Declarative vs. Scripted Pipelines in Jenkins

What is the difference between scripted and declarative pipelines in Jenkins?

Jenkins, a renowned figure in the domain of continuous integration and continuous delivery (CI/CD), has undergone a paradigm shift with the introduction of pipelines. This evolution has generated two main approaches: declarative and scripted pipelines. This comprehensive guide will delve into the nuances of declarative and scripted pipelines, helping you make informed decisions for your projects.

Table of contents

• Evolution of Jenkins pipelines
• Declarative pipelines advantages
• Declarative pipelines disadvantages
• Scripted pipelines advantages
• Scripted pipelines disadvantages
• How to implement declarative pipelines in Jenkins
• How to implement scripted pipelines in Jenkins
• Choosing the right approach
• Best practices for pipeline development in Jenkins
• Conclusion

Evolution of Jenkins pipelines

Jenkins pipelines have evolved significantly to meet the demands of modern software development. Initially relying on freestyle projects, Jenkins introduced pipelines to enable more complex and flexible workflows. Over time, two primary flavors of pipelines have emerged: declarative and scripted. Understanding their differences and use cases is crucial for optimizing your CI/CD processes.

What are declarative pipelines?

Declarative pipelines in Jenkins provide a simplified and opinionated syntax for defining pipelines. The emphasis is on readability and ease of use, making them accessible to developers and non-developers alike. Declarative pipelines aim to abstract away the intricacies of the underlying structure, providing a high-level view of the pipeline flow. They are designed for simplicity and well-suited for projects with straightforward build and deployment requirements.

What are scripted pipelines?

Scripted pipelines offer a more flexible and powerful approach, leveraging the Groovy scripting language. This gives users extensive control over the pipeline logic and the ability to implement complex build and deployment processes. Unlike declarative pipelines, scripted pipelines are more expressive and allow for fine-grained control over every aspect of the pipeline. They are ideal for projects with intricate workflows and advanced automation requirements.

What are the advantages of Jenkins declarative pipeline?

Readability and conciseness

Declarative pipelines boast a concise and easily readable syntax, enhancing accessibility for users with varying levels of expertise. The clear structure promotes collaboration within the team, fostering a shared understanding of the pipeline.

Opinionated structure

Following a predefined structure, declarative pipelines enforce best practices. This opinionated approach simplifies pipeline creation by guiding users to organize workflows in a standardized manner, ensuring consistency across projects.

Ease of adoption

Declarative pipelines are especially well-suited for teams new to Jenkins or CI/CD. Their straightforward syntax and abstraction of complexity make them an excellent starting point, allowing teams to get acquainted with pipeline concepts gradually.

Limited scripting complexity

By design, declarative pipelines restrict the use of complex scripting. This limitation encourages a straightforward, structured approach to pipeline development, ideal for projects requiring less intricate automation.

Built-in step catalog

Declarative pipelines come with a built-in step catalog, providing a convenient way to incorporate standard functionality without extensive scripting. This simplifies the addition of common steps like source code checkout and artifact archiving.

Enhanced security

Declarative pipelines support a more controlled execution environment. With restricted scripting capabilities, they provide an additional layer of security by minimizing the risk of arbitrary code execution, making them suitable for projects with stringent security requirements.

What are the disadvantages of Jenkins declarative pipeline?

Less flexibility

The opinionated nature of declarative pipelines may limit flexibility for projects with unique or complex requirements. Advanced customization may be challenging to achieve within the confines of the declarative syntax, potentially leading to workarounds.

Learning curve for advanced features

While declarative pipelines are easy, mastering advanced features may require transitioning to scripted pipelines. Teams with evolving requirements might need more support as they outgrow the simplicity of the declarative syntax.

Limited expressiveness

While clear and concise, the declarative syntax might need more expressive power for highly customized or intricate CI/CD workflows. Teams with complex automation needs may find themselves constrained.

What are the advantages of Jenkins scripted pipeline?

Unmatched flexibility

Scripted pipelines offer unparalleled flexibility, providing developers with complete programmatic control. This enables the implementation of complex logic, conditional statements, and advanced automation workflows tailored to specific project needs.

Reusability of code

Scripted pipelines support the creation of reusable code snippets and functions. This modularity enhances code organization and allows for the sharing of standard pipeline components across different projects, promoting a more efficient development process.

Advanced logic and customization

Scripted pipelines shine for projects with intricate workflows or specific automation requirements. They allow the implementation of advanced programming constructs, custom functions, and conditional execution, providing a high level of customization.

Parallel execution and optimization

Scripted pipelines excel in parallel execution of tasks, optimizing the overall pipeline runtime. This feature benefits projects with extensive testing suites or complex build processes, significantly reducing overall turnaround time.

Dynamic parameterization

Scripted pipelines support dynamic parameterization, enabling the adjustment of pipeline behavior based on runtime conditions. This flexibility is advantageous for projects requiring adaptive and dynamic automation workflows.

What are the disadvantages of Jenkins scripted pipeline?

Steeper learning curve

Scripted pipelines, leveraging Groovy scripting, come with a steeper learning curve than declarative pipelines. Team members with limited scripting experience may need help to grasp and contribute, necessitating additional training.

Increased complexity and maintenance

The flexibility of scripted pipelines comes at the cost of increased complexity. Managing and maintaining intricate scripts may require higher expertise, and readability could be compromised without proper documentation.

Potential for code duplication

Scripted pipelines may duplicate code across projects without careful organization and modular design. This can impact maintainability and make enforcing consistent practices with diligent code management easier.

Dependency on external libraries

Scripted pipelines might rely on external libraries or plugins, introducing dependencies that must be managed. This can complicate versioning and updates, potentially leading to compatibility issues.

How to implement declarative pipelines in Jenkins

Open Jenkins

Log in to your Jenkins application and navigate to the dashboard.

Create a new pipeline

Click on New Item and select Pipeline. Enter a name for your pipeline and choose the Pipeline option.

Define pipeline script

Select the Pipeline script option in the pipeline section and enter your declarative pipeline script.

Utilize the following example as a template.

pipeline {

    agent any


    environment {

        APP_NAME = 'my-app'

        MAVEN_HOME = tool 'Maven'

    }



    stages {

        stage('Build') {

            steps {

                script {

                    echo "Building ${APP_NAME} using Maven"

                    sh "${MAVEN_HOME}/bin/mvn clean install"

                }

            }

        }



        stage('Test') {

            steps {

                script {

                    echo "Running tests for ${APP_NAME}"

                    sh "${MAVEN_HOME}/bin/mvn test"

                }

            }

        }



        stage('Deploy') {

            steps {

                script {

                    echo "Deploying ${APP_NAME}"

                    sh 'kubectl apply -f deployment.yaml'

                }

            }

        }

    }

}

Explanation of additional elements

• The environment block allows you to set environment variables for the entire pipeline.
• The tool function ensures that the required tools (in this case Maven) are available for the pipeline.
• The script block is used to execute custom Groovy scripts within each stage, providing more flexibility.

Save and run

Save your pipeline configuration and trigger a build to see the declarative pipeline in action. Observe the build stages and results directly from the Jenkins dashboard.

How to implement scripted pipelines in Jenkins

Open Jenkins

Log in to Jenkins and go to the dashboard.

Create a new pipeline

To create a pipeline in Jenkins, click New Item from the dashboard. Then, select Pipeline and enter a name (e.g., MyScriptedPipeline). Finally, click OK to create the Pipeline.

Configure pipeline script from SCM

Select the Pipeline script from SCM in the pipeline section. Choose your version control system as GIT and provide the script path.

Then, fill in the required information:

• Repository URL: Specify the URL of your Git repository.
• Branches to build: Define the branch you want to build (e.g., main).
• Script Path: Specify the path to your scripted pipeline script within the repository (e.g., Jenkinsfile).
• Lightweight checkout: Check this option to obtain the pipeline script contents directly from the SCM without performing a full checkout.

Write script manually

If you prefer to write the scripted pipeline manually, choose Pipeline script instead of Pipeline script from SCM.

In the script editor, enter your scripted pipeline code. Here’s a basic example:

pipeline {

    agent any


    stages {

        stage('Build') {

            steps {

                echo 'Building...'

                // Your build steps here

            }

        }


        stage('Test') {

            steps {

                echo 'Testing...'

                // Your test steps here

            }

        }


        stage('Deploy') {

            steps {

                echo 'Deploying...'

                // Your deployment steps here

            }

        }

    }

}

Script Explanation:

• The pipeline block defines the entire pipeline.
• agent any specifies that the pipeline can run on any available agent.
• stages block contains individual stages of the pipeline (e.g., Build, Test, Deploy).
• Each stage block contains specific steps (echo statements in this example).

Why is this code different from the declarative pipeline script?

Scripted pipelines offer a more flexible and expressive syntax, providing fine-grained control over execution flow. They are ideal for complex and conditional pipeline logic and allow the use of variables and advanced programming constructs.

Save and run

Save your pipeline configuration by clicking the Save button and manually triggering a build, or set up triggers (e.g., webhook, SCM polling) to run the pipeline automatically. Observe the enhanced flexibility and control provided by the scripted approach.

Additional elements

After saving the pipeline configuration, scroll down to the bottom of the page and click the Pipeline Syntax button to navigate to a new page with two tabs:

Snippet Generator

• Under the Steps tab, select a step from the list.
• Configure the step as needed.
• Click Generate Pipeline Script to obtain the scripted pipeline code for the selected step.
• Copy the generated script and incorporate it into your Jenkinsfile.

Declarative Directive Generator

• Switch to the Declarative Directive Generator tab.
• Choose a directive from the dropdown.
• Fill out the form with the desired configuration.
• Click Generate Declarative Directive to obtain the corresponding declarative pipeline code.
• Copy the generated code for use in your Jenkinsfile.

These two generators provide a convenient way to interactively explore and generate snippets of code for your Jenkins scripted or declarative pipelines, allowing you to easily tailor your pipeline configuration on the dedicated Pipeline Syntax page.

Choosing the right approach

The decision between declarative and scripted pipelines hinges on several critical factors, each influencing the efficiency and maintainability of your CI/CD processes.

Project complexity

Declarative pipelines are better suited for simple projects with straightforward build and deployment processes, while complex projects with intricate workflows may benefit from the flexibility of scripted pipelines.

Team expertise

Declarative pipelines may be more accessible for teams with limited scripting experience, while scripted pipelines are ideal for teams with solid background in Groovy scripting.

Control requirements

Assess the level of control needed over your pipeline. Declarative pipelines abstract away some complexity, while scripted pipelines offer fine-grained control.

Maintainability

Consider the long-term maintainability of your pipelines. With their simplicity, declarative pipelines may lead to more maintainable code for specific projects.

Best practices for pipeline development in Jenkins

The effective development and maintenance of Jenkins pipelines involve adherence to best practices. These practices ensure reliability, scalability, and ease of collaboration within your CI/CD processes.

Version control

Storing your pipeline scripts in a version control system (e.g., Git) is crucial for traceability and collaboration. This practice ensures that changes to pipeline configurations are tracked and can be rolled back if necessary.

Pipeline libraries

Consider utilizing shared pipeline libraries for standard functions and steps. This promotes reusability, reduces duplication, and simplifies the maintenance of pipelines across different projects.

Secret management

Effectively manage sensitive information within your pipelines, such as API keys and passwords. Jenkins provides a built-in credential management system; external tools like HashiCorp Vault can enhance security.

Error handling

Implement proper error-handling mechanisms within your pipelines. Use constructs like try-catch blocks to gracefully handle errors and provide meaningful notifications for quick issue identification.

Documentation

Maintain clear and concise documentation for your pipelines. This includes comments within your script and external documentation. A well-documented pipeline aids in understanding, troubleshooting, and onboarding new team members.

Pipeline visualization

Leverage Jenkins plugins like Blue Ocean for enhanced visualization of pipeline stages and results. A visual representation of your pipeline can provide insights into the flow and identify bottlenecks or areas for improvement.

Parallel execution

Optimize your pipelines by leveraging parallel execution of tasks. This can significantly reduce the pipeline runtime, especially for projects with extensive testing suites.

Automated testing for pipelines

Apply the same principles of continuous testing to your pipeline scripts. Implement automated tests for your pipelines to catch potential issues early in development.

Notification and Alerts

Establish notification and alert mechanisms to keep stakeholders informed about the pipeline status. Immediate alerts for failures or issues enable quick responses and minimize downtime in the software delivery pipeline.

Final thoughts on declarative vs. scripted pipelines

Choose between declarative and scripted pipelines in Jenkins based on your project’s needs, team expertise, and required control level. Best practices and Jenkins’ capabilities can streamline pipeline development for efficient and reliable automation. Detailed steps and insights empower you to harness the full potential of Jenkins pipelines for your software delivery.

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• How to create Jenkins pipelines
• Creating pipelines with Jenkins Blue Ocean
• How to use Jenkins shared libraries
• Jenkins monitoring complete guide
• Understanding Jenkins’ architecture
• How to integrate Jenkins with Git and SVN
• Jenkins in test automation

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