Overview

Distributed system such as Kubernetes are designed to be resilient to the failures. More details about Kubernetes High-Availability (HA) may be found at Building High-Availability Clusters

To have a simple view the most of parts of HA will be skipped to describe Kubelet<->Controller Manager communication only.

By default the normal behavior looks like:

Kubelet updates it status to apiserver periodically, as specified by --node-status-update-frequency. The default value is 10s.
Kubernetes controller manager checks the statuses of Kubelet every –-node-monitor-period. The default value is 5s.
In case the status is updated within --node-monitor-grace-period of time, Kubernetes controller manager considers healthy status of Kubelet. The default value is 40s.

Kubernetes controller manager and Kubelet work asynchronously. It means that the delay may include any network latency, API Server latency, etcd latency, latency caused by load on one's control plane nodes and so on. So if --node-status-update-frequency is set to 5s in reality it may appear in etcd in 6-7 seconds or even longer when etcd cannot commit data to quorum nodes.

Failure

Kubelet will try to make nodeStatusUpdateRetry post attempts. Currently nodeStatusUpdateRetry is constantly set to 5 in kubelet.go.

Kubelet will try to update the status in tryUpdateNodeStatus function. Kubelet uses http.Client() Golang method, but has no specified timeout. Thus there may be some glitches when API Server is overloaded while TCP connection is established.

So, there will be nodeStatusUpdateRetry * --node-status-update-frequency attempts to set a status of node.

At the same time Kubernetes controller manager will try to check nodeStatusUpdateRetry times every --node-monitor-period of time. After --node-monitor-grace-period it will consider node unhealthy. Pods will then be rescheduled based on the Taint Based Eviction timers that you set on them individually, or the API Server's global timers:--default-not-ready-toleration-seconds & --default-unreachable-toleration-seconds.

Kube proxy has a watcher over API. Once pods are evicted, Kube proxy will notice and will update iptables of the node. It will remove endpoints from services so pods from failed node won't be accessible anymore.

Recommendations for different cases

Fast Update and Fast Reaction

If --node-status-update-frequency is set to 4s (10s is default). --node-monitor-period to 2s (5s is default). --node-monitor-grace-period to 20s (40s is default). --default-not-ready-toleration-seconds and --default-unreachable-toleration-seconds are set to 30 (300 seconds is default). Note these two values should be integers representing the number of seconds ("s" or "m" for seconds\minutes are not specified).

In such scenario, pods will be evicted in 50s because the node will be considered as down after 20s, and --default-not-ready-toleration-seconds or --default-unreachable-toleration-seconds occur after 30s more. However, this scenario creates an overhead on etcd as every node will try to update its status every 2 seconds.

If the environment has 1000 nodes, there will be 15000 node updates per minute which may require large etcd containers or even dedicated nodes for etcd.

If we calculate the number of tries, the division will give 5, but in reality it will be from 3 to 5 with nodeStatusUpdateRetry attempts of each try. The total number of attempts will vary from 15 to 25 due to latency of all components.

Medium Update and Average Reaction

Let's set --node-status-update-frequency to 20s --node-monitor-grace-period to 2m and --default-not-ready-toleration-seconds and --default-unreachable-toleration-seconds to 60. In that case, Kubelet will try to update status every 20s. So, it will be 6 * 5 = 30 attempts before Kubernetes controller manager will consider unhealthy status of node. After 1m it will evict all pods. The total time will be 3m before eviction process.

Such scenario is good for medium environments as 1000 nodes will require 3000 etcd updates per minute.

In reality, there will be from 4 to 6 node update tries. The total number of of attempts will vary from 20 to 30.

Low Update and Slow reaction

Let's set --node-status-update-frequency to 1m. --node-monitor-grace-period will set to 5m and --default-not-ready-toleration-seconds and --default-unreachable-toleration-seconds to 60. In this scenario, every kubelet will try to update the status every minute. There will be 5 * 5 = 25 attempts before unhealthy status. After 5m, Kubernetes controller manager will set unhealthy status. This means that pods will be evicted after 1m after being marked unhealthy. (6m in total).

In reality, there will be from 3 to 5 tries. The total number of attempt will vary from 15 to 25.

There can be different combinations such as Fast Update with Slow reaction to satisfy specific cases.

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