At first glance, this sounds like one of those things that should be simple. Feed comes in, book gets built, done. Not quite. In practice, reconstructing an order book from incremental depth updates is a bit like trying to keep a chessboard accurate while someone keeps moving pieces across the table every second. You need the snapshot, you need the updates, and you need a way to keep everything in the right order. Miss one step, and the whole thing starts to look off.

What the stream is really saying

An incremental depth update does not hand you the full picture. It gives you changes. A bid gets bigger, an ask disappears, a new level shows up, and suddenly the spread tightens or widens a touch. That is the whole game. The message tells you what changed at a specific price level, not the entire book.

That is why the order book has to be rebuilt locally. You are not just reading the market. You are maintaining a live copy of it. A proper stock market data API usually sends a full snapshot first, then follows up with incremental depth update messages so you can keep the local book in sync.

Start with the snapshot, always

This part matters more than people expect. A snapshot is your anchor. Without it, the updates have nowhere to land.

Once you have the snapshot, each new message has to be applied in sequence. That is where the sequence number earns its keep. If the next message does not follow the last one cleanly, something is wrong. Maybe a packet was missed. Maybe the connection hiccupped. Either way, the book is now suspect, and the safest move is to reload the snapshot rather than pretend nothing happened.

That may sound fussy. It is. But market data is not a place for guesswork.

How the book gets updated

The logic is straightforward, even if the feed itself is a little fussy.

If an update changes a size at an existing price level, overwrite the old size.

If an update removes a level, delete it.

If an update adds a new level, insert it in the correct place.

Then recalculate the top of book. That gives you the current best bid and best ask, which also tells you the bid ask spread. If the spread suddenly looks strange, that is usually your first clue that something went wrong upstream.

In other words, the order book is not a static table. It is a living structure. Every update nudges it a little. Every missing message can throw it out of whack. A little annoying, yes. Also unavoidable.

The part people underestimate

The tricky bit is not the math. It is the housekeeping.

Market data does not always arrive in a neat little line. Sometimes messages come in bursts. Sometimes one update lands late. Sometimes a reconnect happens and the stream picks up again, but not exactly where you thought it would. That is why sequence checking is so important. A local book is only useful when it is trustworthy.

This is also why you should never try to patch over a gap by guessing what might have happened. That road leads straight into bad data, and bad data has a nasty habit of looking fine right until it ruins something important.

What a decent API needs to provide

For this kind of workflow, a stock market data API has to do more than just hand over prices.

It should give you real-time depth data, clear update behavior, low latency, and documentation that actually explains how snapshots and incremental messages fit together. If the feed is vague about sequence handling or depth format, you will spend half your time guessing and the other half cleaning up after the guesses.

That is why the delivery model matters so much. You need to know whether the API gives you snapshots, deltas, or both. You need to know how often updates arrive. You need to know what happens when a sequence number jumps. Small details, big consequences.

Why AllTick fits this use case well

For stock market workflows, AllTick is a very practical choice.

It provides real-time stock market data, tick-by-tick coverage, and order book access across major markets including US stocks, Hong Kong, and China A-shares. The WebSocket feed is built for low-latency streaming, which is exactly what you want when you are maintaining a local order book and watching the spread move in real time.

Another useful point is that AllTick’s order book subscription is designed for live market depth. It is aimed at the here and now, which makes sense for reconstruction work. You are not trying to study some dusty historical ladder from last Tuesday. You are trying to keep a live book alive, moment by moment, level by level.

That is where AllTick makes life easier. It gives you the kind of feed that actually matches the job.

A sensible way to think about the whole process

Load the snapshot. Apply every incremental depth update in order. Watch the sequence number like a hawk. Rebuild the bid side and ask side as the messages arrive. Check the bid ask spread often enough to catch drift before it becomes a headache.

That is really the core of it. Nothing glamorous. Nothing magical. Just a careful chain of updates, applied in the right order, with enough discipline to throw the book away and rebuild it when continuity breaks.

And honestly, that is probably the right mindset. Market data rewards patience more than bravado.

Final takeaway

Reconstructing an order book from incremental depth updates is less about “receiving data” and more about preserving state. The snapshot gives you the starting point. The sequence number keeps you honest. The price level updates keep the local book aligned with the market. The spread tells you whether the result still makes sense.

If you are building this for live stocks, AllTick is a solid recommendation because it combines real-time stock coverage, tick-by-tick delivery, and order book support in a way that fits this exact workflow. It is not flashy. It just does the job, which, in market data, is usually what matters most.