Worker Pod Autoscaler

---

Scale kubernetes pods based on the combination of queue metrics by intelligently querying them only when needed.

Currently the supported Message Queueing Services are:

• AWS SQS
• Beanstalkd

Pull Requests are welcome to add new message queuing services.

---

Install the WorkerPodAutoscaler

Install

Running the below script will create the WPA CRD and install the worker pod autoscaler deployment.

export AWS_REGIONS='ap-south-1,ap-southeast-1'
export AWS_ACCESS_KEY_ID='sample-aws-access-key-id'
export AWS_SECRET_ACCESS_KEY='sample-aws-secret-acesss-key'
./hack/install.sh

Note: AWS_ variables needs to exported only when using SQS and the node role in which the WPA pod runs do not have the required IAM Policy.

Verify Installation

Check the wpa resource is accessible using kubectl

kubectl get wpa

Upgrade

Please follow this document for upgrading Worker Pod Autoscaler.

Example

Do install the wpa crd and wpa deployment before going with the example. (Please check above.)

• Create Deployment that needs to scale based on queue length.

kubectl create -f artifacts/examples/example-deployment.yaml

• Create WPA object (example-wpa) that will start scaling the example-deployment based on SQS queue length.

kubectl create -f artifacts/examples/example-wpa.yaml

This will start scaling example-deployment based on SQS queue length.

Configuration

WPA Resource

apiVersion: k8s.practo.dev/v1
kind: WorkerPodAutoScaler
metadata:
  name: example-wpa
spec:
  minReplicas: 0
  maxReplicas: 10
  deploymentName: example-deployment
  queueURI: https://sqs.ap-south-1.amazonaws.com/{{aws_account_id}}/{{queue_prefix-queue_name-queue_suffix}}
  targetMessagesPerWorker: 2
  secondsToProcessOneJob: 0.03
  maxDisruption: "100%"

Beanstalk's queueURI would be like: beanstalk://beanstalkDNSName:11300/test-tube

WPA Spec Documentation:

Spec	Description	Mandatory
minReplicas	Minimum number of workers you want to run.	Yes
maxReplicas	Maximum number of workers you want to run	Yes
deploymentName	Name of the kubernetes Deployment in the same namespace as WPA object.	No*
replicaSetName	Name of the kubernetes ReplicaSet in the same namespace as WPA object.	No*
queueURI	Full URL of the queue.	Yes
targetMessagesPerWorker	Target ratio between the number of queued jobs(both available and reserved) and the number of workers required to process them. For long running workers with visible backlog, this value may be set to 1 so that each job spawns a new worker (upto maxReplicas).	Yes
secondsToProcessOneJob	For fast running workers doing high RPM, the backlog is very close to zero. So for such workers scale up cannot happen based on the backlog, hence this is a really important specification to always keep the minimum number of workers running based on the queue RPM. (highly recommended, default=0.0 i.e. disabled).	No
maxDisruption	Amount of disruption that can be tolerated in a single scale down activity. Number of pods or percentage of pods that can scale down in a single down scale down activity. Using this you can control how fast a scale down can happen. This can be expressed both as an absolute value and a percentage. (default is the WPA flag --wpa-default-max-disruption).	No

• It is mandatory to set either deploymentName or replicaSetName.

Explained the above specifications with examples:

• targetMessagesPerWorker:

availableMessages=90(backlog), reservedMessages=110(inprocess), and 10 workers are required to process 110+90=200 messages then
targetMessagesPerWorker=110+90/10 = 20

• secondsToProcessOneJob:

secondsToProcessOneJob=0.5
queueRPM=300
min=1
minWorkersBasedOnRPM=Ceil(0.5*300/60)=3, so there will be minium 3 workers running based on the RPM.

• maxDisruption:

min=2, max=1000, current=500, maxDisruption=50%: then the scale down cannot bring down more than 250 pods in a single scale down activity.

min=2, max=1000, current=500, maxDisruption=125: then the scale down cannot bring down more than 125 pods in a single scale down activity.

WPA Controller

Run the workerpodautoscaler

Usage:
  workerpodautoscaler run [flags]

Examples:
  workerpodautoscaler run

Flags:
      --aws-regions string                               comma separated aws regions of SQS (default "ap-south-1,ap-southeast-1")
      --beanstalk-long-poll-interval int                 the duration (in seconds) for which the beanstalk receive message call waits for a message to arrive (default 20)
      --beanstalk-short-poll-interval int                the duration (in seconds) after which the next beanstalk api call is made to fetch the queue length (default 20)
  -h, --help                                             help for run
      --k8s-api-burst int                                maximum burst for throttle between requests from clients(wpa) to k8s api (default 10)
      --k8s-api-qps float                                qps indicates the maximum QPS to the k8s api from the clients(wpa). (default 5)
      --kube-config string                               path of the kube config file, if not specified in cluster config is used
      --metrics-port string                              specify where to serve the /metrics and /status endpoint. /metrics serve the prometheus metrics for WPA (default ":8787")
      --namespace string                                 specify the namespace to listen to
      --queue-services string                            comma separated queue services, the WPA will start with (default "sqs,beanstalkd")
      --resync-period int                                maximum sync period for the control loop but the control loop can execute sooner if the wpa status object gets updated. (default 20)
      --scale-down-delay-after-last-scale-activity int   scale down delay after last scale up or down in seconds (default 600)
      --sqs-long-poll-interval int                       the duration (in seconds) for which the sqs receive message call waits for a message to arrive (default 20)
      --sqs-short-poll-interval int                      the duration (in seconds) after which the next sqs api call is made to fetch the queue length (default 20)
      --wpa-default-max-disruption string                it is the default value for the maxDisruption in the WPA spec. This specifies how much percentage of pods can be disrupted in a single scale down acitivity. Can be expressed as integers or as a percentage. (default "100%")
      --wpa-threads int                                  wpa threadiness, number of threads to process wpa resources (default 10)

Global Flags:
  -v, --v Level   number for the log level verbosity

If you need to enable multiple queue support, you can add queues comma separated in --queue-services. For example, if beanstalkd is started and there is no WPA beanstalk resource present, then nothing happens, until a beanstalk WPA resource is created. Queue poller service only operates on the filtered WPA objects.

--queue-services=sqs,beanstalkd

Troubleshoot (running WPA at scale)

Running WPA at scale require changes in --k8s-api-burst and --k8s-api-qps flags.

WPA makes call to the Kubernetes API to update the WPA resource status. client-go is used as the kubernetes client to make the Kubernetes API calls. This client allows 5QPS and 10Burst requests to Kubernetes API by default. The defaults can be changed by using k8s-api-burst and k8s-api-qps flags.

You may need to increase the --k8s-api-qps and k8s-api-burst if wpa_controller_loop_duration_seconds is greater than 200ms (wpa_controller_loop_duration_seconds>0.200)

For ~800 WPA resources, 100 QPS keeps the wpa_controller_loop_duration_seconds<0.200

WPA Metrics

WPA emits the following prometheus metrics at :8787/metrics.

wpa_controller_loop_count_success{workerpodautoscaler="example-wpa", namespace="example-namespace"} 23140
wpa_controller_loop_duration_seconds{workerpodautoscaler="example-wpa", namespace="example-namespace"} 0.39

wpa_log_messages_total{severity="ERROR"} 0
wpa_log_messages_total{severity="WARNING"} 0

wpa_queue_messages{workerpodautoscaler="example-wpa", namespace="example-namespace", queueName="example-q"} 87
wpa_queue_messages_sent_per_minute{workerpodautoscaler="example-wpa", namespace="example-namespace", queueName="example-q"} 2007

wpa_worker_current{workerpodautoscaler="example-wpa", namespace="example-namespace", queueName="example-q"} 27
wpa_worker_desired{workerpodautoscaler="example-wpa", namespace="example-namespace", queueName="example-q"} 5
wpa_worker_idle{workerpodautoscaler="example-wpa", namespace="example-namespace", queueName="example-q"} 0

go_goroutines{endpoint="workerpodautoscaler-metrics"} 40

Using these metrics, scaling trends can be better analysed, comparing the Replicas Vs Queue:

If you have ServiceMonitor installed in your cluster. You can bring these metrics to Prometheus by running the following:

kubectl create -f artifacts/service.yaml
kubctl create -f artifacts/servicemonitor.yaml

Why make a separate autoscaler CRD ?

Go through this medium post for details.

Kubernetes does support custom metric scaling using Horizontal Pod Autoscaler. Before making this we were using HPA to scale our worker pods. Below are the reasons for moving away from HPA and making a custom resource:

TLDR; Don't want to write and maintain custom metric exporters? Use WPA to quickly start scaling your pods based on queue length with minimum effort (few kubectl commands and you are done !)

1. No need to write and maintain custom metric exporters: In case of HPA with custom metrics, the users need to write and maintain the custom metric exporters. This makes sense for HPA to support all kinds of use cases. WPA comes with queue metric exporters(pollers) integrated and the whole setup can start working with 2 kubectl commands.

2. Different Metrics for Scaling Up and Down: Scaling up and down metric can be different based on the use case. For example in our case we want to scale up based on SQS ApproximateNumberOfMessages length and scale down based on NumberOfMessagesReceived. This is because if the worker jobs watching the queue is consuming the queue very fast, ApproximateNumberOfMessages would always be zero and you don't want to scale down to 0 in such cases.

3. Fast Scaling: We wanted to achieve super fast near real time scaling. As soon as a job comes in queue the containers should scale if needed. The concurrency, speed and interval of sync have been made configurable to keep the API calls to minimum.

4. On-demand Workers: min=0 is supported. It's also supported in HPA.

Release

• Decide a tag and bump up the tag here and create and merge the pull request.

• Get the latest master code.

git clone https://github.com/practo/k8s-worker-pod-autoscaler
cd k8s-worker-pod-autoscaler
git pull origin master

• Build and push the image to public.ecr.aws/practo. Note: ECR push access is required or use a custom registry by adding REGISTRY=public.ecr.aws/exampleorg make push

git fetch --tags
git tag v1.6.0
make push

Note: For every tag major and major minor versions tags also available. For example: v1 and v1.6

• Create a Release in Github. Refer this and create a release. Release should contain the Changelog information of all the issues and pull request after the last release.

• Publish the release in Github 🎉

• For first time deployment use this.

• For future deployments. Edit the image in deployment with the new tag.

kubectl edit deployment -n kube-system workerpodautoscaler

Contributing

It would be really helpful to add all the major message queuing service providers. This interface implementation needs to be written down to make that possible.

• After making code changes, run the below commands to build and run locally.

$ make build
making bin/darwin_amd64/workerpodautoscaler

$ bin/darwin_amd64/workerpodautoscaler run --kube-config /home/user/.kube/config

• Generate CRD generated code at pkg/apis and pkg/generated using:

make generate

• To add a new dependency use go mod vendor
• Dependency management using go modules - https://github.com/liggitt/gomodules/blob/master/README.md
• Get up to speed with go in no time - https://gobyexample.com
• Install go locally - https://golang.org/doc/install
• How to write go code - https://golang.org/doc/code.html

Thanks

Thanks to kubernetes team for making crds and sample controller. Thanks for go-build-template.