Jenkins Pipeline: An Overview

Many industry giants like Expedia, Boeing, and UnitedHealth Group utilize Jenkins for their continuous delivery pipelines. Jenkins has gained immense popularity, particularly in recent years, largely due to its pipeline feature. This guide provides a comprehensive overview of the Jenkins pipeline, Jenkinsfile, and key pipeline concepts. Additionally, we will walk through the process of creating a Jenkins pipeline and provide demonstrations of both declarative and scripted pipelines.

What is a Jenkins Pipeline?

Jenkins is widely recognized for facilitating continuous integration, testing, and deployment, ensuring high-quality software delivery. In the context of continuous delivery (CD), Jenkins employs the Jenkins pipeline feature. Understanding Jenkins pipelines requires a grasp of continuous delivery and its significance.

In simple terms, continuous delivery ensures that software remains in a deployable state at all times. It allows teams to efficiently integrate changes, test them using automation tools, and deploy the builds into production. This streamlined delivery process minimizes delays and enables development teams to respond swiftly to feedback. Continuous delivery, achieved through CI/CD, significantly reduces the cost, time, and risks associated with releasing new software versions. To support CD, Jenkins introduced the pipeline feature, which we will explore in depth.

What is a Jenkins Pipeline?

A Jenkins pipeline consists of a series of automated jobs that facilitate software deployment from a source repository to end users. It provides a structured approach to integrating continuous delivery within the software development lifecycle.

Key Features of Jenkins Pipelines:

• Represent multiple Jenkins jobs within a structured workflow.

• Consist of interconnected jobs that execute in a predefined sequence.

• Improve efficiency in software deployment.

For instance, when developing a small application in Jenkins, three tasks—building, testing, and deployment—can be assigned to separate jobs. The Jenkins pipeline plugin enables execution in an orderly manner. While this method is effective for small applications, it is not ideal for complex pipelines that involve numerous stages, such as unit testing, integration testing, pre-deployment, and monitoring. Managing a large number of jobs increases maintenance costs and complicates execution. To address these challenges, Jenkins introduced the Pipeline project.

Pipeline as Code

One of the key innovations of Jenkins pipelines is the ability to define deployment processes through code. Instead of manually configuring jobs in Jenkins, the entire workflow can be scripted using a Jenkinsfile. This file is stored in a version control system and adheres to the "Pipeline as Code" approach. Below are some of the benefits of using Jenkins pipelines:

Advantages of Jenkins Pipelines:

• Uses Groovy DSL to simplify complex pipeline workflows.

• Jenkinsfile is stored in version control for easy collaboration.

• Supports user input integration for improved UI interaction.

• Resilient to unexpected Jenkins Master restarts.

• Handles complex workflows with conditional loops and parallel execution.

• Can be integrated with various plugins.

What is a Jenkinsfile?

A Jenkinsfile is a text file containing the pipeline script, which can be stored locally or in a source control management (SCM) system like Git. Developers can access, edit, and verify the pipeline code as needed. Written in Groovy DSL, a Jenkinsfile can be created using text editors or directly within the Jenkins UI.

Types of Jenkins Pipelines

Jenkins pipelines follow two primary syntaxes:

1. Declarative Pipeline: A modern approach that simplifies pipeline coding. The pipeline code is stored in a Jenkinsfile within a version control system.

2. Scripted Pipeline: The traditional method of defining pipeline scripts. These scripts are created within the Jenkins UI but are also written in Groovy DSL.

Pipeline Components

1. Pipeline

A user-defined block containing the entire process, including build, test, and deployment stages.

2. Node

Defines the execution environment for the pipeline.

3. Agent

Determines where the pipeline or specific stages run. Types include:

• Any: Runs on any available agent.

• None: No global agent; each stage must define its agent.

• Label: Runs on a labeled agent.

• Docker: Uses a Docker container for execution.

4. Stage

Represents a segment of work within the pipeline, containing multiple steps.

5. Steps

A sequence of commands executed in a defined order within a stage.

Creating a Jenkins Pipeline

Step 1: Log in to Jenkins and select 'New Item' from the dashboard.

Step 2: Provide a pipeline name and select 'Pipeline' as the project type.

Step 3: Scroll down to the pipeline section and choose either a declarative or scripted pipeline.

Step 4a: For a scripted pipeline, select 'Pipeline script' and enter your code.

Step 4b: For a declarative pipeline, choose 'Pipeline script from SCM,' select the SCM type (e.g., Git), and enter the repository URL.

Step 5: Define the Jenkinsfile path, apply changes, and save. Your Jenkins pipeline is now ready.

Declarative Pipeline Demo

In this demo, we will define a declarative pipeline in a Jenkinsfile stored in a Git repository. The pipeline consists of four stages:

• Stage 1: Executes an echo command.

• Stage 2: Uses an input directive to prompt user approval before proceeding.

• Stage 3: Utilizes a conditional "when" directive to execute steps based on branch conditions.

• Stage 4: Runs parallel execution for unit and integration tests.

Scripted Pipeline Demo

In this demonstration, we use a scripted pipeline with a node block. The script defines two stages using a for loop:

• Stage 0: Prints "Hello World" and clones a repository using Git.

• Stage 1: Executes a build job when the condition is met.

Upon execution, the scripted pipeline sequentially runs both stages.

Conclusion

This guide has provided a foundational understanding of Jenkins pipelines, including their components, advantages, and implementation. Stay tuned for a follow-up blog featuring in-depth coding examples and a complete demonstration.

For those interested in DevOps training, Naresh I Technologies offers comprehensive online and classroom training programs. We provide industry-leading DevOps certification training at an affordable price. Contact us today to start your DevOps journey!