The shape
A load balancer accepts traffic and chooses a healthy service instance. The replicas can remain stateless, which makes replacement and horizontal scaling straightforward.
The useful question
How does the balancer know an instance is useful? A process can answer a health check while its database pool is exhausted. Check the dependency and the user visible path, but avoid making every health probe an expensive request.
Stress test
Increase traffic, then slow one replica. Compare round robin with least connections. A slow instance can hold work longer, so equal request counts do not always mean equal load.
Tradeoff
More replicas add capacity only when the constrained dependency can serve them. Measure the full request path before adding instances.