Whiteboard: snapshots lose edits, per-property ops don't
With snapshots the shape is teal but back at x=10: Ravi's upload never contained Asha's move. With per-property operations both changes survive. When both change the colour, the server's order picks one and every screen agrees. The dots are presence cursors: drawn, never saved.
In a collaborative editor several people change one document at once, so clients send small operations instead of whole snapshots, and the system needs a rule (server ordering with OT, or CRDTs) that makes every screen end up the same, plus presence cursors, per-user undo and continuous autosave.
Change the code and press Run (Ctrl+Enter). Predict what will happen first, then break it on purpose and read what changes. Your edits are saved and match the lesson page.
Ready for more? This phase has 5 hands-on exercises with automatic checks.
Practice questions
Write the code in the editor, run it, then open the model answer to compare.
Write transformInsertDelete(insert, del) for a delete { pos, length } that happened at the same time as an insert.
Add a third user to the whiteboard example who changes y, and show that all three changes survive with operations.
Explain it without notes
Why do collaborative apps send operations instead of snapshots?
Explain OT and CRDTs in simple terms and compare them.
How would you design conflict handling for a whiteboard?
How do presence cursors work, and why are they separate from the document?
How do undo and autosave work in a collaborative editor?
Preview
Console
Nothing logged yet.Expected console output
snapshots: {"s1":{"x":10,"color":"teal"}} (Asha's move is lost)
operations: {"s1":{"x":120,"color":"teal"}}
same property: Asha sees teal, Ravi sees teal