Merge pull request #270865 from schaffererin/userstory237419

Expand conceptual docs in Azure fleet docs

Author: American-Dipper

articles/kubernetes-fleet/concepts-fleet.md

@@ -1,7 +1,7 @@
 ---
 title: "Azure Kubernetes Fleet Manager and member clusters"
 description: This article provides a conceptual overview of Azure Kubernetes Fleet Manager and member clusters.
-ms.date: 03/04/2024
+ms.date: 04/01/2024
 author: shashankbarsin
 ms.author: shasb
 ms.service: kubernetes-fleet
@@ -10,21 +10,24 @@ ms.topic: conceptual
 
 # Azure Kubernetes Fleet Manager and member clusters
 
-Azure Kubernetes Fleet Manager (Fleet) solves at-scale and multi-cluster problems for Kubernetes clusters.  This document provides a conceptual overview of fleet and its relationship with its member Kubernetes clusters. Right now Fleet supports joining AKS clusters as member clusters.
+This article provides a conceptual overview of fleets, member clusters, and hub clusters in Azure Kubernetes Fleet Manager (Fleet).
 
-[ ![Diagram that shows relationship between Fleet and Azure Kubernetes Service clusters.](./media/conceptual-fleet-aks-relationship.png) ](./media/conceptual-fleet-aks-relationship.png#lightbox)
+## What are fleets?
 
-## Fleet scenarios
+A fleet resource acts as a grouping entity for multiple AKS clusters. You can use them to manage multiple AKS clusters as a single entity, orchestrate updates across multiple clusters, propagate Kubernetes resources across multiple clusters, and provide a single pane of glass for managing multiple clusters. You can create a fleet with or without a [hub cluster](#what-is-a-hub-cluster-preview).
 
-A fleet is an Azure resource you can use to group and manage multiple Kubernetes clusters. Currently fleet supports the following scenarios:
-  * Create a Fleet resource and group AKS clusters as member clusters.
-  * Orchestrate latest or consistent Kubernetes version and node image upgrades across multiple clusters by using update runs, stages, and groups
-  * Create Kubernetes resource objects on the Fleet resource's hub cluster and control their propagation to member clusters (preview).
-  * Export and import services between member clusters, and load balance incoming L4 traffic across service endpoints on multiple clusters (preview).
+A fleet consists of the following components:
+
+:::image type="content" source="./media/concepts-fleet/fleet-architecture.png" alt-text="This screenshot shows a diagram of the fleet resource, including the hub cluster agent and the member cluster agent.":::
+
+* **fleet-hub-agent**: A Kubernetes controller that creates and reconciles all the fleet-related custom resources (CRs) in the hub cluster.
+* **fleet-member-agent**: A Kubernetes controller that creates and reconciles all the fleet-related CRs in the member clusters. This controller pulls the latest CRs from the hub cluster and consistently reconciles the member clusters to match the desired state.
 
 ## What are member clusters?
 
-You can join Azure Kubernetes Service (AKS) clusters to a fleet as member clusters. Member clusters must reside in the same Microsoft Entra tenant as the fleet. But they can be in different regions, different resource groups, and/or different subscriptions.
+The `MemberCluster` represents a cluster-scoped API established within the hub cluster, serving as a representation of a cluster within the fleet. This API offers a dependable, uniform, and automated approach for multi-cluster applications to identify registered clusters within a fleet. It also facilitates applications in querying a list of clusters managed by the fleet or in observing cluster statuses for subsequent actions. For more information, see [the upstream Fleet documentation](https://github.com/Azure/fleet/blob/main/docs/concepts/MemberCluster/README.md).
+
+You can join Azure Kubernetes Service (AKS) clusters to a fleet as member clusters. Member clusters must reside in the same Microsoft Entra tenant as the fleet, but they can be in different regions, different resource groups, and/or different subscriptions.
 
 ## What is a hub cluster (preview)?
 
@@ -36,7 +39,7 @@ For other scenarios such as Kubernetes resource propagation, a hub cluster is re
 
 The following table lists the differences between a fleet without hub cluster and a fleet with hub cluster:
 
-| Feature Dimension | Without hub cluster | With hub cluster (preview) |
+| Feature dimension | Without hub cluster | With hub cluster (preview) |
 |-|-|-|
 | Hub cluster hosting (preview) | :x: | :white_check_mark: |
 | Member cluster limit | Up to 100 clusters | Up to 20 clusters |
@@ -57,8 +60,9 @@ The fleet resource without hub cluster is currently free of charge. If your flee
 ## FAQs
 
 ### Can I change a fleet without hub cluster to a fleet with hub cluster?
-No during hub cluster preview, to be supported once hub clusters become generally available.
+
+Not during hub cluster preview. This is planned to be supported once hub clusters become generally available.
 
 ## Next steps
 
-* [Create a fleet and join member clusters](./quickstart-create-fleet-and-members.md).
+* [Create a fleet and join member clusters](./quickstart-create-fleet-and-members.md).

articles/kubernetes-fleet/concepts-scheduler-scheduling-framework.md

@@ -0,0 +1,49 @@
+---
+title: "Azure Kubernetes Fleet Manager scheduler and scheduling framework"
+description: This article provides a conceptual overview of the Azure Kubernetes Fleet Manager scheduler and scheduling framework.
+ms.date: 04/01/2024
+author: schaffererin
+ms.author: schaffererin
+ms.service: kubernetes-fleet
+ms.topic: conceptual
+---
+
+# Azure Kubernetes Fleet Manager scheduler and scheduling framework
+
+This article provides a conceptual overview of the scheduler and scheduling framework in Azure Kubernetes Fleet Manager (Fleet).
+
+## What is the scheduler?
+
+The scheduler is a core component in the fleet workload with the primary responsibility of determining scheduling decisions for a bundle of resources based on the latest `ClusterSchedulingPolicySnapshot` generated by the [`ClusterResourcePlacement`](./concepts-resource-propagation.md).
+
+By default, the scheduler operates in *batch mode*, which enhances performance. In this mode, it binds a `ClusterResourceBinding` from a `ClusterResourcePlacement` to multiple clusters whenever possible.
+
+### Batch mode
+
+Scheduling resources within a `ClusterResourcePlacement` involves more dependencies compared to scheduling pods within a Kubernetes Deployment. There are two notable distinctions:
+
+* In a `ClusterResourcePlacement`, multiple replicas of resources can't be scheduled on the same cluster.
+* The `ClusterResourcePlacement` supports different placement types within a single object.
+
+For more information, see [the upstream Fleet Scheduler documentation](https://github.com/Azure/fleet/blob/main/docs/concepts/Scheduler/README.md).
+
+## What is the scheduling framework?
+
+The fleet scheduling framework closely aligns with the native [Kubernetes scheduling framework](https://kubernetes.io/docs/concepts/scheduling-eviction/scheduling-framework/), incorporating several modifications and tailored functionalities to support the fleet workload.
+
+:::image type="content" source="./media/concepts-scheduler-scheduling-framework/scheduling-framework.png" alt-text="This screenshot shows an overview diagram of the fleet scheduling framework.":::
+
+The primary advantage of this framework is its capability to compile plugins directly into the scheduler. Its API facilitates the implementation of diverse scheduling features as plugins, ensuring a lightweight and maintainable core.
+
+The fleet scheduler integrates the following fundamental built-in plugins:
+
+* **Topology spread plugin**: Supports the `TopologySpreadConstraints` in the placement policy.
+* **Cluster affinity plugin**: Facilitates the affinity clause in the placement policy.
+* **Same placement affinity plugin**: Designed specifically for fleet and prevents multiple replicas from being placed within the same cluster.
+* **Cluster eligibility plugin**: Enables cluster selection based on specific status criteria.
+
+For more information, see the [upstream Fleet Scheduling Framework documentation](https://github.com/Azure/fleet/blob/main/docs/concepts/Scheduling-Framework/README.md).
+
+## Next steps
+
+* [Create a fleet and join member clusters](./quickstart-create-fleet-and-members.md).

articles/kubernetes-fleet/index.yml

@@ -20,6 +20,8 @@ landingContent:
         links:
           - text: Fleets and member clusters
             url: concepts-fleet.md
+          - text: Scheduler and scheduling framework
+            url: concepts-scheduler-scheduling-framework.md
           - text: Update orchestration across multiple member clusters
             url: concepts-update-orchestration.md
           - text: Kubernetes resource propagation from hub cluster to member clusters

articles/kubernetes-fleet/media/concepts-fleet/fleet-architecture.png

@@ -20,6 +20,8 @@
   items:
   - name: Fleets and member clusters
     href: concepts-fleet.md
+  - name: Scheduler and scheduling framework
+    href: concepts-scheduler-scheduling-framework.md
   - name: Update orchestration across multiple member clusters
     href: concepts-update-orchestration.md
   - name: Kubernetes workload orchestration