What’s the Difference Between Declarative and Imperative Configuration in IaC?

The rise of cloud computing and modern DevOps practices has made Infrastructure as Code (IaC) an essential part of any software delivery pipeline. IaC allows teams to manage and provision their infrastructure using machine-readable definition files, rather than through manual configuration or interactive tools. This approach brings the benefits of software development—version control, testing, and automation—to the world of infrastructure management. However, within the IaC landscape, a fundamental philosophical difference exists in how teams define and manage their infrastructure: the choice between a declarative and an imperative approach. Understanding this distinction is not just an academic exercise; it is crucial for choosing the right tools, designing efficient workflows, and ensuring the consistency and reliability of your environments. A declarative approach focuses on what the final state of the infrastructure should be, while an imperative approach focuses on how to get there. This blog post will demystify these two concepts, providing a comprehensive breakdown of their differences, their respective pros and cons, and how they shape the way teams build and manage their infrastructure in the cloud.

What Is Infrastructure as Code (IaC)?

Before diving into the differences between declarative and imperative configurations, it is essential to have a clear understanding of Infrastructure as Code (IaC). In the past, managing IT infrastructure involved manually provisioning hardware, installing operating systems, and configuring services. This process was slow, error-prone, and difficult to scale. IaC revolutionizes this by treating infrastructure resources (e.g., virtual machines, databases, networks) as code. This means that infrastructure is defined in files that can be stored in a version control system like Git, allowing for the same benefits as application code: versioning, collaboration, and automated testing. IaC allows teams to automate the entire provisioning process, ensuring consistency across development, staging, and production environments. It is a cornerstone of modern DevOps and Continuous Delivery practices, as it provides the foundation for building repeatable, reliable, and scalable infrastructure. The ability to version-control your infrastructure code also makes it easy to roll back to a previous state in case of a failure, which is a key part of building a resilient system. Without IaC, modern cloud-native architectures would be nearly impossible to manage at scale. It is a core principle that has enabled the rapid growth and complexity of today's digital services.

Declarative IaC: The Desired State Approach

A declarative approach to IaC focuses on the desired state of the infrastructure. In this model, you simply describe the resources you want to have and their final configuration, without specifying the step-by-step process of how to get there. The IaC tool is then responsible for figuring out the necessary actions to achieve that state. For example, you might write a configuration file that says, "I want one virtual machine with a specific operating system, a database instance, and a load balancer." You do not need to specify the commands to create the VM, then the database, and then link them. The tool's job is to read your desired state, compare it to the current state of your infrastructure, and then automatically create, update, or delete resources as needed to match your declaration. This approach is highly idempotent, meaning that running the same configuration file multiple times will always result in the same final state, regardless of the starting point. This makes declarative IaC predictable and reliable. It is the preferred approach for managing complex, long-lived infrastructure and is a key enabler of continuous delivery. Tools that follow a declarative model include Terraform, Kubernetes, and AWS CloudFormation. They empower teams to focus on the "what" of infrastructure, abstracting away the complex "how."

Imperative IaC: The Procedural Approach

In contrast, an imperative approach to IaC focuses on the step-by-step process required to provision infrastructure. It is a procedural model where you explicitly write the commands that the IaC tool must execute to reach a desired state. This is similar to a traditional script where you might write a command to create a virtual machine, followed by a command to install a web server, and then a command to start the web server. The state of the infrastructure is a direct result of the executed commands. If you run the same script twice, it might fail or create duplicate resources, unless you explicitly add logic to handle those scenarios. Imperative tools give you granular control over the provisioning process, which can be useful for complex or highly customized workflows. However, this also means the burden of managing the state, dependencies, and idempotency falls on the user. If a step fails, you must manually intervene to correct the issue and resume the process. This approach is more flexible but also more error-prone and less scalable than a declarative model. Tools that follow an imperative model include Ansible (though it has some declarative features), and scripting languages like Bash or Python that interact with cloud provider APIs or CLIs. It is often used for one-off tasks or for configuring software on pre-existing infrastructure.

What’s the Core Difference Between the Two?

The core difference between declarative and imperative approaches lies in their fundamental philosophy: declarative focuses on the desired end state, while imperative focuses on the sequence of steps to get there. This distinction has far-reaching implications for how you manage your infrastructure. In a declarative model, the tool maintains a record of the current state of your infrastructure (the state file) and compares it to your desired state (the configuration file). It then automatically generates a plan to bridge the gap. This makes it a great choice for managing the lifecycle of long-lived resources like databases and virtual private clouds (VPCs). For a declarative tool, a resource is either present and in the correct state, or it is not. If it is not, the tool creates it. If it is present but in the wrong state, the tool updates it. If it is present but not in the configuration, the tool deletes it. In an imperative model, there is no concept of a desired state or a state file. The tool simply executes a series of commands in the order they are written. This requires the user to manually manage the state of the infrastructure and to write scripts that are robust enough to handle all possible scenarios, including failures and pre-existing resources. This makes it a more flexible but also more brittle approach. The table below provides a clear comparison of the two approaches across a number of key criteria.

Pros and Cons of Each Approach

Each IaC approach has its own set of strengths and weaknesses, making one more suitable for certain use cases than the other. Understanding these trade-offs is essential for making an informed decision about your infrastructure management strategy.

Declarative Pros

• Simplified Management: You focus on the end result, not the steps. The tool handles the logic of creating, updating, or deleting resources.
• Idempotency: The desired state model ensures that running the configuration multiple times will not cause unintended side effects, leading to more reliable and repeatable deployments.
• Automation-Friendly: The state-driven nature of declarative tools makes them ideal for automated CI/CD pipelines. The pipeline can simply apply the configuration, and the tool will handle the rest.
• Scalability: As your infrastructure grows in complexity, a declarative approach scales well because you can manage your entire infrastructure through a single, version-controlled file.

Declarative Cons

• Less Control: The tool abstracts away the steps, which can sometimes make it difficult to debug or to perform highly specific, granular actions.
• Higher Learning Curve: The mental model of managing state and dependencies can be more complex to grasp initially than a simple step-by-step script.

Imperative Pros

• Granular Control: You have full control over the sequence of operations. This can be useful for complex workflows that require a specific order of execution.
• Lower Learning Curve: For those with scripting experience, the procedural nature of imperative tools is often easier to grasp.
• Flexibility: It is easier to write a script for one-off tasks or highly customized configurations that do not fit a standardized declarative model.

Imperative Cons

• No Built-in Idempotency: The user is responsible for scripting the logic to handle all scenarios, which is error-prone and time-consuming.
• State Management is a Burden: You have to manually track the state of your infrastructure, which is difficult and can lead to environment drift.
• Difficult to Debug: When a complex script fails, it can be difficult to determine the exact cause and to resume the process from the point of failure.
• Less Scalable: As infrastructure grows, imperative scripts become unwieldy, difficult to maintain, and prone to errors.

Choosing the Right Approach

The choice between a declarative and an imperative approach is not a matter of one being inherently superior to the other. Instead, it is a matter of choosing the right tool for the job. In most modern cloud environments, a declarative approach is the clear winner for managing the lifecycle of your infrastructure. Tools like Terraform and CloudFormation are designed for this purpose, and their state-driven model provides the consistency and reliability required for a professional software delivery pipeline. The idempotent nature of declarative tools is a critical feature that simplifies automation and reduces the risk of human error. It allows teams to confidently apply changes to their infrastructure, knowing that the tool will handle the logic of updating or creating resources as needed. However, the imperative approach still has its place. It is often used for tasks that involve a specific sequence of commands, such as running a script to configure a virtual machine after it has been created. A hybrid model, where a declarative tool is used for provisioning and an imperative tool is used for configuration, is a common and effective strategy. The goal is to use the right tool for the right task, leveraging the strengths of both approaches to build a robust and reliable system. A declarative tool can be used to manage the infrastructure, while an imperative tool can be used to manage the software on top of it. This separation of concerns allows each tool to excel at its specific job, creating a more cohesive and manageable system overall.

The Future of IaC: A Hybrid Model

As the IaC ecosystem matures, the line between declarative and imperative is becoming increasingly blurred. Many modern tools, such as Ansible, have introduced declarative features to simplify configuration management. Similarly, declarative tools like Terraform often include ways to execute imperative scripts as part of a larger workflow. The future of IaC is not a winner-takes-all scenario but rather a hybrid model that combines the strengths of both approaches. In this model, a declarative tool manages the foundational, long-lived infrastructure, such as networking and compute resources. This ensures consistency and reliability at the core of the system. An imperative tool or script is then used to handle the more dynamic, procedural tasks, such as installing software, running a database migration, or performing a one-off configuration change. This hybrid approach provides the best of both worlds: the consistency and scalability of a declarative model, combined with the flexibility and control of an imperative one. It allows teams to build a robust, reliable, and highly automated software delivery pipeline that can handle the complexity of modern cloud environments. By using a hybrid model, organizations can take advantage of the strengths of each approach while mitigating their weaknesses, creating a system that is both predictable and adaptable. The key is to design a system that uses the right tool for the right job, allowing each to excel at its specific task without creating unnecessary complexity. This model is becoming the industry standard, and understanding it is crucial for anyone working in the DevOps space.

Conclusion

The distinction between declarative and imperative configuration is a cornerstone of Infrastructure as Code (IaC). While a declarative approach focuses on the desired state of the infrastructure, an imperative approach focuses on the step-by-step process of how to get there. The declarative model, championed by tools like Terraform and Kubernetes, provides the benefits of idempotency, scalability, and simplified management, making it the superior choice for managing long-lived, complex infrastructure at scale. The imperative model, while flexible and intuitive for one-off tasks, lacks the built-in reliability and state management of its declarative counterpart. In practice, a hybrid model that combines both approaches—using a declarative tool for provisioning and an imperative tool for configuration—is often the most effective strategy. Ultimately, understanding this difference is key to building a robust, reliable, and highly automated software delivery pipeline that can keep pace with the demands of modern cloud computing and DevOps practices.

Frequently Asked Questions