Which microservices can be scaled horizontally?

Practically any microservice designed according to the principles of statelessness.

Main criteria:

• Stateless (no intermediate state preservation): The service does not store request-specific information between calls. All necessary data is either passed with the request or stored in an external, scalable source (database, cache, message broker).
• Deterministic behavior: With the same input data, the service always produces the same result, regardless of which instance (server) it runs on.
• Independence: The service is not tightly bound to a specific machine or resource that cannot be easily replicated.

Examples of microservices that scale well horizontally:

• Authentication and authorization services.
• HTTP request processing services (API Gateways, backend services).
• Message queue processing services.
• Background task execution services (worker services).
• Report generation services (if data is taken from an external DB).

Stateful microservices are more difficult to scale horizontally and require special approaches such as:

• Partitioning/Sharding: Dividing data among service instances.
• Distributed databases: Using a DBMS that provides scaling and replication.
• Using In-Memory Data Grids: For caching and state processing in a distributed environment.

It is important to understand that even a stateless service requires scalable infrastructure for data storage (database, cache, message broker) and orchestration (Kubernetes, Docker Swarm) for effective horizontal scaling.