godzilla.dev - AI Quant Trader Series - Day 10 - What is an Order Book?

source: https://godzilla.dev/learning/ai_quant_traders_series_10/

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What is an Order Book?¶
Reading time: ~15 minutes
Prerequisites: What is High Frequency Trading, What is Market Microstructure
Focus: understanding the core data structure behind every electronic exchange

Part 1: Introduction¶
Every electronic exchange has one central component.

The Order Book.

Whether you are trading:

Stocks
Futures
Options
ETFs
Cryptocurrencies
every trade begins and ends with the order book.

For quantitative developers, the order book is more than market data.

It is the primary data structure that determines:

Liquidity
Price formation
Execution priority
Market depth
Trading opportunities
Without understanding the order book, it is impossible to understand modern electronic markets.

Part 2: What is an Order Book?¶
An order book is a real-time collection of all active buy and sell orders submitted to an exchange.

It continuously records:

Buy orders (Bids)
Sell orders (Asks)
Prices
Quantities
As orders arrive, are canceled, or are executed, the order book updates immediately.

Unlike historical price charts, the order book represents the current state of market supply and demand.

Part 3: A Simple Order Book¶
A simplified order book might look like this:

        ASK

Price Size

101.30 25

101.20 40

101.10 15

101.00

100.90 18

100.80 35

100.70 12

Price Size

        BID

The highest buying price is called the Best Bid.

The lowest selling price is called the Best Ask.

Together they define the current market.

Part 4: Bid, Ask and Spread¶
Suppose the market looks like:

Best Ask = 101.10

Best Bid = 100.90
The difference is:

Spread = 0.20
The bid-ask spread represents the immediate cost of trading.

Smaller spreads usually indicate:

Higher liquidity
Lower transaction costs
More active markets
Large spreads often signal uncertainty or low liquidity.

Many quantitative strategies continuously monitor spread changes.

Part 5: Order Book Updates¶
The order book changes whenever one of three events occurs.

New Order¶
A participant submits a new buy or sell order.

BUY

100.95

Size 30
The order is inserted into the appropriate price level.

Cancel Order¶
An existing order is removed.

Liquidity decreases.

The market depth changes.

Trade Execution¶
A buy order matches a sell order.

Both orders disappear (fully or partially).

The traded price becomes the latest transaction price.

These three event types generate almost every message published by an electronic exchange.

Part 6: Market Orders vs Limit Orders¶
The order book primarily stores limit orders.

Limit Order¶
A trader specifies:

Price
Quantity
Example:

Buy

10 BTC

at

100,000 USD
The order waits until a seller accepts that price.

Market Order¶
A market order specifies only quantity.

The exchange immediately executes against the best available prices in the order book.

For example:

Market Buy

20 BTC
The exchange consumes liquidity from multiple ask levels until the requested quantity is filled.

Part 7: Market Depth¶
An order book contains more than just the best bid and ask.

It also reveals market depth.

For example:

Ask

101.10 10

101.20 25

101.30 80

101.40 200
Large resting orders often influence market behavior.

Some quantitative strategies analyze:

Depth imbalance
Queue size
Liquidity concentration
to predict short-term price movement.

Part 8: Price-Time Priority¶
Most exchanges use Price-Time Priority.

This means:

Higher prices execute first.

If multiple orders exist at the same price,

the earliest order executes first.

Example:

Trader A

Buy

100.00

09:30:01

Trader B

Buy

100.00

09:30:05
Trader A receives execution before Trader B.

Queue position therefore becomes an important competitive advantage in High Frequency Trading.

Part 9: Why the Order Book Matters¶
Traditional investors mostly observe:

Daily candles
Moving averages
Volume
Quantitative traders often observe:

Best Bid
Best Ask
Queue imbalance
Order flow
Market depth
Trade aggressiveness
These microstructure signals often contain far more information than historical prices alone.

Many HFT strategies never use traditional technical indicators.

Instead, they react directly to order book events.

Part 10: Local Order Books¶
Professional trading systems rarely query the exchange every time they need market information.

Instead, they maintain a local order book.

The process is straightforward:

Exchange

↓

Market Data Feed

↓

Incremental Updates

↓

Local Order Book

↓

Trading Strategy
Maintaining a synchronized local order book dramatically reduces latency and enables strategies to process market events without additional network requests.

Almost every production HFT platform relies on this architecture.

Part 11: Where godzilla.dev Fits¶
Maintaining an accurate local order book is one of the most performance-critical components of a trading system.

A production implementation must:

Decode market data
Process millions of updates
Maintain price levels
Handle incremental messages
Synchronize state
Minimize latency
godzilla.dev provides the infrastructure required to build ultra-low latency trading systems capable of processing order book updates efficiently while exposing a clean interface for quantitative strategy development.

Instead of rebuilding market data infrastructure, developers can focus on designing trading strategies.

Part 12: Key Takeaways¶
The order book is the central data structure of every electronic exchange.

It continuously records:

Buy orders
Sell orders
Available liquidity
Market depth
Execution priority
Understanding the order book is essential for:

High Frequency Trading
Market Making
Execution Algorithms
Statistical Arbitrage
Every market event ultimately becomes an order book update.

What's Next?¶
The next article explores the engine responsible for processing every order submitted to the market:

What is a Matching Engine?