Matching Engine Basics

The matching engine is the system that decides how incoming orders interact with resting orders.

Resting order vs incoming order

When a new order arrives, the engine checks:

1. Is the order valid?
2. Does it cross the current opposite side?
3. If yes, how much executes?
4. If anything remains, should it rest?

Maker and taker

• **maker** = the resting order already on the book
• **taker** = the incoming order that triggers execution

This distinction matters for microstructure analysis and fee models.

FIFO

At one price level, execution priority is usually FIFO:

first in, first out

So if two asks rest at the same price, the older one executes first.

Events vs state

The engine produces two kinds of useful information:

• **state**: the resulting order book
• **events**: accepts, trades, cancellations, top-of-book changes

The best learning workflow is to inspect both.

In this terminal

That is why the terminal includes:

• orderbook views for state
• recent trades for executions
• event log for narrative state changes
• snapshot/debug views for structured inspection