Скачать или смотреть Mastering Kubernetes Dynamic Admission Controller | Series Part-2 || Mutating Admission Controller

A Mutating Admission Controller in Kubernetes is a type of admission controller that intercepts requests to the Kubernetes API server and has the ability to modify the content of objects before they are persisted in the cluster. Unlike validating admission controllers, which can only accept or reject admission requests, mutating admission controllers can dynamically alter the content of the resources being created or modified.

Here's a detailed description of Mutating Admission Controllers:

Overview:
Modification Capability:

Mutating Admission Controllers have the ability to modify the content of Kubernetes resources before they are stored in the etcd datastore.
They allow for changes to be made to the request object, such as adding default values, injecting sidecar containers, or modifying annotations.
Use Cases:

Common use cases include injecting default values or configuration into Pods, sidecar container injection, and setting default labels or annotations.
Flexibility:

Mutating controllers provide flexibility by allowing administrators to customize the behavior of the system based on specific requirements or policies.
Workflow:
Request Handling:

When a user creates or modifies a resource, the API server sends an admission review request to registered Mutating Admission Controllers.
Admission Review Process:

The Mutating Admission Controller processes the request and can modify the content of the resource as needed.
It returns an admission review response that includes the modified resource.
Decision Enforcement:

The API server persists the modified resource in the etcd datastore, enforcing the changes made by the Mutating Admission Controller.
Use Cases:
Default Value Injection:

Mutating controllers can inject default values into resources, ensuring that certain fields are always set, even if not explicitly provided by the user.
Sidecar Injection:

Common in microservices architectures, Mutating Admission Controllers can inject sidecar containers into Pods to provide additional functionality.
Annotation/Label Addition:

Mutating controllers can automatically add annotations or labels to resources based on predefined policies.
Examples:
Pod Defaults:

A Mutating Admission Controller can be used to set default values for fields such as environment variables, volume mounts, or resource requests and limits in Pods.
Istio Sidecar Injection:

In a service mesh like Istio, a Mutating Admission Controller can inject the Istio sidecar container into Pods to enable features like traffic management and security.
Challenges:
Order of Execution:

The order in which Mutating Admission Controllers are executed can impact the final state of the resource, and careful consideration is needed to avoid conflicts.
Testing and Validation:

Proper testing and validation are crucial to ensure that the mutations applied by controllers do not inadvertently break applications or violate policies.
Conclusion:
Mutating Admission Controllers provide a powerful mechanism for dynamically modifying Kubernetes resources before they are persisted in the cluster. They are valuable for enforcing defaults, injecting sidecar containers, and adapting resources to conform to specific policies or requirements. Careful design and testing are essential to ensure that mutations align with the desired behavior without introducing unexpected issues.


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