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Comment by alalonde | Hacker News Reader

You can currently already do good-enough-for-most DOM-level synchronization using any editor, of which we have a few examples. A lot of the examples of RTC out there (apps using TogetherJS for instance) are doing this already. For rich text, we'd like to be able to provide 100% support for a particular editor's capabilities, which necessitates a deeper, custom data model.

One of our value propositions is a unified backend for dealing with any JSON data (and eventually beyond), avoiding binding server-side code to a particular choice of UI component.

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I am curious about this argument - I think CKEditor 5 made a similar one in this epic blog post about how they implemented real-time collaboration (https://ckeditor.com/blog/Lessons-learned-from-creating-a-ri...). We're using Quill.js with ShareDB, which supports JSON structures (which is great, because for us we often have documents with several rich text fields, and other complex structures). So far we've been able to do anything we wanted with Quill, and I've never felt limited by the data structures we have available... (We also do all kinds of other stuff with ShareDB JSON).

I guess one reason you could need custom types would be to ensure consistency - if two keys depend on each other, and one user sets one key, and the other user sets another key, and the document is now invalid, you'd need the engine to be able to reconcile at a higher level?

> (...) if two keys depend on each other, and one user sets one key, and the other user sets another key, and the document is now invalid, you'd need the engine to be able to reconcile at a higher level?

I am not sure if I understand you correctly here, but it's not really that. Could you give me a more concrete example?

The kind of problems for extra types are, for example: user A changes a paragraph to a list item and user B splits it. As a result you'd like to have two list items instead of a list item followed by a paragraph. This is impossible if you don't give more semantic meaning to the operations.

There are other problems though, as you mentioned - with invalid document. For example, you have this kind of a list:

* Foo

__* Bar

__* Baz

User A outdents "Bar" and user B indents "Baz" creating a list like this:

* Foo

* Bar

____* Baz

In CKE5 this is an incorrect list (we don't allow indent differences bigger than one). This cannot be fixed through OT so we fix it in post-fixers which are fired after all the changes are applied.

Author of the linked blog post here.

The example cases for additional types / custom implementation are in this section: https://ckeditor.com/blog/Lessons-learned-from-creating-a-ri...

These content-preservation edge cases weren't possible to solve with what was available (at least at the time we started the project).

Even apart of that, ottypes/json0 was lacking some basic things, like moving objects. I see they came up with a new implementation recently (https://github.com/ottypes/json1) and it allows moving objects. Maybe the new implementation would solve some problems. However, it is in "preview" state, and the last update was 2 months ago, so I am not sure how well it will be maintained.

Also, there are some edge cases when transforming ranges (which CKE5 use to represent, for example, comments on text or content created in track changes mode). I don't want to bury you in difficult to understand examples but if you are interested you might want to check the examples listed in inline codes for this function: https://github.com/ckeditor/ckeditor5-engine/blob/master/src....

As far as Quill.js is concerned, it is based on the linear data model, which brings limitations when it comes to complex features. Transformation algorithms for linear data models are much simpler and there are more implementations and articles in this area. Everything depends on your needs. If Quill.js features set and functionality fit your needs then the solution you chose is correct.

With CKE5 however, we didn't want to go on any compromises. We needed complex structures for our features, and for having a powerful framework - to enable other developers to write whatever feature they want and have those features working in real-time collaboration. We wanted transformation algorithms which will handle all the edge cases. It is true, some of those cases are quite rare. And the old "10/90" mantra applies here, in this case "10% of use cases brings 90% of complexity". But those edge cases happen and we didn't want to disappoint our users.

I think the argument is more about the historical data structures that were used in rich text. A lot of editors either used the DOM, or a very flattened data structure like Google Wave, Quill.js, DraftJS etc. With these flattened data structures it becomes harder to represent complex rich text with things like tables, nested blocks, etc. If you have a nice JSON data structure that is collaborative you can do a lot, and in many / most use cases it is sufficient. However, you can run into use cases where the collaborative data structure will ensure consistency across clients, but violate some semantic constraint on the data.

For example, imagine you have an application that has a list that must contain at least one element. Assume there are two elements in the list. A Shared JSON data structure on its own (that allows for immediate local edits) would to allow two clients to simultaneously delete one element each. The end result is that the client app on both sides will become aware that the constraint was violated only when the remote operation comes in. Resolving this becomes difficult. What is the resolution strategy? Which of the two clients should initiate it? This is a contrived example for sure. But you run into things like this in various use cases, and occasionally you need either new data structures that encode these semantics, or you need an extendable system that allows you to customize constraints, and resolutions.

That said, you can get pretty far with just JSON!

Could you point us to some of those togetherJS apps? I just recently learned about it in the context of collaborative browsing. How is this related to collaborative text stuff?

If you scroll down a bit there are a few examples, including jsfiddle: https://togetherjs.com/

Essentially, they are doing OT on the DOM.

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