Real-Time Collaboration Issues and Solutions | Hacker News Reader

Real-Time Collaboration Issues and Solutions

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24 pointsivan_burazin6 comments

Operational transformation very useful, but it has some drawbacks if you want to do more complicated things with text. Most OT implementations can only get one step out of sync with the server. Client complexity goes way up if you want to do more than that. Also, transformations can only be simple inserts and deletes. These two limitations combine to make it difficult or impossible to search-and-replace, re-indent, or sort lines alphabetically. To do arbitrary transformations of text (or anything) collaboratively, you need different algorithms. Neil Fraser has written extensively on this topic, and suggests an alternative sync strategy.[1]

Disclosure: I'm one of the founders of Floobits[2]. Since native text editors let you do all kinds of crazy stuff with text, we use an algorithm similar to the one described by Neil Fraser.

1. https://neil.fraser.name/writing/sync/

2. https://floobits.com/

I have never heard of an OT implementation with the limitation you state (only being able to keep a single difference), as OT is designed to make that easy to solve: the client can take each new operation it is making and append it to a "difference" buffer, and then transform the difference buffer (which moves contents in its workin copy) by the results from the server when the server acks it's transform. It then can immediately flush this buffer to the server as a new synchronization request. This reuses the exact same logic as any other transform, so it adds almost no code to the client.

As for the limitation that OT only supports inserts and deletes: OT supports anything that can be transformed. If you build an operation "indent area" and then provide a way to partially order that operation against all the other operations (such as "range operation, sort by first position, reverse second, and break ties by operation") you can transform it by moving the position boundaries for inserts and deletes, and when it applies and itself creates a ton of spaces in its interior, that is how it transforms the rest of the operations in the queue. Wave had a number of interesting transforms designed to help it edit structured documents.

Can you explain in more detail how OT limits search & replace for example? I dont see a problem because in our implemetation, text replacement is just anouther insert/delete operation... Or am I missing something?

This is a really nice explanation, thanks for taking the time to post it. We've got our own real time collaborative editor going with code-mirror at https://www.overleaf.com (formerly writeLaTeX), and have experienced many of the same issues!

What are you next plans for it? Be good to have a chat sometime in the new year if you're interested :)

FYI, the image on the front-page of overleaf.com still has the writeLATEX logo in it.

Thanks - yes, we've still got a few things to finish up on the relaunch (we got the technical stuff out of the way first, now have things like that screenshot to go through and update!)

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