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Kubernetes Patterns by Roland Huß, Bilgin Ibryam

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Chapter 9. Daemon Service

The Daemon Service pattern allows placing and running prioritized, infrastructure-focused Pods on targeted nodes. It is used primarily by administrators to run node-specific Pods to enhance the Kubernetes platform capabilities.

Problem

The concept of a daemon in software systems exists at many levels. At an operating system level, a daemon is a long-running, self-recovering computer program that runs as a background process. In Unix, the names of daemons end in “d,” such as httpd, named, and sshd. In other operating systems, alternative terms such as services-started tasks and ghost jobs are used.

Regardless of what they are called, the common characteristics among these programs are that they run as processes and usually do not interact with the monitor, keyboard, and mouse, and are launched at system boot time. A similar concept exists at the application level too. For example, in the JVM daemon threads run in the background and provide supporting services to the user threads. These daemon threads have a low priority, run in the background without a say in the life of the application, and perform tasks such as garbage collection or finalization.

Similarly, there is also the concept of a DaemonSet in Kubernetes. Considering that Kubernetes is a distributed platform spread across multiple nodes and with the primary goal of managing application Pods, a DaemonSet is represented by Pods that run on the cluster nodes and provide some background capabilities ...

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