Which Salesforce integration method would be best for real-time interactions with an external system?

The recommended integration method for real-time interactions with an external system is the SOAP API. This method allows for immediate request-response communication between Salesforce and the external system, enabling quick data exchanges as soon as actions occur. SOAP API is designed for synchronous communication, meaning it establishes a direct connection that waits for a response after a request is made. This is critical for scenarios where real-time data retrieval or updates are necessary, such as updating inventory levels or user records immediately after a transaction.

Batch processing is typically suited for scenarios where large volumes of data need to be processed at once, rather than requiring immediate interaction. Outbound messaging, while useful for sending notifications to external systems based on Salesforce events, operates asynchronously and does not provide immediate feedback. Asynchronous Apex, similar to outbound messaging, allows for non-blocking operations, making it suitable for background processing tasks rather than real-time interactions.

Thus, the SOAP API stands out as the best choice for real-time integrations, ensuring that operations are both timely and efficient.