Keda ScaledObject As Code Using CDK8S

Abstract

keda is Kubernetes Event-driven Autoscaling and it's been used wisely now. In this blog, it provides the way to create Keda scaledobject CRD as code using CDK8S typescript.
With importing Keda CRDs and using CDK8S you can create Keda scaledobjects using your familiar programming languages such as typescript as scale.

Pre-requisite
Overview of Keda
Import Keda CRDs
Write code
Build keda scaledobjects from code
Apply and test
Conclusion

🚀 Pre-requisite

Install typescript, node, and cdk8s as well as projen (optional) which is a tool for managing project configuration as code.
Getting started with cdk8s

🚀 Overview of Keda

KEDA works alongside standard Kubernetes components like the Horizontal Pod Autoscaler and can extend functionality without overwriting or duplication.
KEDA supports multiple triggers within an Scaledobject. Each trigger is exposed split as a metric and the HPA Controller does a MAX between all the metrics.

🚀 Import Keda CRDs

Keda does not provide its CRDs separately so we can find the manifest in GH release section. Here I import the current latest version of keda v2.8.0 and output the imports folder in src/imports

  ⚡ $ cdk8s import https://github.com/kedacore/keda/releases/download/v2.8.0/keda-2.8.0.yaml --output src/imports/
  ------------------------------------------------------------------------------------------------
  A new version 2.0.88 of cdk8s-cli is available (current 2.0.13).
  Run "npm install -g cdk8s-cli" to install the latest version on your system.
  For additional installation methods, see https://cdk8s.io/docs/latest/getting-started
  ------------------------------------------------------------------------------------------------
  Importing resources, this may take a few moments...
  keda.sh
    keda.sh/clustertriggerauthentication
    keda.sh/scaledjob
    keda.sh/scaledobject
    keda.sh/triggerauthentication

Import result

  ⚡ $ tree src/imports/
  src/imports/
  └── keda.sh.ts

  0 directories, 1 file

🚀 Write code

Overview of keda scaledObjects in this post
It's much more convenient to use visual code writing KEDA scaledobject in typescript language. We can read the document and find all references of construct, objects and properties of KEDA CRDs
This blog provides the use case of creating scaledObject (SO) for Apache airflow worker component. It contains 3 triggers (Scalers) in the SO

1. Cron - Scale applications based on a cron schedule.

Airflow server scheduled pipelines and worker components are scaled out at that time, but it takes time to start nodes, join node to cluster and Pod ready (about 2-3mins) so we use cron to pre-scale workers

2. PostgreSQL - Scale applications based on a PostgreSQL query.

This scaler based on the output of the query command to scaleout works, here we count the number of running/queued airflow task instances belonging to the scheduled pipeline (eg. running reports). Divide the count number by Airflow worker concurrency (eg. 16 as default)
targetQueryValue: '1.5' - The result of above calculation divide to this target value to decide how many pods will be scaled out

3. CPU - Scale applications based on cpu metrics

This is optional as it ensures provisioning workers when CPU Utilization is higher than 80%

PostgreSQL Scaler requires TriggerAuthentication to provide a password of airflow user in order to query the database. The credential is get from K8S secret airflow-secret within the airflow namespace

    const pgAuth = new TriggerAuthentication(this, 'KedaPostgresAuthentication', {
      metadata: {
        name: 'keda-airflow-postgresql-auth',
        namespace: 'airflow',
      },
      spec: {
        secretTargetRef: [{
          parameter: 'password',
          name: 'airflow-secret',
          key: 'postgresql-password',
        }],
      },
    });

Some SO specs need to know
- pollingInterval: This is the interval to check each trigger on. By default KEDA will check each trigger source on every ScaledObject every 30 seconds. So to reduce the query connections/workload to airflow database we need to care about this value.
- cooldownPeriod: The period to wait after the last trigger is reported active before scaling the resource back to 0.

🚀 Build keda scaledobjects from code

Source code:

  ⚡ $ tree src/
  src/
  ├── imports
  │   └── keda.sh.ts
  ├── keda-airflow.ts
  └── main.ts

  1 directory, 3 files

Build resource

  ⚡ $ npx projen build
  👾 build » default | ts-node --project tsconfig.dev.json .projenrc.ts
  👾 build » compile | tsc --build
  👾 build » post-compile » synth | cdk8s synth
  No manifests synthesized
  👾 build » test | jest --passWithNoTests --all --updateSnapshot
  No tests found, exiting with code 0
  ----------|---------|----------|---------|---------|-------------------
  File      | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s
  ----------|---------|----------|---------|---------|-------------------
  All files |       0 |        0 |       0 |       0 |
  ----------|---------|----------|---------|---------|-------------------
  👾 build » test » eslint | eslint --ext .ts,.tsx --fix --no-error-on-unmatched-pattern src test build-tools projenrc .projenrc.ts

Manifest yaml file

  ⚡ $ tree dist/
  dist/
  └── keda
      └── airflow-keda-so.yaml

  1 directory, 1 file

🚀 Apply and test

Apply manifest and check the result

  # k apply -f dist/keda/airflow-keda-so.yaml

  # k get so -n airflow
  NAME               SCALETARGETKIND       SCALETARGETNAME   MIN   MAX   TRIGGERS   AUTHENTICATION                 READY   ACTIVE   FALLBACK   AGE
  airflow-worker-1   apps/v1.StatefulSet   airflow-worker    2     12    cron       keda-airflow-postgresql-auth   True    True     False      2d21h

  # k get hpa -n airflow
  NAME                        REFERENCE                    TARGETS                                  MINPODS   MAXPODS   REPLICAS   AGE
  keda-hpa-airflow-worker-1   StatefulSet/airflow-worker   500m/1 (avg), 500m/1 (avg) + 2 more...   2         12        2          2d21h

🚀 Conclusion

Within the Scaledobject class, you can jump to the definition to understand the meaning of each property and also know which required/optional attributes.
We can create a custom construct and based on that provision multiple KEDA scaledobjects with customised specs/meta such as min/max/desired replicas, triggers, trigger authentication, etc.

Vu Dao