In a MetalK8s cluster, almost each component (including applications running on top of it) generates metrics that allow to monitor its health or expose internal information.
These metrics can be scrapped and then displayed to the end user in a fashion way to give him an easily readable overview of the cluster and applications health and status.
Moreover, the metrics can also be used to send alerts depending on their values, allowing to quickly see if there is any issue or strange behavior on a component.
Here, we want to describe the monitoring stack deployed in MetalK8s.
Highly available metric history
Trigger alerts based on metric values
Visualization of metrics through charts
Easily scrape new metrics (from new MetalK8s pods or from workload pods running on top of MetalK8s)
Configuration of alert routing and some existing alerts
Possibility to add extra alerts
Design Design Choices¶
In order to scrape metrics, trigger alerts and visualize metrics history we decided to use Prometheus, AlertManager and Grafana since it is the de facto standard for Kubernetes monitoring and is well integrated with almost any application running on top of Kubernetes.
We use kube-prometheus which embeds the prometheus-operator that deploys the various components but also allows to easily monitor new metrics, add new Grafana dashboards and datasources.
But Prometheus does not work as a cluster which means that to have proper HA we need to duplicate information on several Prometheus instances.
Either each Prometheus instance monitors only a part of all nodes (with some overlaps in order to have HA), but then to visualize information we need to reach the right Prometheus instance depending on the node we want to see.
Either each Prometheus instance monitors all nodes, but even in this case if for whatever reason one Prometheus is down for a short time we will have a “blank” in the resulting graphs while the information is available in the other Prometheus instances.
To solve this we decided to use Thanos, so that we have a Thanos side-car on every Prometheus instances and a global Thanos querier that will query all the Prometheus instances in order to get metrics asked. And then it will deduplicate series, that come from different Prometheus instances, by merging all of them in a single time series. Check the Thanos querier documentation if you want more information about how it works.
Thanos queries are exactly the same as Prometheus ones they both use PromQL.
For the moment we only consider Thanos as a querier and we will not configure any long term retention, but it’s a Thanos capability that can be interesting in the future.
Prometheus can federate other Prometheus instances, it basically fetches all metrics from other Prometheus instances instead of getting them directly from Pod metric endpoints.
It allows to have all metrics available in a single point, with a small network impact.
This approach was rejected because it means we need to store all metrics in yet another Prometheus instance and it does not properly scale if we really want all metrics in this federated Prometheus instance.
Cortex allows to have highly available metrics history, all Prometheus instances just push all metrics to Cortex.
Cortex works almost the same way as a Federated Prometheus except that it runs as a cluster, so it is more scalable since you can have several Cortex replicas that only own a part of the metrics. The Cortex Querier will then retrieve the metrics from the right Cortex instance.
This approach was rejected because it needs a dedicated DB to store the Cortex hash and it consumes more storage, it is a bit excessive for a single Kubernetes cluster (which is the intend of MetalK8s) and Cortex cannot be used as a querier only.
In order to deploy Thanos Prometheus side-car and deploy the Thanos querier that interact with those side-cars we need to add new Salt states in MetalK8s.
Those salt states come from
Kubernetes Prometheus stack helm chart
and Thanos helm chart
as described in the Thanos readme,
then those helm charts are rendered to salt states using the
Thanos will only be used as a querier for the moment so we do not need any specific configuration for it.
Create a Grafana datasource for Thanos querier and use it for all dashboards that need to query Prometheus metrics.
Expose Thanos querier through the metalk8s-ui-proxies-http Ingress object to be able to query Prometheus metrics from the MetalK8s UI.
In the Installation Guide:
Document how to access Grafana
In the Operational Guide:
Document how this monitoring stack works
Document what are the default alerts deployed by MetalK8s
Document how to perform a Prometheus snapshot
Document how to configure Prometheus through CSC
Add test scenarios for monitoring stack using pytest-bdd framework to ensure the correct behavior of this feature.
Ensure that every deployed Pods are properly running after installation
Ensure that metrics are properly scrapped on every Prometheus instances
Ensure that alerts are properly raised
Ensure that we can query metrics from Thanos Querier