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
- Cordons & drains worker nodes before reboot, uncordoning them after
Kubernetes & OS Compatibility
The daemon image contains a 1.7.x k8s.io/client-go and kubectl
binary for the purposes of maintaining the lock and draining worker
nodes. Whilst it has only been tested on a 1.7.x cluster, Kubernetes
typically has good forwards/backwards compatibility so there is a
reasonable chance it will work on adjacent versions; please file an
issue if this is not the case.
Additionally, the image contains a systemctl binary from Ubuntu
16.04 in order to command reboots. Again, although this has not been
tested against other systemd distributions there is a good chance that
it will work.
Installation
To obtain a default installation without Prometheus alerting interlock or Slack notifications:
kubectl apply -f https://github.com/weaveworks/kured/releases/download/1.0.0/kured-ds.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 value alert names to ignore when checking for active alerts
--ds-name string namespace containing daemonset on which to place lock (default "kube-system")
--ds-namespace string name of daemonset on which to place lock (default "kured")
--lock-annotation string annotation in which to record locking node (default "weave.works/kured-node-lock")
--period duration reboot check period (default 1h0m0s)
--prometheus-url string Prometheus instance to probe for active alerts
--reboot-sentinel string path to file whose existence signals need to reboot (default "/var/run/reboot-required")
--slack-hook-url string slack hook URL for reboot notfications
--slack-username string slack username for reboot notfications (default "kured")
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.
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|...$
An important application of this filter will become apparent in the next section.
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.
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.
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