In this architectural scenario, the Integration Architect must navigate two critical technical "bottlenecks": the 9-second API Gateway timeout and the 90-second backend processing time. Since the backend takes significantly longer than the gateway allows for a synchronous connection, a standard Request-Reply pattern will fail. Furthermore, having 3,000 concurrent agents perform synchronous callouts would risk hitting Salesforce's concurrent long-running request limits.

The most scalable and user-friendly solution is to implement an Asynchronous Request-Reply pattern using Platform Events and the empAPI.

When an agent clicks "Check Eligibility," Salesforce sends an initial asynchronous request to the ESB (MuleSoft). The ESB immediately acknowledges receipt with a 202 Accepted status, freeing up the Salesforce UI thread and avoiding the API Gateway’s 9-second timeout. Once the backend eligibility system completes its 90-second process, MuleSoft acts as a client to Salesforce, performing a Remote Call-In to publish a specific Platform Event containing the result and the original Request ID.

On the frontend, the Lightning UI uses the empAPI (Enterprise Messaging Platform API) to subscribe to the streaming channel for that Platform Event. Because the empAPI uses CometD technology to maintain a single long-lived connection, it can efficiently push the response to the agent's screen the moment it arrives, without requiring the agent to manually refresh or click a "Check Update" button (as suggested in Option B). This provides a "real-time" feel despite the long backend latency.

Option A is non-viable because synchronous polling would exacerbate the load on the API Gateway and likely lead to governance limit issues within Salesforce. By using Platform Events and empAPI, the architect ensures the solution remains within Salesforce's execution limits while providing a seamless, automated experience for a high-volume call center environment.