108 lines
5.3 KiB
Markdown
108 lines
5.3 KiB
Markdown
# 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](https://kubernetes.io/docs/admin/high-availability/)
|
||
|
||
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:
|
||
|
||
1. Kubelet updates it status to apiserver periodically, as specified by
|
||
`--node-status-update-frequency`. The default value is **10s**.
|
||
|
||
2. Kubernetes controller manager checks the statuses of Kubelet every
|
||
`–-node-monitor-period`. The default value is **5s**.
|
||
|
||
3. 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 master 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](https://github.com/kubernetes/kubernetes/blob/release-1.5/pkg/kubelet/kubelet.go#L102).
|
||
|
||
Kubelet will try to update the status in
|
||
[tryUpdateNodeStatus](https://github.com/kubernetes/kubernetes/blob/release-1.5/pkg/kubelet/kubelet_node_status.go#L312)
|
||
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](https://kubernetes.io/docs/concepts/scheduling-eviction/taint-and-toleration/#taint-based-evictions)
|
||
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.
|