The matching engine is also called the cross engine. Here are the primary responsibilities of the matching engine:

Maintain the order book for each symbol. An order book is a list of buy and sell orders for a symbol. We explain the construction of an order book in the Data models section later.

Match buy and sell orders. A match results in two executions (fills), with one each for the buy and sell sides. The matching function must be fast and accurate.

Distribute the execution stream as market data.

A highly available matching engine implementation must be able to produce matches in a deterministic order. That is, given a known sequence of orders as an input, the matching engine must produce the same sequence of executions (fills) as an output when the sequence is replayed. This determinism is a foundation of high availability which we will discuss at length in the deep dive section.