Kubernetes 07 - Imperative vs Declarative Approaches in Kubernetes

Kubernetes, offers two primary approaches to managing your infrastructure: imperative and declarative. Understanding the difference between these approaches is important for effectively managing your Kubernetes clusters. In this article, we’ll explore both methods, their pros and cons, and when to use each approach.

Understanding Imperative vs Declarative Approaches

Before diving into Kubernetes-specific examples, let’s understand the general concept of imperative and declarative approaches using a simple analogy.

The Cofffee Analogy

Imagine you want a cup of coffee:

• Imperative approach: You focus on how to make the coffee. You might say, “I boil water, scoop out 42 grams of medium-fine grounds, pour over 700 grams of water, etc.”
• Declarative approach: You focus on what you want. You might say to a barista, “I’d like a cup of coffee.” The barista (acting as the engine) works through the steps, including retrying if necessary, and is only finished when you have your cup of coffee.

Now, let’s see how these concepts apply to Kubernetes.

Kubernetes Imperative Approach

In Kubernetes, the imperative approach involves using commands that focus on how to achieve a desired state. You typically use these commands when you know the current state of your cluster and want to make specific changes.

Examples of Imperative Commands

• kubectl run: Create a new deployment
• kubectl create deployment: Create a deployment with more options
• kubectl update: Update an existing resource

Pros of the Imperative Approach

1. Easier to get started: For beginners, imperative commands are often more intuitive.
2. Direct control: You have precise control over each action.
3. Ideal for quick tasks: Perfect for one-off tasks or quick fixes.

Cons of the Imperative Approach

1. Difficult to automate: Each change requires a specific command, making automation challenging.
2. Less consistent: It’s easier to make mistakes or forget steps when manually entering commands.
3. No clear history: It’s harder to track changes over time.

Kubernetes Declarative Approach

The declarative approach in Kubernetes focuses on what you want your cluster to look like, rather than how to get there. You describe the desired state in a YAML file, and Kubernetes figures out how to achieve that state.

Examples of Declarative Commands

• kubectl apply -f my-resources.yaml: Apply the configuration in a YAML file

This command applies the configuration specified in my-resources.yaml, regardless of the current state of the cluster.

Pros of the Declarative Approach

1. Easier to automate: You can version control your YAML files and apply them consistently.
2. Idempotent: Running the same command multiple times produces the same result.
3. Better for complex setups: You can manage multiple related resources in a single file or directory.

Cons of the Declarative Approach

1. Steeper learning curve: Requires understanding YAML syntax and Kubernetes resource definitions.
2. More initial setup: Creating a YAML file takes more time than running a quick command.

When to Use Each Approach

Use the imperative approach when:

• You’re learning Kubernetes and want to understand how things work.
• You need to quickly create or modify a resource for testing or debugging.
• You’re performing one-time tasks that don’t need to be repeated or version-controlled.

Use the declarative approach when:

• You’re setting up production environments.
• You want to implement GitOps practices.
• You need to manage complex applications with multiple interconnected resources.
• You want to ensure consistency across multiple clusters or environments.

Three Management Approaches

Kubernetes offers three main approaches to managing resources. Each has its own use cases, advantages, and drawbacks. Let’s explore them in detail:

• Imperative Commands Examples: kubectl run, kubectl expose, kubectl scale, kubectl edit, kubectl create deployment
  • In this approach, you directly tell Kubernetes what to do using specific commands. It’s like giving step-by-step instructions to achieve a desired state.
  • Ideal for: Development environments, learning purposes, and personal projects
  • Pros:
    • Easiest to learn and use for beginners
    • Provides immediate results
    • Great for quick experimentation
  • Cons:
    • Hardest to manage over time
    • Doesn’t provide a clear history of changes
    • Difficult to reproduce or automate
• Imperative Object Configuration Examples: kubectl create -f file.yml, kubectl replace -f file.yml, kubectl delete -f file.yml
  • This approach uses YAML files to define objects, but you still use imperative commands to create, update, or delete these objects. It’s a step towards more structured management.
  • Ideal for: Small production environments or scenarios where you manage one file per command
  • Pros:
    • Allows you to store your configuration in git-based YAML files
    • Provides a balance between simplicity and reproducibility
  • Cons:
    • Can be hard to automate
    • Requires managing multiple commands for updates
• Declarative Object Configuration Examples: kubectl apply -f file.yml, kubectl apply -f directory/, kubectl diff -f file.yml
  • In this approach, you define the desired state of your system in YAML files, and Kubernetes figures out how to achieve that state. You primarily use the kubectl apply command, which can create or update resources as needed.
  • Ideal for: Production environments, complex setups, and situations requiring easy automation
  • Pros:
    • Best for production use
    • Easiest to automate
    • Allows managing multiple files or entire directories at once
    • Provides a clear history of changes when combined with version control
  • Cons:
    • Can be harder to understand for beginners
    • Sometimes difficult to predict the exact changes that will occur

The Most Important Rule

Regardless of which approach you choose, the most crucial rule is “Don’t mix the three approaches”

Mixing these approaches can lead to confusion, conflicts, and unexpected results. For example, if you create a deployment using an imperative command and then try to update it using a declarative approach, you might encounter conflicts or lose track of the current state.

Choose the approach that best fits your needs and stick with it consistently. As you become more comfortable with Kubernetes and your projects grow in complexity, you’ll likely find yourself moving from imperative commands towards declarative object configuration.

Remember, while imperative commands are great for learning and quick tasks, declarative approaches become invaluable as your projects scale and require more rigorous management and version control.