What is the single-threaded process shared across the server cluster that accepts requests from VizQL?

The correct answer is the Cache Server. The Cache Server functions as a single-threaded process within Tableau Server that is specifically designed to handle requests from VizQL, which is Tableau's query language used for rendering visualizations.

When a user interacts with a Tableau dashboard, VizQL sends requests for data and visual elements. The Cache Server plays a critical role in optimizing response times and performance by storing frequently accessed data in memory. This allows for faster retrieval and reduces the load on the primary data sources.

In a distributed server environment, the Cache Server ensures that multiple VizQL requests can be processed efficiently, facilitating a smooth interactive experience for end users. The architecture is such that the Cache Server improves scalability and responsiveness by leveraging cached data rather than having to query the underlying databases for every request.

The other components, while integral to Tableau Server's functionality, do not specifically perform this role in relation to VizQL requests. For example, the Metrics Service is focused on collecting usage metrics, the File Store manages file storage and data extracts, and the Backgrounder handles tasks like data extract refreshes and subscription deliveries. None of these fulfill the specific function of managing requests from VizQL like the Cache Server does.