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Jenkins Agent (Slave / Node)

This document explains Jenkins Agents, which are responsible for executing builds and tests. Agents do the real work in Jenkins.

What Is a Jenkins Agent?​

A Jenkins Agent (formerly Slave) is a machine or container that:

  • Executes pipeline stages
  • Runs builds and tests
  • Hosts required tools
  • Manages workspaces

Agents are controlled by the Jenkins Controller.

Agent Responsibilities​

An agent is responsible for:

  • Running build steps
  • Executing test automation
  • Creating and managing workspaces
  • Hosting tools (Java, Maven, Node, Docker, browsers)
  • Sending build results back to the controller

Agent Types​

Static Agents​

  • Permanently available machines
  • Preconfigured with tools
  • Suitable for stable workloads

Dynamic Agents​

  • Created on demand
  • Short-lived
  • Common in Docker/Kubernetes setups

Agent Connectivity Methods​

SSH Agents​

  • Controller connects to agent via SSH
  • Most common setup
  • Preferred for Linux agents

JNLP Agents​

  • Agent initiates outbound connection
  • Useful behind firewalls
  • Requires Java on agent

Executors on Agents​

  • Executors define how many jobs an agent can run in parallel
  • Configured per agent
  • More executors = more resource usage

Best Practice​

  • Start with 1–2 executors per agent
  • Scale agents horizontally instead of increasing executors

Node Labels​

  • Labels are used to target specific agents
  • Common use cases:
    • OS-specific builds
    • Browser-specific testing
    • Heavy vs lightweight jobs

Example​

agent { label 'linux && docker' }

Workspace on Agents​

  • Workspaces are created on agents
  • Each job and branch has its own workspace
  • Parallel builds use separate workspace directories

Common Problems​

  • Dirty workspaces
  • Disk space exhaustion
  • Tool version conflicts

Scaling Strategies​

  • Prefer more agents over more executors
  • Use labels to isolate workloads
  • Use dynamic agents for burst workloads

Security Best Practices​

  • Agents should have minimal permissions
  • Do not store secrets on agents
  • Use Jenkins credentials binding
  • Isolate agents by purpose

Interview Traps​

  • Where does execution happen?
    → On agents

  • Can agents run without controller?
    → No

  • Is it safe to run builds on controller?
    → No, not in production