Declarative Programming

Declarative programming is a popular approach to infrastructure as code. You define the desired end-state of the final configuration and the IaC solution figures out how to get there. Declarative programming is highly idempotent, or repeatable, which means you can execute your IaC commands over and over again and still achieve the same result. The declarative paradigm also adapts well to configuration drift – the inevitable, slow changes to your infrastructure over time – because the IaC tool’s provisioning steps are not explicitly defined.

The biggest drawback of the declarative approach is that you give up a lot of control over the individual steps in the provisioning process. It’s also not the best choice for small fixes and updates that can be handled by a simple CLI (command line interface) script – declarative programming can overcomplicate matters and slow down the process.  

Pros and Cons of Imperative Programming

Imperative programming, on the other hand, requires more scripting knowledge because you must write commands for every provisioning step. This gives you control over how you accomplish infrastructure tasks, which is ideal when you need to make small changes, optimize for a specific purpose, or account for software quirks.

The biggest initial challenge to the imperative approach is that it requires a high level of skill with the programming language, which infrastructure teams in the initial stages of the DevOps journey may not have yet. Imperative IaC scripts are often less idempotent as well — your predefined steps can lead to different results depending on the environment. Plus, imperative IaC scripts are so explicit that an error with one step can cause the whole thing to fail. 

Declarative vs Imperative Programming Comparison:

  Declarative Programming Advantages

Imperative Programming Advantages

Requires less coding skill

Control over every step of the process

Highly repeatable

Ideal for simple or one-off tasks

Adapts well to configuration drift

Follows a familiar, step-by-step approach

Declarative Programming Disadvantages

Imperative Programming Disadvantages

Less control over the process

Requires a lot of programming knowledge

Can overcomplicate simple tasks

Less idempotent

May be harder to conceptualize

Explicitness makes it error-prone

Overall, many organizations seeking to fully automate and orchestrate their DevOps infrastructure prefer the declarative approach. You can use declarative IaC tools to create highly repeatable and adaptable configuration scripts without years of coding experience. However, if you’re easing into infrastructure automation with a few CLI scripts, or you don’t need a full orchestration platform, the imperative approach is often simpler and easier to manage.

Comparing declarative vs imperative programming for IaC is just one step in the DevOps infrastructure automation process.