kubespray/docs/ha-mode.md

HA endpoints for K8s

The following components require a highly available endpoints:

• etcd cluster,
• kube-apiserver service instances.

The former provides the etcd-proxy service to access the cluster members in HA fashion.

The latter relies on a 3rd side reverse proxies, like Nginx or HAProxy, to achieve the same goal.

Etcd

Etcd proxies are deployed on each node in the k8s-cluster group. A proxy is a separate etcd process. It has a localhost:2379 frontend and all of the etcd cluster members as backends. Note that the access_ip is used as the backend IP, if specified. Frontend endpoints cannot be accessed externally as they are bound to a localhost only.

The etcd_access_endpoint fact provides an access pattern for clients. And the etcd_multiaccess (defaults to false) group var controlls that behavior. When enabled, it makes deployed components to access the etcd cluster members directly: http://ip1:2379, http://ip2:2379,.... This mode assumes the clients do a loadbalancing and handle HA for connections. Note, a pod definition of a flannel networking plugin always uses a single --etcd-server endpoint!

Kube-apiserver

K8s components require a loadbalancer to access the apiservers via a reverse proxy. Kargo includes support for an nginx-based proxy that resides on each non-master Kubernetes node. This is referred to as localhost loadbalancing. It is less efficient than a dedicated load balancer because it creates extra health checks on the Kubernetes apiserver, but is more practical for scenarios where an external LB or virtual IP management is inconvenient.

This option is configured by the variable loadbalancer_apiserver_localhost. you will need to configure your own loadbalancer to achieve HA. Note that deploying a loadbalancer is up to a user and is not covered by ansible roles in Kargo. By default, it only configures a non-HA endpoint, which points to the access_ip or IP address of the first server node in the kube-master group. It can also configure clients to use endpoints for a given loadbalancer type. The following diagram shows how traffic to the apiserver is directed.

Note: Kubernetes master nodes still use insecure localhost access because there are bugs in Kubernetes <1.5.0 in using TLS auth on master role services. This makes backends receiving unencrypted traffic and may be a security issue when interconnecting different nodes, or maybe not, if those belong to the isolated management network without external access.

A user may opt to use an external loadbalancer (LB) instead. An external LB provides access for external clients, while the internal LB accepts client connections only to the localhost, similarly to the etcd-proxy HA endpoints. Given a frontend VIP address and IP1, IP2 addresses of backends, here is an example configuration for a HAProxy service acting as an external LB:

listen kubernetes-apiserver-https
  bind <VIP>:8383
  option ssl-hello-chk
  mode tcp
  timeout client 3h
  timeout server 3h
  server master1 <IP1>:443
  server master2 <IP2>:443
  balance roundrobin

And the corresponding example global vars config:

apiserver_loadbalancer_domain_name: "lb-apiserver.kubernetes.local"
loadbalancer_apiserver:
  address: <VIP>
  port: 8383

This domain name, or default "lb-apiserver.kubernetes.local", will be inserted into the /etc/hosts file of all servers in the k8s-cluster group. Note that the HAProxy service should as well be HA and requires a VIP management, which is out of scope of this doc. Specifying an external LB overrides any internal localhost LB configuration.

Note: In order to achieve HA for HAProxy instances, those must be running on the each node in the k8s-cluster group as well, but require no VIP, thus no VIP management.

Access endpoints are evaluated automagically, as the following:

Endpoint type	kube-master	non-master
Local LB	http://lc:p	https://lc:sp
External LB, no internal	https://lb:lp	https://lb:lp
No ext/int LB (default)	http://lc:p	https://m[0].aip:sp

Where:

• m[0] - the first node in the kube-master group;
• lb - LB FQDN, apiserver_loadbalancer_domain_name;
• lc - localhost;
• p - insecure port, kube_apiserver_insecure_port
• sp - secure port, kube_apiserver_port;
• lp - LB port, loadbalancer_apiserver.port, defers to the secure port;
• ip - the node IP, defers to the ansible IP;
• aip - access_ip, defers to the ip.