Controller, agents and executors

Jenkins CI/CD Course · lesson 2 of 15 · 5 min read

Jenkins schedules on one controller and builds on agents, and executors are the slots that decide throughput.

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Key points

  • The controller serves the UI on port 8080, holds all config under JENKINS_HOME and schedules work.
  • An agent is any machine that runs builds; it attaches over SSH or inbound TCP, commonly on port 50000.
  • An executor is one build slot on a node, so four executors means four builds at a time on that machine.
  • A fresh install gives the built-in node 2 executors, and the hardening advice is to lower that to 0.
  • Labels are how a stage asks for a machine, and a label no online node carries leaves the build queued forever.
  • Jenkins LTS 2.479.1 and later require Java 17 or Java 21 on the controller; Java 11 is no longer supported.

The controller should schedule and store, never build; throughput comes from executors on real agents.

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This is a reading copy. The full lesson — with the visual explainer, the interactive lab and a Run button for the code — lives in the Jenkins CI/CD Course course, and every lesson in it is listed on the Jenkins CI/CD Course contents page.