Orders Can Arrive Between Executions
So far, large orders have been shown executing in uninterrupted sequences.
This can suggest that execution happens “all at once.”
In practice, execution proceeds step by step, and the book may change between those steps.
New orders can arrive between executions.
When they do, execution resumes using the updated book, without changing any execution rules.
This lesson demonstrates that behavior explicitly.
Current State
At this moment, the book contains a single ask price.
No trade occurs yet because the best bid $16 does not overlap the ask $22
Stage 1. A Buy Order Arrives
Event: At a buyer submits an order to buy 6 apples at $25
The buy price overlaps the ask.
Execution can proceed.
Stage 2. First Execution Occurs
Execution 1
2 apples are exchanged at $22
- A - the ask @ $22 is fully removed.
- B - the bid @ $25 amount is reduced from 6 to 4 apples.
- C - a trade is recorded at
The buy order still has 4 apples remaining.
No additional execution occurs at this point. Because the asks ledger is empty, a new ask order must be recorded before any further execution can occur.
Stage 3. A New Order Arrives Between Executions
Before the next execution step occurs, the book changes.
Event: At a seller submits an order to sell 4 apples at $18
This order is now the best available ask.
No execution occurs at the moment of arrival.
The book is simply updated.
Stage 4. Execution Resumes Using the Updated Book
Execution resumes using the current book state.
Execution 2
4 apples are exchanged at $18
- D - the ask @ $18 is fully removed.
- E - the bid @ $25 is fully executed and removed.
- F - a trade is recorded at
The buy order is now fully executed.
Execution Summary
During this sequence:
- execution occurred in multiple steps
- the book changed between executions
- a newly arrived order altered the next execution price
No execution rules were changed.
Each execution step simply used the current contents of the book at the moment it occurred.
Conclusion
Execution does not occur “all at once.”
It proceeds step by step.
Between those steps, the book may change.
When new orders arrive between executions:
- they are recorded immediately
- they may alter which prices are executed next
- execution resumes using the updated book
This behavior requires no special logic.
It follows directly from the same price-first, time-second rules already established.
From this point forward, execution should be understood as an interleaving of events not as a single continuous action.