Which pattern should be used to pull information from related objects with outbound messages?

The correct choice is the Callback pattern. This pattern is ideal for pulling information from related objects with outbound messages because it leverages the asynchronous nature of outbound messaging.

In a scenario where Salesforce sends messages to an external system, the Callback pattern facilitates a response that allows the external system to request additional information about related objects. When an event occurs in Salesforce that triggers an outbound message, the external system can process that message and, if needed, make a subsequent call back to Salesforce to retrieve more detailed data that may be located in related objects.

The Callback pattern effectively maintains the flow of information without overwhelming the system with constant polling, ensuring that only relevant data is requested when necessary. This is particularly advantageous in situations where related information is not always immediately available or needs to be queried based on the context provided by the outbound message.

Other patterns are less suited for this specific requirement. Polling would require regular checks for updates, potentially leading to inefficiencies. The Mashup pattern involves embedding and displaying data together from different sources, which is not the focus of pulling additional related information. Streaming focuses on real-time updates instead of structured requests for related data on-demand. Thus, the Callback pattern is the most fitting method for this scenario.