Which of the following is NOT a core component of the Market Data Distribution System (MDDS)?

The correct answer highlights that a database is not considered a core component of the Market Data Distribution System (MDDS). The MDDS primarily focuses on the distribution and handling of market data, and while databases are essential for storing and managing data, they are not intrinsic to the operational aspects of data distribution in the way that other components are.

The cache plays a crucial role in temporarily storing market data to facilitate quick access and minimize latency for users who require real-time information. The feed handler is vital for managing how data is ingested from various sources and then formatted for distribution, effectively bridging the source data with the system's output. A WAN gateway is essential for enabling communication over wide area networks, which is critical for propagating market data to various locations efficiently.

In this context, the database, while it might be utilized alongside the MDDS, does not directly pertain to the functionality of distributing and managing real-time market data. This distinction makes it clear why the database is not classified as a core component of the MDDS.