# KEDA — Kubernetes Event-Driven Autoscaling

> CNCF-graduated autoscaler that scales Kubernetes workloads to and from zero using 70+ event sources like Kafka, SQS, Prometheus and Redis.

## Install

Save as a script file and run:

# KEDA — Kubernetes Event-Driven Autoscaling

## Quick Use
```bash
# Install via Helm
helm repo add kedacore https://kedacore.github.io/charts
helm upgrade --install keda kedacore/keda -n keda --create-namespace

# Scale a Deployment on Kafka lag
cat <<YAML | kubectl apply -f -
apiVersion: keda.sh/v1alpha1
kind: ScaledObject
metadata:
  name: kafka-scaler
spec:
  scaleTargetRef:
    name: consumer
  minReplicaCount: 0
  maxReplicaCount: 50
  triggers:
  - type: kafka
    metadata:
      bootstrapServers: kafka:9092
      topic: orders
      consumerGroup: workers
      lagThreshold: "100"
YAML
```

## Introduction
KEDA (Kubernetes Event-Driven Autoscaler) is a CNCF graduated project that extends Kubernetes Horizontal Pod Autoscaler with 70+ external event sources — queues, streams, databases, cloud services — and adds the critical ability to scale workloads all the way down to zero when there is no work. It is additive: HPA, VPA, and cluster autoscalers continue to do their jobs.

## What KEDA Does
- Scales any Deployment, StatefulSet, Job, or custom resource based on external metrics.
- Supports 70+ scalers: Kafka, RabbitMQ, SQS, Azure Service Bus, Prometheus, Postgres, Redis, Pulsar, NATS, GCP Pub/Sub, Kubernetes workload, CPU/memory, cron, and more.
- Scales to and from zero — pods are fully terminated until work appears.
- Delivers external metrics through a Kubernetes External Metrics API adapter so existing HPAs can use them.
- Runs ScaledJobs for queue-consuming batch work where each message becomes a Job.

## Architecture Overview
KEDA is two small components: the Operator and the Metrics Server. The Operator watches ScaledObject/ScaledJob CRs and manages HPAs for each target. The Metrics Server implements Kubernetes' external metrics API; HPA reads metrics from KEDA, which in turn polls scalers (Kafka lag, Prometheus queries, etc.). When the metric drops to zero and the cooldown passes, KEDA sets replicas to 0; when a poll returns non-zero, it bumps to the `minReplicaCount` and lets HPA take over.

## Self-Hosting & Configuration
- Official Helm chart covers CRDs, RBAC, and the two pods.
- Use `TriggerAuthentication` and `ClusterTriggerAuthentication` for secrets — never inline them.
- `pollingInterval`, `cooldownPeriod`, and `idleReplicaCount` are the three knobs that matter most.
- For private Kafka/RabbitMQ, pair with `SecretTargetRef` so credentials stay in cluster secrets.
- KEDA HTTP Add-On (separate install) lets you scale to zero HTTP services with a traffic interceptor.

## Key Features
- Scale-to-zero for deployment workloads, not just Jobs.
- 70+ first-class scalers plus a generic Prometheus/Metrics API scaler.
- CNCF graduated — production proven at Microsoft, Alibaba, Shopify, and others.
- Non-invasive: does not replace HPA, it produces metrics for it.
- ScaledJobs enable one-pod-per-message batch processing.

## Comparison with Similar Tools
- **Kubernetes HPA** — CPU/memory only unless extended; KEDA supplies the extension.
- **KNative Serving** — scales HTTP to zero, but is a full serverless stack.
- **OpenFaaS** — function-platform with built-in scaling; KEDA is workload-agnostic.
- **Karpenter** — node autoscaler, complementary to KEDA (which scales pods).
- **Prometheus Adapter** — exposes Prometheus metrics to HPA; KEDA covers that plus many other sources.

## FAQ
**Q:** Does KEDA replace HPA?
A: No. KEDA creates the HPA for you and feeds it external metrics.

**Q:** Can it scale StatefulSets?
A: Yes, and custom resources that implement the scale subresource.

**Q:** How does scale-to-zero handle in-flight work?
A: KEDA waits for the metric to stay at zero for `cooldownPeriod` before scaling down.

**Q:** Is there a way to scale HTTP services to zero?
A: Yes, the KEDA HTTP Add-On adds an interceptor that wakes pods on request.

## Sources
- https://github.com/kedacore/keda
- https://keda.sh/docs/latest/


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Source: https://tokrepo.com/en/workflows/298b2541-38d7-11f1-9bc6-00163e2b0d79
Author: Script Depot