In the realm of software engineering, the concepts of containerization and orchestration are fundamental to understanding the modern application deployment landscape. This glossary entry will delve into the intricacies of 'Admission Webhooks in Operators', a topic that sits at the intersection of these two key areas.
Operators, in the context of Kubernetes, are a method of packaging, deploying, and managing a Kubernetes application. They are built with the understanding of the application's state and the knowledge of how to move from one state to another. Admission webhooks, on the other hand, are HTTP callbacks that receive admission requests, process them, and return admission responses.
Definition of Key Terms
Before we delve into the details, it's crucial to define some key terms that will be used throughout this glossary entry. These terms form the backbone of our understanding of admission webhooks in operators, containerization, and orchestration.
Firstly, 'Containerization' is the process of encapsulating an application along with its environment into a container with its own operating system. This ensures that the application runs smoothly across different computing environments. 'Orchestration', on the other hand, is the automated configuration, coordination, and management of computer systems, services, and applications. It's the process of managing lifecycles of containers, especially in large, dynamic environments.
Admission Webhooks
Admission webhooks are a feature in Kubernetes that intercepts requests to the Kubernetes API server prior to persistence of the object, but after the request is authenticated and authorized. They come in two flavors: mutating admission webhooks and validating admission webhooks.
Mutating admission webhooks are invoked first, and can modify objects sent to the API server to enforce custom defaults. Validating admission webhooks are invoked next, and can reject requests to enforce custom policies. Both types receive AdmissionReview objects from the API server and return AdmissionReview objects in response.
Operators
In the context of Kubernetes, an Operator is a method of packaging, deploying, and managing a Kubernetes application. A Kubernetes application is an application that is both deployed on Kubernetes and managed using the Kubernetes APIs and kubectl tooling.
To build an Operator, you need an understanding of Kubernetes and software development. This understanding is codified into software that is actively watching over your environment and that can take the necessary steps towards the desired state.
History of Admission Webhooks and Operators
The history of admission webhooks and operators is intertwined with the history of Kubernetes itself. Kubernetes, an open-source platform designed to automate deploying, scaling, and operating application containers, was first announced by Google in 2014. Since then, it has become the de facto standard for container orchestration.
Admission webhooks were introduced in Kubernetes 1.9, released in December 2017, as a way to extend Kubernetes with custom logic. Operators, on the other hand, were introduced by CoreOS in 2016 as a way to extend Kubernetes' functionality, allowing it to manage stateful applications and complex deployments.
Evolution of Admission Webhooks
Admission webhooks have evolved significantly since their introduction. Initially, they were used to enforce security policies, validate objects, and apply default values. Over time, they have been extended to cover a wider range of use cases, including complex validation logic, defaulting fields, and injecting sidecar containers.
The evolution of admission webhooks has been driven by the growing complexity of applications deployed on Kubernetes and the need for greater control over how these applications are configured and managed.
Evolution of Operators
Operators have also evolved significantly since their introduction. Initially, they were used to manage stateful applications, such as databases, that required specific knowledge to manage. Over time, they have been extended to manage a wider range of applications and provide a way to encode operational knowledge into software.
The evolution of operators has been driven by the growing complexity of applications deployed on Kubernetes and the need for automated management of these applications.
Use Cases of Admission Webhooks in Operators
Admission webhooks in operators have a wide range of use cases, from enforcing security policies and validating objects to managing complex deployments and stateful applications. They provide a way to extend Kubernetes with custom logic, allowing you to tailor the platform to your specific needs.
One common use case is enforcing security policies. For example, you can use an admission webhook to enforce that all pods in a cluster have a certain set of labels, or that all containers in a pod have resource limits set. This allows you to ensure that your clusters are configured in a secure and consistent manner.
Validating Objects
Another common use case is validating objects. For example, you can use an admission webhook to validate that the configuration of a custom resource conforms to a certain schema, or that certain fields of a pod spec are set in a certain way. This allows you to ensure that your objects are configured correctly and prevent misconfigurations that could lead to errors or security vulnerabilities.
For instance, you can use a validating admission webhook to check that all deployments in a namespace have a certain annotation, or that all services of a certain type have a certain label. This allows you to enforce best practices and prevent misconfigurations.
Managing Complex Deployments
Admission webhooks in operators can also be used to manage complex deployments. For example, you can use an operator to manage a stateful application, such as a database, that requires specific knowledge to manage. The operator can use admission webhooks to enforce policies, validate objects, and manage the lifecycle of the application.
For instance, you can use an operator to manage a Cassandra cluster. The operator can use admission webhooks to validate the configuration of the cluster, enforce policies such as anti-affinity rules, and manage the lifecycle of the cluster, including scaling up and down, rolling updates, and failure recovery.
Examples of Admission Webhooks in Operators
Let's now delve into some specific examples of admission webhooks in operators. These examples will illustrate how these concepts can be applied in real-world scenarios.
One example is the Prometheus Operator, an open-source project that provides Kubernetes native deployment and management of Prometheus and related monitoring components. The Prometheus Operator uses admission webhooks to validate the configuration of Prometheus instances and enforce policies such as resource limits.
etcd Operator
Another example is the etcd Operator, an open-source project that automates the deployment, scaling, and management of etcd clusters on Kubernetes. The etcd Operator uses admission webhooks to validate the configuration of etcd clusters and enforce policies such as anti-affinity rules.
The etcd Operator also uses admission webhooks to manage the lifecycle of etcd clusters, including scaling up and down, rolling updates, and failure recovery. This allows you to manage etcd clusters with the same kubectl tooling you use to manage other resources in your Kubernetes cluster.
Strimzi Operator
A third example is the Strimzi Operator, an open-source project that provides a way to run an Apache Kafka cluster on Kubernetes or OpenShift in various deployment configurations. The Strimzi Operator uses admission webhooks to validate the configuration of Kafka clusters and enforce policies such as resource limits and security configurations.
The Strimzi Operator also uses admission webhooks to manage the lifecycle of Kafka clusters, including scaling up and down, rolling updates, and failure recovery. This allows you to manage Kafka clusters with the same kubectl tooling you use to manage other resources in your Kubernetes cluster.
Conclusion
Admission webhooks in operators are a powerful tool for extending Kubernetes with custom logic. They provide a way to enforce security policies, validate objects, and manage complex deployments and stateful applications. With the right understanding and use, they can greatly enhance the power and flexibility of Kubernetes.
Whether you're a software engineer looking to deepen your understanding of Kubernetes, or a DevOps professional looking to automate your deployments, understanding admission webhooks in operators is a crucial step in your journey. We hope this glossary entry has provided you with a comprehensive understanding of this topic.