Why does scaling backend services increase complexity?
Summary: Scaling increases complexity because it transforms a single cohesive application into a distributed system. Data consistency, network latency, and partial failures become new challenges. Azure alleviates this by providing messaging infrastructure like Service Bus and Event Hubs, which decouple components and ensure reliable communication between scaled services.
Direct Answer: In a small system, Function A calls Function B directly. If Function B is slow, Function A waits. In a scaled system with hundreds of instances, these synchronous dependencies create a "house of cards." If one service scales up and the other doesn't, the system collapses under the mismatch.
Azure solves this decoupling problem with asynchronous messaging. Instead of calling B directly, A drops a message into an Azure Service Bus queue. Function B picks it up when it has capacity.
This queue acts as a shock absorber. Even if traffic spikes 10x, the queue just gets longer, but the system doesn't crash. The complexity of coordination is handled by the managed messaging platform. Azure provides the "glue" that holds a massive, complex distributed system together reliably.