Jenkins is a free, open-source automation server. In plain words: it is a program that does boring, repeated software chores for you, automatically and on time.

Let us unpack that sentence. Open-source means the recipe for the program is public. Anyone can read it, use it for free, and suggest improvements. Automation means a computer does a task by itself instead of a human clicking buttons. A server is a computer (or a program on a computer) that stays switched on and waits for work to arrive.

So what chores does Jenkins automate? Mostly the chores that happen after a programmer writes code. The code must be put together into a working program, which is called a build. The program must be checked with automatic tests to make sure nothing is broken. Finally, the program may need to be delivered to the place where real users can use it, which is called a deployment. Jenkins does all of this by following instructions that the team writes down once.

Two big words: CI and CD

Continuous Integration (CI) means that every time someone adds new code, the whole project is automatically built and tested. “Continuous” means it happens all the time, not once a month. “Integration” means joining everyone’s work together.

Continuous Delivery (CD) takes it one step further. After the code passes its tests, it is automatically prepared and sent toward the real world, so releasing a new version becomes a calm, repeatable routine instead of a stressful event. Some teams go even further and push every passing change straight to users; that is usually called continuous deployment.

Jenkins has a surprising story with a friendly split, a new name, and a big community that kept it alive for more than twenty years.

The story begins with a programmer named Kohsuke Kawaguchi, who worked at Sun Microsystems. He was tired of breaking the shared code and only finding out hours later. So he built a small tool that would check the code automatically. He first called it Hudson. Other developers loved it, and it spread quickly because it was free and easy to extend.

To understand why Jenkins matters, picture life before it.

In the old days, a team of programmers would each work alone for weeks. Near the release date they would try to join all their work together. This moment was so painful that people gave it a nickname: integration hell. Pieces did not fit, hidden bugs appeared everywhere, and releases were late. Someone had to build the program by hand on their own laptop, and if that person was sick, nobody else knew the exact steps.

Manual work causes three big troubles. First, it is slow, because humans must wait and click. Second, it is error-prone, because humans forget steps, especially at midnight before a release. Third, it is not repeatable, because the steps live in one person’s head.

Jenkins answers these troubles with one simple habit: check early, check often, check automatically. If a programmer breaks something at 10:02 in the morning, Jenkins shows a red warning at 10:07, while the change is still fresh in their mind. Fixing a small mistake found in five minutes is far cheaper than hunting for it in a giant pile of changes found a month later.

A handful of words unlock everything else in Jenkins. Learn these and the rest feels natural.

What is a source control system?

Jenkins does not store your code itself. Code lives in a source control system such as Git, which remembers every change ever made and who made it. Popular hosting websites for Git include GitHub, GitLab, and Bitbucket. Jenkins watches these places, pulls a fresh copy of the code, and gets to work.

Jenkins uses a controller and agents design. One brain, many hands.

The controller is a Java program that serves the Jenkins website, stores all configuration, keeps the build history, decides which build runs where, and talks to plugins. The agents do the heavy lifting: compiling code, running tests, packaging results. Agents can be physical machines, virtual machines in the cloud, or short-lived containers that appear for one build and vanish afterward.

flowchart LR
  DEV["Developer"] -->|"push code"| SCM["Git Repository"]
  SCM -->|"webhook"| LB["Reverse Proxy and Load Balancer"]
  LB --> CTRL["Jenkins Controller"]
  CTRL -->|"assign job"| A1["Agent 1: Linux VM"]
  CTRL -->|"assign job"| A2["Agent 2: Windows VM"]
  CTRL -->|"assign job"| A3["Agent 3: Kubernetes Pod"]
  A1 --> ART["Artifact Repository"]
  A2 --> ART
  A3 --> ART
  CTRL --> NOTE["Email and Chat Notifications"]
  CTRL --- HOME[("JENKINS_HOME Storage")]

Walking through the picture

A developer pushes code to the Git repository. The repository sends a small message called a webhook (an automatic “hey, something changed!” notification over the web) to Jenkins. In a real company the message first passes through a reverse proxy or load balancer, a front-door program that adds secure HTTPS and hands traffic to the controller. The controller puts the work in a waiting line and gives it to a free agent. The agent builds and tests, then stores its result in an artifact repository, a warehouse for finished packages. Finally the controller sends a message to email or chat so humans know the outcome.

Where does Jenkins keep its memory?

Everything Jenkins knows, including job settings, build logs, plugin files, and credentials, lives in a folder called JENKINS_HOME on the controller. This detail matters a lot: if that folder is lost, Jenkins forgets everything. That is why serious teams back it up regularly and store it on reliable disks.

How agents connect

Agents talk to the controller in two common ways. In the first, the controller reaches out to the agent using SSH, a secure remote login method. In the second, the agent starts the conversation itself and connects inward to the controller, which is helpful when the agent sits behind a strict firewall. Many modern teams also use the Kubernetes plugin, where each build gets a brand-new temporary container as its agent. When the build ends, the container is thrown away, so every build starts clean.

Let us follow one build from the doorbell ringing to the final report.

A clever trick: durable pipelines

Pipelines can run for hours, and computers sometimes restart. Jenkins saves the state of a running pipeline to disk as it goes, so after a controller restart the pipeline can pick up where it stopped instead of starting over. Think of saving your game progress at each checkpoint.

Why plugins load at startup

When the controller starts, it reads the plugin folder and loads each add-on into memory. Each plugin can add pages, buttons, build steps, and triggers. This is why a Jenkins with hundreds of plugins takes longer to start, and why a broken plugin can sometimes stop Jenkins from starting properly.

A pipeline is your delivery recipe written as a simple text file that lives beside your code.

That text file is called a Jenkinsfile. Because it is stored in the same repository as the application, the delivery process gets the same benefits as code: every change is recorded, teammates can review it, and you can travel back to an older version if a new recipe goes wrong. This idea is often called Pipeline as Code.

Stages and steps

A pipeline is divided into stages, which are big named phases such as Checkout, Build, Test, and Deploy. Each stage holds steps, the small individual actions. Stages appear as neat boxes in the Jenkins screen, so anyone can see at a glance which phase is running and which one failed.

flowchart LR
  C["Checkout Code"] --> B["Build"]
  B --> T["Automated Tests"]
  T --> P["Package Artifact"]
  P --> S["Deploy to Staging"]
  S --> AP{"Human Approval"}
  AP -->|"approved"| PR["Deploy to Production"]
  AP -->|"rejected"| STOP["Stop and Notify"]
  T -->|"tests fail"| FAIL["Mark Build Failed and Alert Team"]

Declarative versus scripted

Jenkins offers two writing styles for pipelines. Declarative is a tidy, structured form with a fixed layout, which is easier for beginners and easier for Jenkins to check for mistakes. Scripted is a free-form style built on a full programming language called Groovy, which is more powerful but easier to make messy.

Multibranch pipelines and shared libraries

A multibranch pipeline scans a repository and automatically creates a pipeline for every branch and every pull request that contains a Jenkinsfile. When a branch is deleted, its pipeline disappears too. A shared library is a central collection of reusable pipeline pieces. If fifty projects all need the same “deploy to production” routine, the company writes it once and every project borrows it. That is like a school having one official lunch menu instead of every classroom inventing its own.

Here is the same journey as a conversation between the main characters.

sequenceDiagram
  participant D as Developer
  participant G as Git Server
  participant J as Jenkins Controller
  participant A as Build Agent
  participant R as Artifact Repository
  D->>G: Push a new commit
  G->>J: Send webhook notification
  J->>J: Place build in the queue
  J->>A: Assign build to a free executor
  A->>G: Download the source code
  A->>A: Compile and run tests
  A->>R: Upload the finished package
  A-->>J: Report success or failure
  J-->>D: Send notification

Notice that the controller never compiles anything itself in a healthy setup. It only coordinates. The agent pulls the code directly from Git and pushes the package directly to the repository. Keeping heavy work off the controller is one of the most important habits in Jenkins, and we will return to it in the best practices chapter.

What happens to old builds?

Every build leaves behind logs and files. Left alone, they would slowly fill the disk. Jenkins lets you set a retention policy, such as “keep only the last thirty builds” or “delete logs older than sixty days”. This is like cleaning out your fridge on a schedule so the shelves never overflow.

Plugins are the reason Jenkins can talk to almost any tool on earth. They are also the reason it needs careful care.

Out of the box, Jenkins is fairly small. It becomes powerful when you add plugins. There are plugins for Git and GitHub, for building with Maven, Gradle, and npm, for running Docker containers, for launching Kubernetes agents, for scanning code quality, for sending Slack or email messages, and for hundreds of other jobs. Plugins are written by a large community, and a well-known plugin site lets you browse them.

The plugin trade-off

Jenkins holds the keys to your code and your servers, so protecting it is not optional.

Think about what Jenkins can do: it can read source code, it can log in to cloud accounts, and it can deploy programs to real servers. That makes it a very attractive target for attackers. A safe Jenkins follows several layers of protection.

Who are you, and what may you do?

Authentication proves who a person is, usually by connecting Jenkins to the company login system through single sign-on. Authorization decides what each proven person may do. Good setups use role-based rules, so a tester can run builds but cannot change system settings, and only a few administrators can install plugins.

Protecting secrets

Builds often need passwords, tokens, and keys. Jenkins has a built-in credentials store that keeps these secrets encrypted and hands them to a build only when needed, hiding them from the log output. The golden rule is simple: never write a password in plain text inside a pipeline file or a job setting. Many companies go further and connect Jenkins to a dedicated secrets manager so that secrets are short-lived and rotate automatically.

Keep the brain safe

Because the controller holds every secret, teams set its own executor count to zero so no build ever runs on it. Builds run only on agents. If a malicious or careless build runs on an agent, the damage stays in that throwaway machine instead of reaching the controller.

Other important protections

• Keep Jenkins and plugins updated. The Jenkins project publishes security advisories, and attackers love old versions.
• Use HTTPS. Put the reverse proxy in front so traffic is encrypted.
• Use the script sandbox. Pipeline scripts run in a restricted mode, and unusual actions need an administrator’s approval.
• Limit network reach. Agents should reach only the systems they truly need.
• Audit activity. Record who changed what, so surprises can be traced.

A healthy Jenkins is watched like a patient in a clinic: temperature, heartbeat, and daily notes.

Monitoring means watching a system’s health signals so you can react before users complain. For Jenkins, teams watch several things:

Tools that help

Jenkins has plugins that expose its numbers in standard formats. Many teams collect them with Prometheus, a popular metrics collector, and draw graphs and alerts with Grafana, a dashboard tool. Build logs can be sent to a central log system such as the ELK stack (Elasticsearch, Logstash, Kibana) so that engineers can search across thousands of builds. Each build also keeps its own console output, which is the first place to look when something fails.

No tool is perfect. Knowing both sides helps you choose wisely, and helps you explain your choice.

How Jenkins compares with other tools

Newer services such as GitHub Actions and GitLab CI/CD are built into code hosting platforms, so there is almost nothing to install. Jenkins is a separate server that you control. That difference drives most of the trade-off.

A helpful way to think about it: built-in CI is like renting a furnished apartment, while Jenkins is like owning a house. Owning gives you freedom to rebuild any room, but you also fix the roof.

A pattern is a proven way of solving a repeated problem. An anti-pattern is a tempting shortcut that usually hurts later.

Patterns that work

An anti-pattern to avoid

These habits separate a calm Jenkins from a chaotic one.

Best practices

1. Use pipelines, not hand-made freestyle jobs, for anything important. Pipelines are visible, repeatable, and reviewable.
2. Keep the controller light. No builds there, and only the plugins you truly need.
3. Back up JENKINS_HOME often, and practice restoring it. A backup you have never tested is only a hope.
4. Pin and test upgrades. Try new Jenkins and plugin versions in a copy first, then move to production.
5. Prefer the LTS line for production. Long-Term Support versions change more slowly and receive careful fixes.
6. Make builds fast. Cache downloads, run independent tests at the same time (called running in parallel), and split giant projects into smaller ones.
7. Keep secrets in the credentials store or an external secrets manager, never in the code.
8. Set retention limits so old builds do not eat the disk.
9. Give every build a clear name and notification so people know exactly what failed and where to look.

Common mistakes

Jenkins is used across startups, banks, telecom companies, and giant internet firms.

A typical journey for a small team

A team of five starts with one Jenkins machine running both controller and workers. As the team grows, builds start waiting. They move to a controller plus several cloud agents. Later they switch to containers created on demand, save money by shutting idle workers down, and store their pipeline files in Git. Jenkins grew with them instead of forcing a rewrite.

You started with a kitchen full of friends and finished with a full picture of one of the most influential tools in software delivery.