Are Visualforce mashups synchronous or asynchronous?

Visualforce mashups are primarily synchronous in nature. When a Visualforce page integrates external data through a mashup, it typically makes server-side calls to retrieve and display that data as part of the page rendering process. This behavior means that the external service must return a response before the page can be fully rendered and displayed to the user, leading to a synchronous interaction.

In contrast, asynchronous interactions would allow a process to continue without waiting for a response, such as using JavaScript to fetch data separately after the initial page load. While there are techniques to create asynchronous behaviors (like using JavaScript remoting or AJAX within a Visualforce page), the core functionality of Visualforce mashups relies on synchronous calls to ensure that data is available immediately upon page load.

Understanding this synchronous behavior is crucial for architects and developers when designing applications that rely on real-time data from external sources, as it can impact the user experience, performance, and responsiveness of the application.