The redesign replaced the monolithic handler with a three-layer event-driven pipeline on Azure serverless infrastructure.

Ingestion Layer — Entry point, Service Bus topic, subscription routing

Ingestion: A lightweight Azure Function accepts webhooks, normalizes payloads into a generic envelope, and pushes to a Service Bus topic. Response time: milliseconds. The URL convention /incoming/{vendor}/{servicetype}/{*requesttype} made vendor onboarding a config change, not an infrastructure change.
Routing: The Service Bus topic fans out via subscriptions — SMS to SMS processors, email to email, voice to voice — each with independent retry policies and dead-letter queues.

Processing Layer — Processor microservices, actions, storage backends

Processing: Vendor-scoped processor functions own the full event lifecycle — routing, business logic, and storage writes — with Azure SQL for transactional data, Cosmos DB for audit trails, and Azure Storage for overflow.
Result
Callback latency dropped from 90–120 seconds to sub-second. Inbound volume fell from ~75M to expected range — confirming the majority of prior load was vendor retries. Customer-reported delays resolved completely.

Architectural debt compounds under load. The handler worked at lower volumes. Recognizing a boundary problem vs. a code problem determined the right investment.

Fast acknowledgment is a force multiplier. Decoupling receipt from processing eliminated the retry cascade and cut total system load by ~65%.

Isolation enables independent scaling. Per-channel processors turned capacity planning from an all-or-nothing upgrade into a targeted decision.