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
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.
