FIFO order book - Numo Docs

The book is a central limit order book with price-time priority. Better prices fill first. At the same price, the order that arrived first fills first.

Sort order

Side	First by	Then by
Bids	Highest limit price	Earliest `created_at`
Asks	Lowest limit price	Earliest `created_at`

Priority is set when the order is created. Nothing you do later moves it up the queue. To change price or size, cancel the order and submit a new one, which joins the back of the queue at its price.

What fills against what

The matcher walks bids from the best down. For each bid, it walks asks from the best up, until the prices no longer cross. It takes the first pair that can settle. A pair is passed over, and the next one tried, when:

the later order is post_only, so it would be the taker. See Order types.
both orders belong to the same subaccount. See Self-trade prevention.
the pair is waiting out a retry after the chain refused it. See When a fill would revert.

Read the book

GET /v1/book returns the current depth.
The websocket stream pushes updates as the book changes.