Aisalkyn Aidarova

Posted on Feb 6

Why Companies Still Use Jenkins

#automation #cicd #devops #opensource

What Jenkins is

Jenkins is an open-source automation server used to automate:

code integration (CI)
testing
security checks
builds
deployments
infrastructure tasks

Jenkins does not build code itself — it orchestrates tools:

Maven, Gradle, npm
Docker
kubectl
Terraform
Shell scripts

Think of Jenkins as:

“The automation brain that connects everything together.”

Why companies STILL use Jenkins

Even with GitHub Actions, GitLab CI, etc., Jenkins is still everywhere because:

1. Vendor-neutral

Works with any cloud (AWS, Azure, GCP, on-prem)
Not locked to GitHub or GitLab

2. Extreme flexibility

Custom pipelines
Custom agents
Custom networking
Air-gapped environments

3. Enterprise & legacy systems

Banks, insurance, government
Old systems + new cloud together
Jenkins fits hybrid environments well

4. Plugin ecosystem

Thousands of plugins
Almost every DevOps tool integrates with Jenkins

Interview line

“Jenkins is used when organizations need full control, flexibility, and integration across heterogeneous environments.”

2. When Jenkins Is the Right Tool — and When It’s Not

Use Jenkins when:

You need custom CI/CD logic
You run on-prem or hybrid infrastructure
You integrate many tools (Terraform, Ansible, EKS, legacy apps)
Security and network control matters
You want pipeline logic independent of Git provider

Do NOT use Jenkins when:

Pipelines are very simple
Team wants zero maintenance
GitHub-only workflows are enough
Small startup with minimal CI needs

Interview comparison

Jenkins gives power and flexibility; GitHub Actions gives simplicity and convenience.

3. Freestyle Jobs vs Pipelines (VERY IMPORTANT)

Freestyle Jobs

What they are

UI-based jobs
Click-configured
One step after another

Why they exist

Easy to learn
Good for:
- beginners
- quick scripts
- operational tasks
- legacy Jenkins setups

DevOps reality

Freestyle jobs do not scale well:

Hard to version
Hard to review
Hard to reproduce

Interview rule

Freestyle jobs are for learning and quick tasks, not production pipelines.

Pipelines

What pipelines are

Defined as code
Written in Jenkinsfile
Stored in Git

Why pipelines are preferred

Version-controlled
Reproducible
Reviewable via PR
Same pipeline across environments

Pipeline structure (mental model)

Agent → Stages → Steps

4. CI vs CD (This Is Always Asked)

Continuous Integration (CI)

CI means:

Code is merged frequently
Automated build runs
Tests are executed
Failures stop the pipeline

CI answers:

“Is this code safe to merge?”

Typical CI stages:

Checkout
Build
Unit tests
Linting
Security checks

Continuous Delivery / Deployment (CD)

CD means:

Code is automatically prepared for deployment
Deployment may be:
- manual approval (Delivery)
- automatic (Deployment)

CD answers:

“Can we safely release this?”

Typical CD stages:

Package artifact
Deploy to dev
Deploy to staging
Deploy to prod (with approval)

Interview explanation

CI protects the codebase.
CD protects production.

5. Jenkinsfile — Pipeline as Code

What Jenkinsfile is

A Jenkinsfile is a declarative definition of your pipeline.

It lives:

inside the Git repository
next to application code

This is called Pipeline as Code.

Why Pipeline as Code matters

No clicking in UI
Auditable
Versioned
Reproducible
Disaster-recoverable

Interview line

“In production, I always use Jenkinsfile instead of UI-configured jobs.”

Typical Jenkinsfile structure

pipeline {
    agent any

    environment {
        ENV = "dev"
    }

    stages {
        stage('Checkout') {
            steps {
                checkout scm
            }
        }

        stage('Build') {
            steps {
                echo 'Building application'
            }
        }

        stage('Test') {
            steps {
                echo 'Running tests'
            }
        }

        stage('Deploy') {
            steps {
                echo 'Deploying to environment'
            }
        }
    }
}

6. Handling Failures, Logs, and Credentials

Failures

How Jenkins handles failure

Non-zero exit code = failure
Pipeline stops immediately
Downstream stages do not run

Why this is good

Fail fast
Protects production
Prevents bad deployments

Interview line

“A red pipeline is a safety mechanism, not a problem.”

Logs

Where logs are

Console Output per build
Logs per stage
Timestamped

How DevOps uses logs

Debug failures
Identify flaky tests
Root-cause analysis

Key skill

Being able to read logs quickly matters more than writing pipelines.

Credentials

DevOps rule

Never hardcode secrets.

Jenkins credentials system

Stored securely
Injected at runtime
Masked in logs

Types:

Username/password
SSH keys
Tokens
Certificates

Interview line

“Secrets are managed by Jenkins credentials, never committed to Git.”

7. How Jenkins Fits Into Real DevOps Flow

Real production flow

Developer
  ↓
GitHub
  ↓
Webhook
  ↓
Jenkins
  ↓
Build + Test + Security
  ↓
Artifact Registry
  ↓
Deploy (EKS / EC2 / ECS)

Jenkins is the automation glue.

8. How to Talk About Jenkins Confidently in Interviews

30-second answer (memorize this)

“Jenkins is a CI/CD automation server used to integrate code, run automated tests, enforce policies, and deploy applications.
I use Jenkinsfile for Pipeline-as-Code, integrate GitHub using webhooks, manage secrets with Jenkins credentials, and design pipelines that fail fast and support multiple environments.”