What microservices can be horizontally scaled?

Any microservice can be horizontally scaled if it is designed following these principles:

• Statelessness: The microservice should be stateless, or its state should be stored in an external storage (database, cache, message queue). This allows running multiple independent instances of the service.
• Idempotency: The microservice operations should be idempotent so that repeated execution does not lead to unwanted side effects. This is important when handling requests with multiple service instances.
• Independence: The microservice should be independent of the specific location or identifier of the instance.
• Batch processing/asynchrony: It is advisable to use asynchronous or batch processing to reduce load on individual instances and to distribute work more efficiently.
• Transparency: The microservice should be easily discoverable and monitorable so that the scaling system can manage its instances correctly.

Examples of microservices that are often horizontally scaled:

• API Gateway: Handles incoming requests and routes them to the appropriate services.
• Data processing services: Perform calculations or data transformations.
• User services: For example, authentication or user profile services.
• Message queue services: Allow asynchronous task processing.

Restrictions on horizontal scaling may arise if the microservice has tight dependencies on local state, external systems with limitations on concurrent connections, or is not designed considering distributed load.

# Example configuration for horizontal scaling in Kubernetes
apiVersion: autoscaling/v2
kind: HorizontalPodAutoscaler
metadata:
  name: my-microservice-hpa
spec:
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: my-microservice-deployment
  minReplicas: 2 # Minimum number of instances
  maxReplicas: 10 # Maximum number of instances
  metrics:
    - type: Resource
      resource:
        name: cpu
        target:
          type: Utilization
          averageUtilization: 50 # Scale when CPU utilization reaches 50%