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docs: Consul K8s Overview update (#12575)

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* docs: Consul K8s Overview update

Co-authored-by: trujillo-adam <47586768+trujillo-adam@users.noreply.github.com>
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66
website/content/docs/k8s/architecture.mdx
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---
layout: docs
page_title: Consul on Kubernetes Architecture
description: >-
A high level overview of Consul on Kubernetes Architecture
---
# Architecture
This topic describes the architecture, components, and resources associated with Consul deployments to Kubernetes. Consul employs the same architectural design on Kubernetes as it does with other platforms (see [Architecture](/docs/architecture)), but Kubernetes provides additional benefits that make operating a Consul cluster easier.
Refer to the standard [production deployment guide](https://learn.hashicorp.com/consul/datacenter-deploy/deployment-guide) for important information, regardless of the deployment platform.
## Server Agents
The server agents are deployed as a `StatefulSet` and use persistent volume
claims to store the server state. This also ensures that the
[node ID](/docs/agent/options#_node_id) is persisted so that servers
can be rescheduled onto new IP addresses without causing issues. The server agents
are configured with
[anti-affinity](https://kubernetes.io/docs/concepts/configuration/assign-pod-node/#affinity-and-anti-affinity)
rules so that they are placed on different nodes. A readiness probe is
configured that marks the pod as ready only when it has established a leader.
A Kubernetes `Service` is registered to represent the servers and exposes ports that are requried to communicate to the Consul server pods.
The servers utilize the DNS address of this service to join a Consul cluster, without requiring any other access to the Kubernetes cluster. Additional consul servers may also utilize non-ready endpoints which are published by the Kubernetes service, so that servers can utilize the service for joining during bootstrap and upgrades.
Additionally, a **PodDisruptionBudget** is configured so the Consul server
cluster maintains quorum during voluntary operational events. The maximum
unavailable is `(n/2)-1` where `n` is the number of server agents.
-> **Note:** Kubernetes and Helm do not delete Persistent Volumes or Persistent
Volume Claims when a
[StatefulSet is deleted](https://kubernetes.io/docs/concepts/workloads/controllers/statefulset/#stable-storage),
so this must done manually when removing servers.
## Client Agents
The client agents are run as a **DaemonSet**. This places one agent
(within its own pod) on each Kubernetes node.
The clients expose the Consul HTTP API via a static port (8500)
bound to the host port. This enables all other pods on the node to connect
to the node-local agent using the host IP that can be retrieved via the
Kubernetes downward API. See
[accessing the Consul HTTP API](/docs/k8s/installation/install#accessing-the-consul-http-api)
for an example.
We do not use a **NodePort** Kubernetes service because requests to node ports get randomly routed
to any pod in the service and we need to be able to route directly to the Consul
client running on our node.
-> **Note:** There is no way to bind to a local-only
host port. Therefore, any other node can connect to the agent. This should be
considered for security. For a properly production-secured agent with TLS
and ACLs, this is safe.
We run Consul clients as a **DaemonSet** instead of running a client in each
application pod as a sidecar because this would turn
a pod into a "node" in Consul and also causes an explosion of resource usage
since every pod needs a Consul agent. Service registration should be handled via the
catalog syncing feature with Services rather than pods.
-> **Note:** Due to a limitation of anti-affinity rules with DaemonSets,
a client-mode agent runs alongside server-mode agents in Kubernetes. This
duplication wastes some resources, but otherwise functions perfectly fine.

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website/content/docs/k8s/index.mdx
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@ -17,17 +17,6 @@ This section documents the official integrations between Consul and Kubernetes.
## Use Cases
**Running a Consul server cluster:** The Consul server cluster can run directly
on Kubernetes. This can be used by both nodes within Kubernetes as well as
nodes external to Kubernetes, as long as they can communicate to the server
nodes via the network.
**Running Consul clients:** Consul clients can run as pods on every node
and expose the Consul API to running pods. This enables many Consul tools
such as envconsul, consul-template, and more to work on Kubernetes since a
local agent is available. This will also register each Kubernetes node with
the Consul catalog for full visibility into your infrastructure.
**Consul Connect Service Mesh:**
Consul can automatically inject the [Consul Connect](/docs/connect)
sidecar into pods so that they can accept and establish encrypted
@ -45,82 +34,13 @@ to use Consul service discovery to discover and connect to Kubernetes services.
native integrations provided by Consul itself, any other tool built for
Kubernetes can choose to leverage Consul.
## Architecture
Consul runs on Kubernetes with the same
[architecture](/docs/architecture)
as other platforms. There are some benefits Kubernetes can provide
that eases operating a Consul cluster and we document those below. The standard
[production deployment guide](https://learn.hashicorp.com/consul/datacenter-deploy/deployment-guide) is still an
important read even if running Consul within Kubernetes.
Each section below will outline the different components of running Consul
on Kubernetes and an overview of the resources that are used within the
Kubernetes cluster.
### Server Agents
The server agents are run as a **StatefulSet**, using persistent volume
claims to store the server state. This also ensures that the
[node ID](/docs/agent/options#_node_id) is persisted so that servers
can be rescheduled onto new IP addresses without causing issues. The server agents
are configured with
[anti-affinity](https://kubernetes.io/docs/concepts/configuration/assign-pod-node/#affinity-and-anti-affinity)
rules so that they are placed on different nodes. A readiness probe is
configured that marks the pod as ready only when it has established a leader.
A **Service** is registered to represent the servers and expose the various
ports. The DNS address of this service is used to join the servers to each
other without requiring any other access to the Kubernetes cluster. The
service is configured to publish non-ready endpoints so that it can be used
for joining during bootstrap and upgrades.
Additionally, a **PodDisruptionBudget** is configured so the Consul server
cluster maintains quorum during voluntary operational events. The maximum
unavailable is `(n/2)-1` where `n` is the number of server agents.
-> **Note:** Kubernetes and Helm do not delete Persistent Volumes or Persistent
Volume Claims when a
[StatefulSet is deleted](https://kubernetes.io/docs/concepts/workloads/controllers/statefulset/#stable-storage),
so this must done manually when removing servers.
### Client Agents
The client agents are run as a **DaemonSet**. This places one agent
(within its own pod) on each Kubernetes node.
The clients expose the Consul HTTP API via a static port (8500)
bound to the host port. This enables all other pods on the node to connect
to the node-local agent using the host IP that can be retrieved via the
Kubernetes downward API. See
[accessing the Consul HTTP API](/docs/k8s/installation/install#accessing-the-consul-http-api)
for an example.
We do not use a **NodePort** Kubernetes service because requests to node ports get randomly routed
to any pod in the service and we need to be able to route directly to the Consul
client running on our node.
-> **Note:** There is no way to bind to a local-only
host port. Therefore, any other node can connect to the agent. This should be
considered for security. For a properly production-secured agent with TLS
and ACLs, this is safe.
We run Consul clients as a **DaemonSet** instead of running a client in each
application pod as a sidecar because this would turn
a pod into a "node" in Consul and also causes an explosion of resource usage
since every pod needs a Consul agent. Service registration should be handled via the
catalog syncing feature with Services rather than pods.
-> **Note:** Due to a limitation of anti-affinity rules with DaemonSets,
a client-mode agent runs alongside server-mode agents in Kubernetes. This
duplication wastes some resources, but otherwise functions perfectly fine.
## Getting Started With Consul and Kubernetes
There are several ways to try Consul with Kubernetes in different environments.
**Tutorials**
- The [Getting Started with Consul Service Mesh track](https://learn.hashicorp.com/tutorials/consul/service-mesh?utm_source=WEBSITE&utm_medium=WEB_IO&utm_offer=ARTICLE_PAGE&utm_content=DOCS)
- The [Getting Started with Consul Service Mesh track](https://learn.hashicorp.com/tutorials/consul/service-mesh-deploy?in=consul/gs-consul-service-mesh?utm_source=WEBSITE&utm_medium=WEB_IO&utm_offer=ARTICLE_PAGE&utm_content=DOCS)
provides guidance for installing Consul as service mesh for Kubernetes using the Helm
chart, deploying services in the service mesh, and using intentions to secure service
communications.

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website/data/docs-nav-data.json
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@ -398,6 +398,10 @@
"title": "Overview",
"path": "k8s"
},
{
"title": "Architecture",
"path": "k8s/architecture"
},
{
"title": "Get Started",
"routes": [

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