Understanding the architecture of Jenkins is crucial for effectively using and managing Jenkins in your projects. This section will cover the key components and their interactions within the Jenkins ecosystem.

1. Jenkins Master

   • Role: The Jenkins master is the central control unit of Jenkins. It is responsible for managing:
     • Scheduling build jobs.
     • Dispatching builds to the nodes (slaves) for the actual job execution.
     • Monitoring the nodes (slaves) and recording the build results.
     • Providing the user interface for configuring Jenkins and viewing the build results.
   • Components:
     • Web UI: The interface through which users interact with Jenkins.
     • Configuration: Stores job configurations, build logs, and other metadata.
     • Scheduler: Manages the queue of build jobs and assigns them to available nodes.

2. Jenkins Nodes (Slaves)

   • Role: Nodes are the machines that perform the actual build tasks. They can be physical, virtual, or cloud-based.
   • Components:
     • Executor: A slot for running a build. Each node can have multiple executors, allowing it to run multiple builds concurrently.
     • Workspace: A directory on the node where the build process takes place.

3. Build Jobs

   • Role: A build job is a task or a set of tasks that Jenkins executes. It can be a simple task like compiling code or a complex pipeline involving multiple stages.
   • Types:
     • Freestyle Projects: Basic build jobs with a simple configuration.
     • Pipeline Projects: Advanced jobs defined using a domain-specific language (DSL) for more complex workflows.

4. Plugins

   • Role: Plugins extend the functionality of Jenkins. They can add new build steps, integrate with other tools, or provide additional user interface elements.
   • Examples:
     • SCM Plugins: Integrate with version control systems like Git, SVN, etc.
     • Build Tools Plugins: Integrate with build tools like Maven, Gradle, etc.
     • Notification Plugins: Send build notifications via email, Slack, etc.

Below is a simplified diagram of Jenkins architecture:

1. Job Scheduling: When a build job is triggered (manually or automatically), the Jenkins master schedules it and places it in the build queue.
2. Job Dispatching: The master dispatches the job to an available node based on the node's labels and availability.
3. Job Execution: The node executes the job in its workspace, using its executors to run the build steps.
4. Result Reporting: The node reports the build results back to the master, which records them and updates the user interface.

Let's create a simple Jenkins job to understand the interaction between the master and nodes.

1. Create a New Job:

   • Go to the Jenkins dashboard.
   • Click on "New Item".
   • Enter a name for your job (e.g., "HelloWorldJob").
   • Select "Freestyle project" and click "OK".

2. Configure the Job:

   • In the job configuration page, scroll down to the "Build" section.
   • Click on "Add build step" and select "Execute shell" (or "Execute Windows batch command" if on Windows).
   • Enter the following command:

     echo "Hello, World!"
     

3. Save and Build:

   • Click "Save" to save the job configuration.
   • Click "Build Now" to trigger the job.

4. View the Results:

   • Once the build is complete, click on the build number in the "Build History" section.
   • Click on "Console Output" to see the result of the build.

• The Jenkins master schedules the "HelloWorldJob" and places it in the build queue.
• The master dispatches the job to an available node.
• The node executes the shell command echo "Hello, World!" in its workspace.
• The node reports the build result back to the master, which updates the Jenkins dashboard.

In this section, we covered the key components of Jenkins architecture, including the Jenkins master, nodes, build jobs, and plugins. We also explored how these components interact with each other through a practical example. Understanding this architecture is fundamental for effectively using Jenkins in your projects. In the next section, we will dive into the Jenkins dashboard and explore its various features.