Jenkins - The CI/CD Server That Won't Die

Jenkins is the automation server that refuses to die. Started by Kohsuke Kawaguchi in 2004 when continuous integration was still a buzzword, it's now the backbone keeping half the internet's deployments running. The UI looks like Windows XP threw up, but it works, and that's why companies like Netflix, LinkedIn, and Twitter still use it.

The current LTS version is 2.516.2 (August 2025), and you'll need Java 17+ to run it. Yes, Jenkins survived the Java 8 to Java 11 migration that killed half the enterprise Java ecosystem.

What Jenkins Actually Does

Jenkins takes your code, builds it, tests it, and deploys it automatically. Simple concept, complex execution. You define jobs (builds), Jenkins runs them when shit changes in your repo, and hopefully your stuff ends up deployed without breaking production.

The secret sauce isn't the pretty UI (because there isn't one) - it's the plugin ecosystem. Need to deploy to AWS? There's a plugin. Want Slack notifications when builds fail? Plugin. Need GitHub integration? Docker support? Kubernetes agents? Someone probably wrote a plugin for it.

How This Beast Actually Works

Master-Agent Architecture: The Jenkins controller (they stopped calling it "master" because corporate sensitivity) manages the web UI and coordinates everything. The agents do the actual work - building code, running tests, deploying stuff. This means you can have one ugly web interface managing builds across a dozen different servers.

The controller eats memory like Chrome with too many tabs open. Plan accordingly.

Pipeline as Code: Modern Jenkins uses Jenkinsfiles to define builds. It's basically Groovy scripts that describe what your build should do. Store it in your repo, version it with your code, and now your build process is reproducible instead of "works on Dave's machine."

Plugin Hell: Jenkins has 1,800+ plugins. This is both amazing and terrifying. You can integrate with anything, but plugin dependency conflicts will ruin your weekend. Some plugins haven't been updated since 2018 but somehow still work. Others get updated and break everything.

When Jenkins Makes Sense

Jenkins thrives in complex environments where you need maximum flexibility and don't mind the maintenance overhead. If you're running microservices across multiple clouds with different deployment requirements, Jenkins can orchestrate that chaos.

It's also perfect when you have legacy systems that newer CI tools don't understand. Jenkins has plugins for mainframes, proprietary databases, and weird enterprise tools that GitHub Actions has never heard of.

The downside? You become a Jenkins admin whether you want to or not. Someone on your team will spend 20% of their time keeping it running, updating plugins, and explaining why the UI looks like it's from the Bush administration.

Here's how Jenkins works so you don't screw up your setup like I did the first time. Understanding the architecture matters because when Jenkins breaks (and it will), you need to know where to look.

The Controller vs Agent Setup

Jenkins uses a distributed architecture where one server (the controller) manages everything and other machines (agents) do the actual work. Think of it like a construction foreman who tells workers what to do but doesn't swing the hammer.

The Controller: This is the main Jenkins server with the ugly web UI. It stores all your job configurations, manages the build queue, and coordinates everything. The controller lives in $JENKINS_HOME directory, which will grow to consume all available disk space if you don't configure log rotation.

The Agents: These machines actually run your builds. They check out code, run tests, build artifacts, and deploy stuff. You can have Windows agents, Linux agents, Docker agents, cloud agents - whatever you need. The beauty is you can have one shitty-looking web interface managing builds across completely different environments.

Connecting Agents (Where Things Get Fun)

SSH Method: Jenkins SSHs into your agents to start builds. Works great until your network admin changes firewall rules without telling anyone. Needs SSH keys set up properly, which means someone will inevitably screw up file permissions and spend an afternoon debugging why chmod 600 matters.

Inbound Method: Agents connect back to the controller. Good for agents behind NAT or firewalls. Uses WebSocket connections now instead of the old JNLP nonsense that never worked reliably. Still occasionally breaks when corporate proxies get updated.

Cloud Agents: Spin up agents on-demand in AWS, Azure, or Kubernetes. Sounds amazing until you get the cloud bill and realize you've been spinning up c5.xlarge instances to build "Hello World" applications. Configure resource limits or cry later.

What You Actually Need to Run This

Minimum Resources: The docs say 256MB RAM and 1GB disk. This is like saying a Ferrari needs "some gas" - technically true but completely useless for reality.

Real Requirements: Start with 8GB RAM for the controller and at least 4 cores. Jenkins is single-threaded for most operations, so it's slower than modern CI tools. The web UI gets sluggish with more than 500 jobs, and you'll restart it monthly to clear memory leaks.

Disk Space: The $JENKINS_HOME directory grows forever. Build logs, artifacts, workspace checkouts - it all accumulates. Set up log rotation immediately or your disk will fill up on a Friday evening. The worst part? Jenkins stores everything as XML files, so debugging means grepping through thousands of XML config files.

The Directory Structure Nobody Tells You About

Your $JENKINS_HOME contains:

• Job configs (XML files that make you wish for JSON)
• Build histories (every build ever, unless you configure retention)
• Plugin data (some plugins create their own storage nightmares)
• Secrets and credentials (encrypted, but backup the keys)
• Workspace checkouts (can be huge, clean these regularly)

Pro tip: The secrets/ directory contains encryption keys. If you lose these, all your stored credentials become useless and you get to re-enter every password manually.

When It All Goes Wrong

The controller will randomly use 100% CPU for no apparent reason. Build agents will go offline and nobody knows why. Plugin updates will break working jobs. The build queue will get stuck and require a restart to fix.

This isn't a bug, it's Jenkins. Plan for monthly maintenance windows and make friends with whoever manages your servers, because you'll be restarting this thing regularly.

The good news? Once you understand how it works, Jenkins is surprisingly reliable for the amount of complexity it manages. Just don't expect it to be elegant.