Problem Statement
Explain Jenkins master-agent architecture in detail. Discuss components, communication, scaling strategies, and best practices for production deployments.
Explanation
Jenkins master is the central server managing the entire Jenkins environment. Master responsibilities include hosting the Jenkins UI (web interface), storing configuration (jobs, pipelines, system settings), scheduling builds and distributing to agents, monitoring agent health, collecting and displaying build results, managing plugins, and handling authentication/authorization. Master runs on JVM and stores data in JENKINS_HOME directory containing jobs, build history, plugins, and configuration.
Agents (nodes) are worker machines executing builds. Agent types include permanent agents (always connected machines), cloud agents (dynamically provisioned on demand in AWS, Azure, GCP, Kubernetes), and Docker agents (ephemeral containers for isolated builds). Agents require Java runtime, can run on various OS (Linux, Windows, macOS), and connect to master via SSH, JNLP, or as controller-initiated connections.
Communication between master and agents uses TCP/IP with authentication and optional encryption. Master sends build jobs to agents with workspace and environment details. Agents execute builds, stream console output back to master, and report results. Agents can have labels (tags like 'linux', 'docker', 'python3') allowing pipelines to target specific agent types: agent { label 'docker' } runs on agents with 'docker' label.
Scaling strategies include horizontal scaling by adding more agents for parallel builds, vertical scaling by increasing agent resources (CPU, RAM), using cloud-based dynamic agents that scale automatically based on build queue depth, Kubernetes plugin for auto-scaling agent pods, and Docker plugin for on-demand container agents. Master should not execute builds in production to remain responsive and avoid resource contention.
Best practices: separate master and agents (never run builds on master in production), use labels for agent targeting, implement master high availability with active-passive setup or clustering plugins, backup JENKINS_HOME regularly, use Configuration as Code plugin for master configuration, monitor agent connectivity and health, set executor count on master to 0, use cloud agents for scaling, implement resource limits on agents, and secure communication with SSL/TLS. Understanding architecture enables building robust, scalable Jenkins deployments.