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DevOps Day 78: Conditional Deployments with Jenkins Pipelines

Today's task was an advanced exercise in Jenkins Pipeline scripting. I built upon my knowledge of pipelines and agent nodes to create a parameterized deployment job.

The challenge was to create a single pipeline that could deploy either the master branch or the feature branch of a web application, depending on a choice made by the user at runtime. This required me to combine three key concepts: Pipeline Parameters, Conditional Logic (using if/else in Groovy), and Git Branch Selection.

Table of Contents

The Task

My objective was to create a Jenkins pipeline job named nautilus-webapp-job. The requirements were: 1. Source Code: Use the web_app repository hosted on Gitea (user sarah). 2. Agent Node: Use the Storage Server (ststor01) agent, deploying to /var/www/html. 3. Parameter: The job must accept a String Parameter named BRANCH. 4. Condition: - If BRANCH is master, deploy the master branch. - If BRANCH is feature, deploy the feature branch. 5. Verification: The app must be accessible at the main URL https://<LBR-URL>.

My Step-by-Step Solution

The solution involved configuring the agent (if not already done), adding credentials, and writing the conditional pipeline script.

Phase 1: Configuring the Agent Node (Prerequisite)

If the Storage Server node was not already configured from the previous task: 1. I logged into Jenkins as admin. 2. I went to Manage Jenkins > Nodes > New Node. 3. Node Name: Storage Server, Type: Permanent Agent. 4. Remote root directory: /var/www/html. 5. Labels: ststor01. 6. Launch method: SSH, Host: ststor01, Credentials: natasha (Bl@kW). 7. I verified the agent launched successfully.

Phase 2: Creating the Credentials

  1. I went to Manage Jenkins > Credentials.
  2. I added a new Username with Password credential for Gitea:
    • Username: sarah
    • Password: Sarah_pass123
    • ID: git-creds (This ID is referenced in my script).

Phase 3: Creating the Pipeline Job

  1. I created a new Pipeline job named nautilus-webapp-job.
  2. In the configuration, I checked "This project is parameterized".
  3. I added a String Parameter:
    • Name: BRANCH
    • Default Value: master (A safe default).

Phase 4: Writing the Conditional Pipeline Script

I scrolled down to the Pipeline script section and wrote the following Groovy code. This script uses the params.BRANCH variable to dynamically select the branch and includes debug steps.

pipeline{
    agent{
        label 'ststor01'
    }

    stages{
        stage('Deploy'){
            steps{
                script{
                    if(params.BRANCH != 'master' && params.BRANCH != 'feature'){
                        error('**** Invalid Branch Parameter. Only master or feature is allowed. ****')
                    }
                    git branch: params.BRANCH,
                        url: 'http://git.stratos.xfusioncorp.com/sarah/web_app.git'

                    sh'''
                        cp -r * /var/www/html
                    '''
                }
            }
        }
    }
}

Phase 5: Execution and Verification

  1. I clicked Save.
  2. I clicked Build with Parameters.
  3. I entered master and clicked Build. I checked the Console Output to confirm "Deploying MASTER branch..." was printed.
  4. I ran it again, entered feature, and clicked Build. I checked the Console Output to confirm "Deploying FEATURE branch..." was printed.
  5. Finally, I clicked the App button to confirm the website was loading correctly from the root URL.

Troubleshooting: Pipeline Not Running Logic

If your pipeline finishes with SUCCESS but the Console Output only shows "Start of Pipeline" and "End of Pipeline" with no actual steps executed:

  1. Check Node Labels: Ensure the agent { label 'ststor01' } matches the label on your Storage Server node exactly. If it doesn't match, Jenkins might be waiting for a node that doesn't exist or skipping the block.
  2. Check Parameter Names: Verify your String Parameter is named BRANCH (all caps). If it's named Branch, params.BRANCH will be null, and the logic might be skipped (though the else block should catch it).
  3. Check Syntax: Ensure your script block is correctly nested inside steps, which is inside stage.

Why Did I Do This? (The "What & Why")

- Parameterized Pipelines: Hardcoding values (like branch names) makes automation brittle. By using parameters (params.BRANCH), I created a single, flexible pipeline that can handle multiple scenarios. This allows developers to deploy different versions of the app (e.g., for testing a new feature) without needing to modify the Jenkinsfile. - Conditional Deployment: Real-world deployments are rarely linear. We often need logic like "If this is Production, do X; if this is Staging, do Y." The script block within the declarative pipeline allowed me to use standard Groovy if/else statements to implement this logic. - Dynamic Git Checkout: Instead of letting the pipeline configuration handle the SCM checkout (which usually defaults to one branch), I explicitly used the git step inside my stages. This gave me total control to switch branches dynamically based on the user's input.

Deep Dive: The Conditional Pipeline Script

Here is the breakdown of the key logic in my script.

pipeline{
    agent{
        label 'ststor01'
    }

    stages{
        stage('Deploy'){
            steps{
                script{
                    if(params.BRANCH != 'master' && params.BRANCH != 'feature'){
                        error('**** Invalid Branch Parameter. Only master or feature is allowed. ****')
                    }
                    git branch: params.BRANCH,
                        url: 'http://git.stratos.xfusioncorp.com/sarah/web_app.git'

                    sh'''
                        cp -r * /var/www/html
                    '''
                }
            }
        }
    }
}

Common Pitfalls

- Missing script Block: In Declarative Pipelines (the ones that start with pipeline { ... }), you cannot just write if (...) directly inside steps. You must wrap imperative logic like loops and conditionals inside a script { ... } block. - Case Sensitivity: The stage name Deploy must be exact. deploy or Deployment will fail the validation. - Parameter Name Mismatch: The parameter is defined as BRANCH. Accessing it as params.branch (lowercase) would fail because parameters are case-sensitive. - Credentials ID: Ensure the credentialsId used in the script matches exactly the ID you gave when creating the credentials in Jenkins.

Exploring the UI Used

- Build with Parameters: This replaces the "Build Now" button when a job has parameters. It creates a form for user input. - Pipeline Syntax: I used this tool (link at bottom of pipeline editor) to generate the correct syntax for the git checkout step, ensuring I got the URL and credentials format right.