Admission control proposal
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# Kubernetes Proposal - Admission Control
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**Related PR:**
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| Topic | Link |
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| ----- | ---- |
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## Background
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High level goals:
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* Enable an easy-to-use mechanism to provide admission control to cluster
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* Enable a provider to support multiple admission control strategies or author their own
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* Ensure any rejected request can propagate errors back to the caller with why the request failed
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* Enable usage of cluster resources to satisfy admission control criteria
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* Enable admission controller criteria to change without requiring restart of kube-apiserver
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Policy is focused on answering if a user is authorized to perform an action.
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Admission Control is focused on if the system will accept an authorized action.
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The Kubernetes cluster may choose to dismiss an authorized action based on any number of admission control strategies they choose to author and deploy:
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1. Quota enforcement of allocated desired usage
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2. Pod black-lister to restrict running specific images on the cluster
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3. Privileged container checker
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4. Host port reservation
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5. Volume validation - e.g. may or may not use hostDir, etc.
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6. Min/max constraint checker for pod requested resources
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7. ...
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This proposal therefore attempts to enumerate the basic design, and describe how any number of admission controllers could be injected.
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## kube-apiserver
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The kube-apiserver takes the following OPTIONAL arguments to enable admission control
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| Option | Behavior |
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| ------ | -------- |
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| admission_controllers | List of addresses (ip:port, dns name) to invoke for admission control |
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| admission_controller_service | Service label selector to resolve for admission control (namespace/labelKey/labelValue) |
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If the list of addresses to invoke for admission control are provided as a label selector, the kube-apiserver will update the list
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of admission control services at a regular interval.
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Upon an incoming request, the kube-apiserver performs the following basic flow:
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1. Authorize the request, if authorized, continue
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2. Invoke the Admission Control REST API for each defined address, if all return true, continue
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3. RESTStorage processes request
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4. Data is persisted in store
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If there is no configured admission control address, then by default, all requests are admitted.
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Admission control is enforced on POST/PUT operations, but is ignored on GET/DELETE operations.
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## Admission Control REST API
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An admission controller satisfies a stable REST API invoked by the kube-apiserver to satisfy requests.
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| Action | HTTP Verb | Path | Description |
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| ---- | ---- | ---- | ---- |
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| CREATE | POST | /admissionController | Send a request for admission to evaluate for admittance or denial |
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The message body to the admissionController includes the following:
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1. requesting user identity
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2. action to perform
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3. proposed resource to create/modify (if any)
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If the request for admission is satisfied, return a HTTP 200.
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If the request for admission is denied, return a HTTP 403, the response must include a reason for why the response failed.
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## System Design
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The following demonstrates potential cluster setups using an external list of admission control endpoints.
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Request
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+
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+---------------|----------+
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| API Server | |
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|---------------|----------|
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| v |
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| +--------+ |
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| | Policy | | +---------------------+---+
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| ++-------+ | |Endpoints |
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+---------+ | | | |-------------------------|
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|Scheduler|<---+| v | |E1. Quota Enforcer |
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+---------+ | +----------------------+ | |E2. Capacity Planner |
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| | Admission Controller +-------->|E3. White-lister |
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| +----+-----------------+ | |... |
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| | | +-------------------------+
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| +v-------------+ |
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| | REST Storage | |
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| +--------------+ |
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+----------------+---------+
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v
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+--------------+
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| Data Store |
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|--------------|
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+--------------+
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The following demonstrates potential cluster setup that uses services to fulfill admission control.
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In this context, the cluster itself is used to provide HA admission control, and pods may choose to
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invoke the API Server to determine if a request is or is not admissible.
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Request +--------+
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+ |Pods... |
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| |--------|
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| | <-------+
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+---------------|----------+ | | |
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| API Server | | | | |
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|---------------|----------| +---+----+ |
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| v | | |
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| +--------+ |<---------------+ |
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| | Policy | | +-------------------------------------+
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| ++-------+ | |Service (ns=infra, labelKey=admitter)|
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+---------+ | | | |-------------------------------------|
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|Scheduler|<---+| v | |Service1 |
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+---------+ | +----------------------+ | |Service2 |
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| | Admission Controller +-------->|Service3 |
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| +----+-----------------+ | |... |
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| | | +-------------------------------------+
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| +v-------------+ |
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| | REST Storage | |
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| +--------------+ |
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+----------------+---------+
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v
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+--------------+
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| Data Store |
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|--------------|
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+--------------+
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