Understanding the different types of Jenkins pipelines - scripted and declarative

Jenkins is an open-source automation server that allows developers to automate various tasks in the software development lifecycle. It provides support for continuous integration and continuous delivery (CI/CD) pipelines, which are essential for efficiently building, testing, and deploying software.

When working with Jenkins pipelines, you have two main options: scripted and declarative. Both types offer similar functionality but differ in their syntax and approach. In this article, we will explore the differences between scripted and declarative pipelines and help you understand when to choose one over the other.

Scripted Pipeline

A scripted pipeline in Jenkins is written in Groovy, a powerful scripting language that runs on the Java Virtual Machine (JVM). It provides full flexibility and control over your pipeline by allowing you to script each stage and step explicitly.

Scripted pipelines are defined in a single Jenkinsfile, which contains the complete pipeline definition. You have the freedom to write custom code, use loops, conditionals, and any Groovy feature to define your pipeline logic. This flexibility is useful for complex scenarios where you need fine-grained control or have existing scripts that you want to integrate into your pipeline.

Here's an example of a scripted pipeline:

node {
    stage('Build') {
        // Code to build your application
    }
    stage('Test') {
        // Code to run tests
    }
    stage('Deploy') {
        // Code to deploy your application
    }
}

A scripted pipeline relies on a Jenkins agent node to execute the stages. Each stage defines a specific task or set of tasks to be performed. Inside each stage, you can call various Jenkins functions or write custom Groovy code.

While scripted pipelines offer great flexibility, they can become more complicated to maintain as pipelines grow in size and complexity. The lack of structure can make it harder to understand the flow of execution and can lead to more error-prone code.

Declarative Pipeline

Declarative pipelines, introduced in Jenkins 2.0, provide a more structured and opinionated way of defining pipelines. They use a declarative DSL (Domain-Specific Language) and promote a more pipeline-centric approach.

Instead of writing custom code, you define your pipeline using a set of predefined blocks and parameters. These blocks represent the different stages, steps, and conditions of your pipeline. The declarative syntax enforces a defined structure and helps maintain consistency across pipelines.

Here's an example of a declarative pipeline:

pipeline {
    agent any
    stages {
        stage('Build') {
            steps {
                // Code to build your application
            }
        }
        stage('Test') {
            steps {
                // Code to run tests
            }
        }
        stage('Deploy') {
            steps {
                // Code to deploy your application
            }
        }
    }
}

In a declarative pipeline, the pipeline block serves as the entry point and defines the main structure. Inside the stages block, you define individual stages, and within each stage, you specify the steps to be executed.

Declarative pipelines offer several advantages, including better visualization of the pipeline structure, built-in error handling, and easier plugin integration. They encourage a more consistent and maintainable approach to pipeline definition.

Which one to choose?

Choosing between scripted and declarative pipelines depends on your specific needs and preferences. Here are a few points to consider:

• If you require fine-grained control, complex logic, or integration with existing scripts, a scripted pipeline can provide the flexibility you need.
• If you prefer a more structured and opinionated approach, a declarative pipeline can simplify pipeline definition and maintenance.
• Declarative pipelines are recommended for new users or teams looking for a standardized approach or those transitioning from legacy Jenkins jobs.

In many cases, a combination of both scripted and declarative pipelines can be used, taking advantage of each type's strengths.

Ultimately, the choice between the two types of pipelines comes down to your specific requirements, familiarity with Groovy scripting, and the complexity of your software development process.

Conclusion

Jenkins pipelines are a powerful way to automate your CI/CD processes. Understanding the differences between scripted and declarative pipelines is crucial for making informed decisions when designing your pipeline strategy. Both types offer their own advantages and determining which one is best for you depends on your specific needs and development environment.

Regardless of the pipeline type you choose, Jenkins provides a robust and extensible framework to ensure smooth automation of your software delivery pipeline.