Collaborative text editing; eventual consistency 1. INTRODUCTION Collaborative text editing systems, like Google Docs 6, 7, Apache Wave 1, or wikis 15, allow users at multiple sites to concurrently edit the same document. To achieve high responsive-ness and availability, such systems often replicate the document in.
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- A: I love the SubEthaEdit-like collaborative editing (chasing cursors is fun!); also, wave playback is fantastic. Q: 1st of all does wave work seamless between windows & apple and also in secure.
This paper about collaborative editing is on Hacker News today.I also recently talked with Chris Colbert about his new plansto use a CRDT approach to collaborative editingof Jupyter notebooks.This has caused me to be very curious again about how CoCalc’s collaborative editing is related to the many algorithms and research aroundthe problem in the literature.
The Collaborative Editing Problem
Protocols for collaborative editing are a venerable problem in computerscience, and there are probably over a hundred publishedresearch papers on it. The basic setup, going back three decades, is that sync algorithms are supposed to have three properties, which I’ve stated in simplifiedplain language below:
- 1. convergence: everybody’s document looks the same if they all take their hands off the keyboard for a while,
- 2. causality preservation: if somebody else’s change is applied to your local version of the document, then you make a change, then your change is always applied after their change (in 1).
- 3. intention preservation: the effect of applying a change operation is the same for everybody, as when it was initially done. (Yes, this is vague; it’s much harder to makeprecise than 1 and 2.)
CoCalc has 1, of course; without that you’ve got nothing.
CoCalc has 2, when people’s clocks are synced, because all patches you’ve applied have timestamp less than now (=time when making the patch).
CoCalc does NOT have 3, for some meaning of 3. Patches are applied on a “best effort basis”. So instead of our changes being “insert the word ‘foo’ at position 7”, they are more vague, e.g., apply this patch with this context using these parameters to determine Levenshtein distance between strings. With intention preservation, if the operation is “insert word ‘foo’ at position 7”, definitely that’s exactly what happens whenever anybody does it (‘foo’ will appear in the document) – it does not depend at all on context. With diffmatchpatch patches (which we use in CoCalc), the effect of the patch depends very much on the document you’re applying the patch to. If there is insufficient context, then ‘foo’ might not get inserted at all.
Similar remark apply to how I designed the structured object sync in CoCalc, which is used, e.g., for CoCalc Jupyter Notebooks;it also applies patches on a best effort basis.
OT = operational transforms
This is a protocol that in theory has all of 1-3. Of course thereare many, many specific versions of OT. The hard part is ensuring 3,and it can be complicated. The problem to be solved makes sense,and it can be done. The details (and implementingthem) are certainly nontrivial to think about conceptually…There’s many academic research papers on OT,and it’s implemented (well) in many productionsystems.
In OT, the data structure that defines the document is simple (e.g., just a text string), and the operations are simple, but applying them in a meaningful way is very hard. This paper on HN that I mentioned above argues that OTis much more popular in production systems than CRDT.
CRDT = commutative replicated data type
This also does 1-3. It sets everything up so thedata structure that defines the document is very complicated(and verbose Together 3 3 7 3. ), but it’s always possible to merge documents ina consistent way. What is difficult gets pushed to differentplaces in the protocol than OT, but it’s still quite hard,and there are subtle issues involved with any non-toy implementation.
What about CoCalc’s approach…?
CoCalc’s text editing does synchronization as follows. Eachuser periodically computes a timestamped patch, then broadcastsit to everybody else editing the same file. When patches arrive,each user computes the current state of the document as theresult of applying all patches in timestamp order.If everybody stops editing, then they all agree on the samedocument.
This protocol satisfies 1 and 2, but not 3. The reason is thatpatches are applied on a best-effort basis using the diff-match-patchalgorithm. For example, a patch made from deleting a single letterin a document can, when applied to a different document endup deleting multiple letters (or none). Basically, CoCalc replacesall the very hard work needed for 3 that OT and CRDT’s have with a notion of applying patches on a “besteffort” basis. The behavior is well defined (because of the timestamps),but may be surprising when multiple people do simultaneous nearbyedits in a document.
The paper says:
“There are two basic ways to propagate localedits: one is to propagate the edits as operations [12,38,50,51,73]; the other is to propagate the editsas states [13]. Most real-time co-editors, including those based on OT and CRDT, have adoptedthe operation approach for propagation for communication efficiency, among others. The operationapproach is assumed for all editors discussed in the rest of this paper”.
Here [13] is N. Fraser’s paper on Differential Sync. This was the syncalgorithm in the first version of CoCalc, and was the inspiration forwhat CoCalc currently does.
In CoCalc, the data structure that defines the document is simple (just a text string, say), and the operations are less simple (computing diffs, defining patches), and applying them in a meaningful way issomewhat difficult (it’s what the diffmatchpatch library does).This approach is very easy to think about and generalize, since itis self contained and a local problem. After all, I mostly described the algorithm in a single paragraph above!
In CoCalc, we compute diffs of arbitrary documents periodically,much like how React.js DOM updates work.This seems to not be needed in OT or CRDT, which instead track theactual operations performed by users (i.e., type a character,delete something).Computing diffs has bad complexity in general, but very goodcomplexity in many cases that matter in practice (that’sthe trick behind React). Diffs involve observing state periodically, rather than tracking changes.
OT and CRDT really are solving a much harder problem than we solve. This is similar to how git uses the trick of “assume sha1 hashes don’t collide” to solve a much easier problem than the much harder problems other revision control systems like Darcs solve.
An Example in which CoCalc violates the intention preservation requirement
There is a nice example to illustrate how CoCalc fails for this third “user intention” requirement. This is called “the TP2 puzzle”. You can try the following in both CoCalc and Overleaf (which probably does some OT algorithm):
- Type in some blank lines, then “abcd”, then blank lines
- Open three windows on the doc you’re editing.
- Disconnect your Internet
- In each of the three window, make these changes, in order:
- abcxd (put x after c)
- abycd (put y before c)
- acd (delete b)
- Reconnect and watch. The experts agree that the “correct” intention preserving convergent state is “aycxd” (which overleaf produces), but CoCalc will produce “acxd”.
I do NOT consider this a bug in CoCalc – it’s doing exactly what is implemented, and what I as the author of the realtime sync systemintended.The issue is that the patch to delete “b” has “a” and “cd” as surroundingcontext, and if you look at how diffmatchpatch patch application works, this is a case where it just deletes everything inside the context.
Evidently, Google Wave also had issues with TP2 because fully implementing OT is…
Bigasoft total video converter 5 4 0 9. “… hard! In fact, almost all published algorithms that claim to satisfy TP2 have been shown to be flawed.”
More details…
The “famous” TP2 puzzle for CoCalc ends up like this (in at least 1 of the 6 possibilities!).
Start with
then add an x and a y on either side of b, and delete b.
In one order, end up with
The patches are:
Applying the “delete b” patch, also deletes the y:
Looking at the source code of diffmatchpatch, this is just what DMP does. If there is a lot more badness and the strings are bigger, it’ll refuse to delete. It really is a sort of “best effort application of patches” with parameters and heuristics; no magic there.
Subethaedit 4 0 1 – Collaborative Text Editing Using Bonjour Download
Conclusion
I viewed the problem of realtime synchronizedediting of text to be primarily one of synchronization. Thegoal is that if multiple people are editing a fileat once, then they take their hands of the keyboard,everybody quickly converges to looking at the same document.Moreover, that document should reflect what peopleare typing – it should not be random gibberish!By relaxing condition 3 “intention preservation”, whichis hard to rigorously define anyways and very hard to satisfy,we obtain a very simple algorithm to implement and reasonabout. So that’s what CoCalc uses. I had battledfor a while with other approaches, and decidedon switching to this very simple approach, since I wantedsomething that worked solidly in practice and would beeasy to implement and extend.
Editing and Proofreading Worksheets
Correct your own works as well as the work of others is a critical skill to command at all levels of education. You should always be working to improve your skills on this topic. We give you a wide range of activities to work on here.
How Many Mistakes Can You Find?
A story about two twin boys with a number of errors. Go get them!
A story about two twin boys with a number of errors. Go get them!
Rewriting and Editing Short Passages: At the Farm
A story about Jim and Andy's farm.
A story about Jim and Andy's farm.
Editing Informal Letters
A simple 4 sentence letter, so what Holly missed.
A simple 4 sentence letter, so what Holly missed.
Editing and Proofreading an Informational Memo
These will become familiar to students quickly. They are sent home all the time.
These will become familiar to students quickly. They are sent home all the time.
Editing a Short Letter of Complaint
Make sure you work on your grammar before you complain to anyone.
Make sure you work on your grammar before you complain to anyone.
Rewriting Poorly Written Sentences
We give you some messed up sentences, we want you to make them squeaky clean.
We give you some messed up sentences, we want you to make them squeaky clean.
Editing Sentences
There are some really bad errors in the sentences that we give you.
There are some really bad errors in the sentences that we give you.
Choosing Proper Sentences
Which of the three sentences is written properly?
Which of the three sentences is written properly?
Grammar and Contractions in Sentences
Which sentence works best?
Which sentence works best?
Rewriting Paragraphs
We have you correct an entire paragraph to step up our use of this data.
We have you correct an entire paragraph to step up our use of this data.
Using the (Write) Right Homophone
We have you pick out which Homophones are way off.
We have you pick out which Homophones are way off.
Vgurusoft video player 1 6 0 6. Editing Stories
These stories have a number of errors, find them all.
These stories have a number of errors, find them all.
Proofread and Fix Each Paragraph
These are extended passages for you to work on.
These are extended passages for you to work on.
Daily Writing Edit - Cats
We give you three quick entries to edit that all relate to cats.
We give you three quick entries to edit that all relate to cats.
Daily Writing Edit - Dogs
The same concept as above, except this time we focus on dogs.
The same concept as above, except this time we focus on dogs.
Editing and Proofreading a Letter of Complaint
The reader should write a letter of complaint back to the author.
The reader should write a letter of complaint back to the author.