Apa itu Book builder?

A book builder maintains the limit order book (One Quant Book 10, chapter 1) of an instrument from the feed handler’s events: from a market-by-order feed (Book 1, chapter 19) it keeps every resting order and aggregates them into price levels; from a market-by-price feed it keeps the levels directly. It answers level-1 and level-2 queries (Book 1, chapter 28) after every event, and knows when its book cannot be trusted.