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ha-mode.md

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HA endpoints for K8s

The following components require a highly available endpoints:

  • etcd cluster,
  • kube-apiserver service instances.

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

Etcd

The etcd_access_endpoint fact provides an access pattern for clients. And the etcd_multiaccess (defaults to True) group var controls that behavior. 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.

Kube-apiserver

K8s components require a loadbalancer to access the apiservers via a reverse proxy. Kubespray 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 (defaults to True). You may also define the port the local internal loadbalancer uses by changing, nginx_kube_apiserver_port. This defaults to the value of kube_apiserver_port. It is also important to note that Kubespray will only configure kubelet and kube-proxy on non-master nodes to use the local internal loadbalancer.

If you choose to NOT use the local internal loadbalancer, 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 Kubespray. 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.

Image

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. 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>:6443
  server master2 <IP2>:6443
  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 (default) http://lc:p https://lc:nsp
External LB, no internal https://lb:lp https://lb:lp
No ext/int LB 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
  • nsp - nginx secure port, nginx_kube_apiserver_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.