kured - Kubernetes Reboot Daemon
- Introduction
- Kubernetes & OS Compatibility
- Installation
- Configuration
- Operation
- Building
- Frequently Asked/Anticipated Questions
- Getting Help
Introduction
Kured (KUbernetes REboot Daemon) is a Kubernetes daemonset that performs safe automatic node reboots when the need to do so is indicated by the package management system of the underlying OS.
- Watches for the presence of a reboot sentinel e.g.
/var/run/reboot-required - Utilises a lock in the API server to ensure only one node reboots at a time
- Optionally defers reboots in the presence of active Prometheus alerts or selected pods
- Cordons & drains worker nodes before reboot, uncordoning them after
Kubernetes & OS Compatibility
The daemon image contains versions of k8s.io/client-go and k8s.io/kubectl (the binary of kubectl in older releases) for the purposes of maintaining the lock and draining worker nodes. Kubernetes aims to provide forwards and backwards compatibility of one minor version between client and server:
| kured | kubectl | k8s.io/client-go | k8s.io/apimachinery | expected kubernetes compatibility |
|---|---|---|---|---|
| master | 1.19.4 | v0.19.4 | v0.19.4 | 1.18.x, 1.19.x, 1.20.x |
| 1.6.0 | 1.19.4 | v0.19.4 | v0.19.4 | 1.18.x, 1.19.x, 1.20.x |
| 1.5.1 | 1.18.8 | v0.18.8 | v0.18.8 | 1.17.x, 1.18.x, 1.19.x |
| 1.4.4 | 1.17.7 | v0.17.0 | v0.17.0 | 1.16.x, 1.17.x, 1.18.x |
| 1.3.0 | 1.15.10 | v12.0.0 | release-1.15 | 1.15.x, 1.16.x, 1.17.x |
| 1.2.0 | 1.13.6 | v10.0.0 | release-1.13 | 1.12.x, 1.13.x, 1.14.x |
| 1.1.0 | 1.12.1 | v9.0.0 | release-1.12 | 1.11.x, 1.12.x, 1.13.x |
| 1.0.0 | 1.7.6 | v4.0.0 | release-1.7 | 1.6.x, 1.7.x, 1.8.x |
See the release notes for specific version compatibility information, including which combination have been formally tested.
Versions >=1.1.0 enter the host mount namespace to invoke systemctl reboot, so should work on any systemd distribution.
Installation
To obtain a default installation without Prometheus alerting interlock or Slack notifications:
latest=$(curl -s https://api.github.com/repos/weaveworks/kured/releases | jq -r .[0].tag_name)
kubectl apply -f "https://github.com/weaveworks/kured/releases/download/$latest/kured-$latest-dockerhub.yaml"
If you want to customise the installation, download the manifest and edit it in accordance with the following section before application.
Configuration
The following arguments can be passed to kured via the daemonset pod template:
Flags:
--alert-filter-regexp regexp.Regexp alert names to ignore when checking for active alerts
--blocking-pod-selector stringArray label selector identifying pods whose presence should prevent reboots
--ds-name string name of daemonset on which to place lock (default "kured")
--ds-namespace string namespace containing daemonset on which to place lock (default "kube-system")
--end-time string schedule reboot only before this time of day (default "23:59:59")
-h, --help help for kured
--lock-annotation string annotation in which to record locking node (default "weave.works/kured-node-lock")
--lock-ttl duration expire lock annotation after this duration (default: 0, disabled)
--message-template-drain string message template used to notify about a node being drained (default "Draining node %s")
--message-template-reboot string message template used to notify about a node being rebooted (default "Rebooting node %s")
--period duration reboot check period (default 1h0m0s)
--prefer-no-schedule-taint string Taint name applied during pending node reboot (to prevent receiving additional pods from other rebooting nodes). Disabled by default. Set e.g. to "weave.works/kured-node-reboot" to enable tainting.
--prometheus-url string Prometheus instance to probe for active alerts
--reboot-days strings schedule reboot on these days (default [su,mo,tu,we,th,fr,sa])
--reboot-sentinel string path to file whose existence signals need to reboot (default "/var/run/reboot-required")
--slack-channel string slack channel for reboot notfications
--slack-hook-url string slack hook URL for reboot notfications
--slack-username string slack username for reboot notfications (default "kured")
--start-time string schedule reboot only after this time of day (default "0:00")
--time-zone string use this timezone for schedule inputs (default "UTC")
Reboot Sentinel File & Period
By default kured checks for the existence of /var/run/reboot-required every sixty minutes; you can override these values with --reboot-sentinel and --period. Each replica of the daemon uses a random offset derived from the period on startup so that nodes don't all contend for the lock simultaneously.
Setting a schedule
By default, kured will reboot any time it detects the sentinel, but this may cause reboots during odd hours. While service disruption does not normally occur, anything is possible and operators may want to restrict reboots to predictable schedules. Use --reboot-days, --start-time, --end-time, and --time-zone to set a schedule. For example, business hours on the west coast USA can be specified with:
--reboot-days=mon,tue,wed,thu,fri
--start-time=9am
--end-time=5pm
--time-zone=America/Los_Angeles
Times can be formatted in numerous ways, including 5pm, 5:00pm 17:00, and 17. --time-zone represents a Go time.Location, and can be UTC, Local, or any entry in the standard Linux tz database.
Note that when using smaller time windows, you should consider shortening the sentinel check period (--period).
Blocking Reboots via Alerts
You may find it desirable to block automatic node reboots when there are active alerts - you can do so by providing the URL of your Prometheus server:
--prometheus-url=http://prometheus.monitoring.svc.cluster.local
By default the presence of any active (pending or firing) alerts will block reboots, however you can ignore specific alerts:
--alert-filter-regexp=^(RebootRequired|AnotherBenignAlert|...$
See the section on Prometheus metrics for an important application of this filter.
Blocking Reboots via Pods
You can also block reboots of an individual node when specific pods are scheduled on it:
--blocking-pod-selector=runtime=long,cost=expensive
Since label selector strings use commas to express logical 'and', you can specify this parameter multiple times for 'or':
--blocking-pod-selector=runtime=long,cost=expensive
--blocking-pod-selector=name=temperamental
In this case, the presence of either an (appropriately labelled) expensive long running job or a known temperamental pod on a node will stop it rebooting.
Try not to abuse this mechanism - it's better to strive for restartability where possible. If you do use it, make sure you set up a RebootRequired alert as described in the next section so that you can intervene manually if reboots are blocked for too long.
Prometheus Metrics
Each kured pod exposes a single gauge metric (:8080/metrics) that indicates the presence of the sentinel file:
# HELP kured_reboot_required OS requires reboot due to software updates.
# TYPE kured_reboot_required gauge
kured_reboot_required{node="ip-xxx-xxx-xxx-xxx.ec2.internal"} 0
The purpose of this metric is to power an alert which will summon an operator if the cluster cannot reboot itself automatically for a prolonged period:
# Alert if a reboot is required for any machines. Acts as a failsafe for the
# reboot daemon, which will not reboot nodes if there are pending alerts save
# this one.
ALERT RebootRequired
IF max(kured_reboot_required) != 0
FOR 24h
LABELS { severity="warning" }
ANNOTATIONS {
summary = "Machine(s) require being rebooted, and the reboot daemon has failed to do so for 24 hours",
impact = "Cluster nodes more vulnerable to security exploits. Eventually, no disk space left.",
description = "Machine(s) require being rebooted, probably due to kernel update.",
}
If you choose to employ such an alert and have configured kured to probe for active alerts before rebooting, be sure to specify --alert-filter-regexp=^RebootRequired$ to avoid deadlock!
Slack Notifications
If you specify a Slack hook via --slack-hook-url, kured will notify you immediately prior to rebooting a node:
We recommend setting --slack-username to be the name of the environment, e.g. dev or prod.
Alternatively you can use the --message-template-drain and --message-template-reboot to customize the text of the message, e.g.
--message-template-drain="Draining node %s part of *my-cluster* in region *xyz*"
Overriding Lock Configuration
The --ds-name and --ds-namespace arguments should match the name and namespace of the daemonset used to deploy the reboot daemon - the locking is implemented by means of an annotation on this resource. The defaults match the daemonset YAML provided in the repository.
Similarly --lock-annotation can be used to change the name of the annotation kured will use to store the lock, but the default is almost certainly safe.
Operation
The example commands in this section assume that you have not overriden the default lock annotation, daemonset name or namespace; if you have, you will have to adjust the commands accordingly.
Testing
You can test your configuration by provoking a reboot on a node:
sudo touch /var/run/reboot-required
Disabling Reboots
If you need to temporarily stop kured from rebooting any nodes, you can take the lock manually:
kubectl -n kube-system annotate ds kured weave.works/kured-node-lock='{"nodeID":"manual"}'
Don't forget to release it afterwards!
Manual Unlock
In exceptional circumstances, such as a node experiencing a permanent failure whilst rebooting, manual intervention may be required to remove the cluster lock:
kubectl -n kube-system annotate ds kured weave.works/kured-node-lock-
NB the
-at the end of the command is important - it instructskubectlto remove that annotation entirely.
Automatic Unlock
In exceptional circumstances (especially when used with cluster-autoscaler) a node which holds lock might be killed thus annotation will stay there for ever.
Using --lock-ttl=30m will allow other nodes to take over if TTL has expired (in this case 30min) and continue reboot process.
Building
Kured now uses Go Modules, so build instructions vary depending on where you have checked out the repository:
Building outside $GOPATH:
make
Building inside $GOPATH:
GO111MODULE=on make
You can find the current preferred version of Golang in the go.mod file.
If you are interested in contributing code to kured, please take a look at our development docs.
Frequently Asked/Anticipated Questions
Why is there no latest tag on Docker Hub?
Use of latest for production deployments is bad practice - see here for details. The manifest on master refers to latest for local development testing with minikube only; for production use choose a versioned manifest from the release page.
Getting Help
If you have any questions about, feedback for or problems with kured:
- Invite yourself to the Weave Users Slack.
- Ask a question on the #kured slack channel.
- File an issue.
- Join us in our monthly meeting, every fourth Wednesday of the month at 16:00 UTC.
We follow the CNCF Code of Conduct.
Your feedback is always welcome!
