kube-metrics-adapter

Kube Metrics Adapter is a general purpose metrics adapter for Kubernetes that can collect and serve custom and external metrics for Horizontal Pod Autoscaling.

It discovers Horizontal Pod Autoscaling resources and starts to collect the requested metrics and stores them in memory. It's implemented using the custom-metrics-apiserver library.

Here's an example of a HorizontalPodAutoscaler resource configured to get requests-per-second metrics from each pod of the deployment myapp.

apiVersion: autoscaling/v2beta1
kind: HorizontalPodAutoscaler
metadata:
  name: myapp-hpa
  annotations:
    # metric-config.<metricType>.<metricName>.<collectorName>/<configKey>
    metric-config.pods.requests-per-second.json-path/json-key: "$.http_server.rps"
    metric-config.pods.requests-per-second.json-path/path: /metrics
    metric-config.pods.requests-per-second.json-path/port: "9090"
spec:
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: myapp
  minReplicas: 1
  maxReplicas: 10
  metrics:
  - type: Pods
    pods:
      metricName: requests-per-second
      targetAverageValue: 1k

The metric-config.* annotations are used by the kube-metrics-adapter to configure a collector for getting the metrics. In the above example it configures a json-path pod collector.

Building

This project uses Go modules as introduced in Go 1.11 therefore you need Go >=1.11 installed in order to build. If using Go 1.11 you also need to activate Module support.

Assuming Go has been setup with module support it can be built simply by running:

export GO111MODULE=on # needed if the project is checked out in your $GOPATH.
$ make

Collectors

Collectors are different implementations for getting metrics requested by an HPA resource. They are configured based on HPA resources and started on-demand by the kube-metrics-adapter to only collect the metrics required for scaling the application.

The collectors are configured either simply based on the metrics defined in an HPA resource, or via additional annotations on the HPA resource.

Pod collector

The pod collector allows collecting metrics from each pod matched by the HPA. Currently only json-path collection is supported.

Supported metrics

Metric	Description	Type
custom	No predefined metrics. Metrics are generated from user defined queries.	Pods

Example

This is an example of using the pod collector to collect metrics from a json metrics endpoint of each pod matched by the HPA.

apiVersion: autoscaling/v2beta1
kind: HorizontalPodAutoscaler
metadata:
  name: myapp-hpa
  annotations:
    # metric-config.<metricType>.<metricName>.<collectorName>/<configKey>
    metric-config.pods.requests-per-second.json-path/json-key: "$.http_server.rps"
    metric-config.pods.requests-per-second.json-path/path: /metrics
    metric-config.pods.requests-per-second.json-path/port: "9090"
spec:
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: myapp
  minReplicas: 1
  maxReplicas: 10
  metrics:
  - type: Pods
    pods:
      metricName: requests-per-second
      targetAverageValue: 1k

The pod collector is configured through the annotations which specify the collector name json-path and a set of configuration options for the collector. json-key defines the json-path query for extracting the right metric. This assumes the pod is exposing metrics in JSON format. For the above example the following JSON data would be expected:

{
  "http_server": {
    "rps": 0.5
  }
}

The json-path query support depends on the github.com/oliveagle/jsonpath library. See the README for possible queries. It's expected that the metric you query returns something that can be turned into a float64.

The other configuration options path and port specifies where the metrics endpoint is exposed on the pod. There's no default values, so they must be defined.

Prometheus collector

The Prometheus collector is a generic collector which can map Prometheus queries to metrics that can be used for scaling. This approach is different from how it's done in the k8s-prometheus-adapter where all available Prometheus metrics are collected and transformed into metrics which the HPA can scale on, and there is no possibility to do custom queries. With the approach implemented here, users can define custom queries and only metrics returned from those queries will be available, reducing the total number of metrics stored.

One downside of this approach is that bad performing queries can slow down/kill Prometheus, so it can be dangerous to allow in a multi tenant cluster. It's also not possible to restrict the available metrics using something like RBAC since any user would be able to create the metrics based on a custom query.

I still believe custom queries are more useful, but it's good to be aware of the trade-offs between the two approaches.

Supported metrics

Metric	Description	Type	Kind
custom	No predefined metrics. Metrics are generated from user defined queries.	Object	any

Example

This is an example of an HPA configured to get metrics based on a Prometheus query. The query is defined in the annotation metric-config.object.processed-events-per-second.prometheus/query where processed-events-per-second is the metric name which will be associated with the result of the query.

It also specifies an annotation metric-config.object.processed-events-per-second.prometheus/per-replica which instructs the collector to treat the results as an average over all pods targeted by the HPA. This makes it possible to mimic the behavior of targetAverageValue which is not implemented for metric type Object as of Kubernetes v1.10. (It will most likely come in v1.12).

apiVersion: autoscaling/v2beta1
kind: HorizontalPodAutoscaler
metadata:
  name: myapp-hpa
  annotations:
    # metric-config.<metricType>.<metricName>.<collectorName>/<configKey>
    metric-config.object.processed-events-per-second.prometheus/query: |
      scalar(sum(rate(event-service_events_count{application="event-service",processed="true"}[1m])))      
    metric-config.object.processed-events-per-second.prometheus/per-replica: "true"
spec:
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: custom-metrics-consumer
  minReplicas: 1
  maxReplicas: 10
  metrics:
  - type: Object
    object:
      metricName: processed-events-per-second
      target:
        apiVersion: v1
        kind: Service
        name: event-service
      targetValue: 10 # this will be treated as targetAverageValue

Skipper collector

The skipper collector is a simple wrapper around the Prometheus collector to make it easy to define an HPA for scaling based on ingress metrics when skipper is used as the ingress implementation in your cluster. It assumes you are collecting Prometheus metrics from skipper and it provides the correct Prometheus queries out of the box so users don't have to define those manually.

Supported metrics

Metric	Description	Type	Kind
requests-per-second	Scale based on requests per second for a certain ingress.	Object	Ingress

Example

This is an example of an HPA that will scale based on requests-per-second for an ingress called myapp.

apiVersion: autoscaling/v2beta1
kind: HorizontalPodAutoscaler
metadata:
  name: myapp-hpa
spec:
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: myapp
  minReplicas: 1
  maxReplicas: 10
  metrics:
  - type: Object
    object:
      metricName: requests-per-second
      target:
        apiVersion: extensions/v1beta1
        kind: Ingress
        name: myapp
      targetValue: 10 # this will be treated as targetAverageValue

Note: As of Kubernetes v1.10 the HPA does not support targetAverageValue for metrics of type Object. In case of requests per second it does not make sense to scale on a summed value because you can not make the total requests per second go down by adding more pods. For this reason the skipper collector will automatically treat the value you define in targetValue as an average per pod instead of a total sum.

AWS collector

The AWS collector allows scaling based on external metrics exposed by AWS services e.g. SQS queue lengths.

Supported metrics

Metric	Description	Type
sqs-queue-length	Scale based on SQS queue length	External

Example

This is an example of an HPA that will scale based on the length of an SQS queue.

apiVersion: autoscaling/v2beta1
kind: HorizontalPodAutoscaler
metadata:
  name: myapp-hpa
spec:
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: custom-metrics-consumer
  minReplicas: 1
  maxReplicas: 10
  metrics:
  - type: External
    external:
      metricName: sqs-queue-length
      metricSelector:
        matchLabels:
          queue-name: foobar
          region: eu-central-1
      targetAverageValue: 30

The matchLabels are used by kube-metrics-adapter to configure a collector that will get the queue length for an SQS queue named foobar in region eu-central-1.

The AWS account of the queue currently depends on how kube-metrics-adapter is configured to get AWS credentials. The normal assumption is that you run the adapter in a cluster running in the AWS account where the queue is defined. Please open an issue if you would like support for other use cases.