pg_crdt VS multiversion-concurrency-control

Cool, I'll check out your repos. Supabase also as Postgres CRDT (https://github.com/supabase/pg_crdt).

But, in general, I think searching in and across documents and database storage is going to a thorny problem even with existing, hyper-optimized CRDT algorithms. For example, if you just store your Yjs document as a binary blob in a Postgres column, Postgres becomes the bottleneck and all the fancy range-tree or b-tree optimizations are no longer that helpful on the server. Plus, it'd be great to have an document edit just be a tiny insert into the database that can be streamed to other clients rather than a big row update.

This all may because the focus of CRDTs seems to be rooted in a fully decentralized, P2P system but many developers want collaborative text editing in a more traditional client-server model.

I'm part of the Supabase Realtime team. Thanks for doing this! We've been wanting to do it internally ever since launching Broadcast. We've also been experimenting with what a very integrated CRDT solution looks like (e.g. pg_crdt).

This is an experimental extension for CRDTs, `pg_crdt`[0]. It supports Yjs/Yrs and Automerge.

The linked blog post describes how we're thinking about this extension in a Supabase context. Ideally this "Show HN" generates some discussion/interest, both here and in the github discussions [1].

I want to emphasise this part from the blog post[2]: "pg_crdt has not been released onto the Supabase platform (and it may never be). We’re considering many options for offline-sync/support and, while CRDTs will undoubtedly factor in, we’re not sure if this is the right approach."

[0] GitHub repo: https://github.com/supabase/pg_crdt

[1] Discussions: https://github.com/supabase/pg_crdt/discussions

[2] Blog post: https://supabase.com/blog/postgres-crdt

The code is in this repository https://github.com/samsquire/multiversion-concurrency-control in MultiplexingThread.java and MultiplexProgramParser.java

https://github.com/samsquire/multiversion-concurrency-contro...

And I implemented a 3 way text diff with myers algorithm based on https://blog.jcoglan.com/2017/02/12/the-myers-diff-algorithm...

https://github.com/samsquire/text-diff

I implemented an eventually consistent mesh protocol that uses timestamps to provide last write wins

I think I lean towards per-thread sharding instead of mutex based or lock free data structures except for lockfree ringbuffers.

You can get embarassingly parallel performance if you split your data by thread and aggregate periodically.

If you need a consistent view of your entire set of data, that is slow path with sharding.

In my experiments with multithreaded software I simulate a bank where many bankaccounts are randomly withdrawn from and deposited to. https://github.com/samsquire/multiversion-concurrency-contro...

I get 700 million requests per second due to the sharding of money over accounts.

If you want some sample code to implement MVCC, I implemented MVCC in multithreaded Java as a toy example

https://github.com/samsquire/multiversion-concurrency-contro...

First read TransactionC.java then read MVCC.java

I wrote an unrolled switch statement in Java to simulate eager async/await across treads.

https://github.com/samsquire/multiversion-concurrency-contro...

The goal is that a compiler should generate this for you. This code is equivalent to the following:

   task1:

https://doc.rust-lang.org/book/ch16-00-concurrency.html

I've been working on implementing Java async/await state machine with switch statements and a scheduling loop. If the user doesn't await the async task handle, then the task's returnvalue is never handled. This is similar to the Go problem with the go statement.

https://github.com/samsquire/multiversion-concurrency-contro...

If your async call returns a handle and

yield value2++

https://github.com/samsquire/multiversion-concurrency-contro...

I am still working on allowing multiple coroutines to be in flight in parallel at the same time. At the moment the tasks share the same background thread.

I asked this stackoverflow question regarding C++ coroutines, as I wanted to use coroutines with a thread pool.

https://stackoverflow.com/questions/74520133/how-can-i-pass-...

This is very interesting. Thank you for submitting this and thank you for working on this.

I am highly interested in parallelism and high concurrency. I implemented multiversion concurrency control in Java.

https://github.com/samsquire/multiversion-concurrency-contro...

I am curious how to handle replication with high concurrency. I'm not sure how you detect dangerous reads+writes to the same key (tuples/fields) across different replica machines. In other words, multiple master.

I am aware Google uses truetime and some form of timestamp ordering and detection of interfering timestamps. But I'm not sure how to replicate that.

I began working on an algorithm to synchronize database records, do a sort, then a hash for each row where hash(row) = hash(previous_row.hash + row.data)

Then do a binary search on hashes matching/not matching. This is a synchronization algorithm I'm designing that requires minimal data transfer but multiple round trips.

The binary search would check the end of the data set for hash(replica_a.row[last]) == hash(replica_b.row[last]) then split the hash list in half and check the middle item, this shall tell you which row and which columns are different.