Sqlite3 WebAssembly

Something that would be really fun would be to run SQLite in-memory in a browser but use the same tricks as Litestream and Cloudflare Durable Objects (https://simonwillison.net/2024/Oct/13/zero-latency-sqlite-st...) to stream a copy of the WAL log to a server (maybe over a WebSocket, though intermittent fetch() POST would work too).

Then on subsequent visits use that server-side data to rehydrate the client-side database.

From https://sqlite.org/forum/info/50a4bfdb294333eec1ba4749661934... is looks like WAL mode is excluded from the default SQLite WASM build so you would have to go custom with that.

Are you thinking something like https://electric-sql.com/

Isn't a more advanced/production version of this what the combo of couchdb and pouchdb do since several years ago?

Yeah this kind of thing has certainly been tried before, I feel like SQLite WASM plus WAL might be an interesting twist on the idea.

There are many layers of that's not how it works at play here.

In-memory SQLite databases don't use WAL. Wasm (and browser Wasm, in particular) doesn't support anything like the shared memory APIs SQLite wants for its WAL mode.

Litestream requires a very precise WAL setup to work (which just so happens to work with the default native SQLite setup, but is hard to replicate with Wasm).

Cloudflare Durable Objects may have been inspired by Litestream but works very differently (as do LiteFS, Turso, etc…)

The general idea of streaming changes from SQLite would work, but it's a lot of work, and the concurrency model of in-browser Wasm will make it challenging to implement.

(I wrote that forum post some time ago, and have WAL working in a server side Wasm build of SQLite, but none of the options to make it work would make much sense, or be possible, in browser)

What’s the catch with this thing?

As someone who uses sqlite fairly regularly, but doesn't understand what most of those paragraphs mean, do you have any recommendations for learning resources?

I'm gathering that I need to learn about: - WAL - Shared Memory APIs - Concurrency models - Durable Objects?

Then don't use in-memory sqlite? Use file backed sqlite but have your wasm implementation of those "system calls" just be to memory?

I dunno, feels like you're coming down too hard.

WAL: Write ahead log, common strategy for DBs (sqlite, postgres, etc.) to improve commit performance. Instead of fsync()ing every change, you just fsync() a log file that contains all the changes and then you can fsync() the actual changes at your leisure

Shared memory API: If you want to share (mutable) data between multiple processes, you need some kind of procedure in place to manage that. How do you get a reference to the data to multiple processes, how do you make sure they don't trample each other's writes, etc.

Concurrency model: There are many different ways you can formalize concurrent processes and the way they interact (message passing, locking, memory ordering semantics, etc.). Different platforms will expose different concurrency primitives that may not work the same way as other platforms and may require different reasoning or code structure

Durable objects - I think this is some Cloudflare service where they host data that can be read or modified by your users

This is all from memory, but IME, GPT is pretty good for asking about concepts at this level of abstraction

If you like, solving these sort of problems, we are tackling them at Fireproof.

Our database API is modeled on CouchDB and MongoDB, but our storage abstractions are along the lines of what you need to build the multi writer WAL you describe.

More details here https://jsr.io/@fireproof/encrypted-blockstore

The security model is challenging, as it relies on Postgres users for iam. Your users essentially log directly into your db

File backed SQLite in a browser? Do you mean like OPFS?

https://sqlite.org/wasm/doc/trunk/persistence.md#opfs-wal

Again, just because the all the Lego pieces sound like they should all just fit together, doesn't mean that they will.

The VFS mechanism was primarily designed to make SQLite easy to port to multiple OSes. WAL mode is hard to port everytime you step away from a more traditional OS.

“We have SQLite in the browser, let's just stream the WAL like the Litestream hack” does not add up.

It's not impossible, but it surely took a lot of effort at Cloudflare (or at Fly, or Turso) to get there. And neither of them opened it up sufficiently to help us do the same. In a browser.

Isn’t Postgres a fairly capable IAM provider, all things considered? I’d their access control mechanisms at least as much as a run of the mill external backend’s.

Thank you!

And side note on your last point - I've been burned too many times by confident hallucinations to trust my foundational learning to GPT. I hope someday that will improve, but for now ChatGPT is as trustworthy as an evening chat with someone at the bar.

... Someone who has been drinking since happy hour.

I haven't looked but I bet a lot of the WAL complexity comes down to supporting consistency and durability guarantees, neither of which you necessarily need for your in-browser use case.

For basic auth it works well, but the challenge comes when you need to integrate with oidc, need to enforce mfa, enable sso etc. session invalidation is also quite complicated.

You need an identity middle man in front of the Postgres identity to tackle these and validate that the session is still active. Last time I looked at electric it was a big challenge to integrate such a service. This might have improved since then however

I think OrbitDB [1] is the closest we have to a functional version of this right now? Unsure, but the concept is really cool!

[1]: https://github.com/orbitdb/orbitdb/blob/main/docs/GETTING_ST...

hi simon. i direct messaged you on twitter about a PoC i did of this in aug 2022, but never heard back - i thought you might have been interested. my twitter handle is justjs14.

i have some code i would have to dig out that did this very thing - it allows you to open a SQLite db in browser using sqlite (with a VFS) compiled to wasm (not the official WASM build), make changes and both push and pull WALs to and from a server (or indeed browser to browser would be possible both manually or over WebRTC). it even works with github pages if you give the browser client a github token to work with.

if you are interested, feel free to ping me and i can see if i can get this up and running from scratch again. i did a ton of experiments with this approach around then and i think it could be useful for a subset of applications at least.

there's also a working demo of the pull functionality only here: https://just.billywhizz.io/sqlite/demo/#https://just.billywh...

LiteVFS can be compiled in a browser and sync with LiteFS cloud: https://github.com/superfly/litevfs

It needs to be run from a worker, though

I was curious and Grok 2 seemed to do pretty good: https://x.com/i/grok/share/c2qCdF2wwIx7AHz0U1f2u8dTO

You can see what this means specifically from the docs: https://electric-sql.com/docs/guides/auth

WAL - Shared Memory APIs: https://sqlite.org/wal.html

concurrency model of in-browser Wasm: https://medium.com/@quincarter/wasm-is-awsm-and-multi-thread...

Cloudflare Durable Objects: https://www.cloudflare.com/developer-platform/durable-object...

This sounds like what meteor was 10+ years ago but with sqlite instead of mongodb.

Nothing like it, actually.

If you'd like a trustworthy overview, the book Designing Data-Intensive Applications by Martin Kleppmann is a classic. I really hope we get an updated version, but the fundamentals all still hold anyway.

Unlike other opinions here I do think it is technically feasible to stream a copy of the WAL - it just has to be implemented in the VFS. "Shared memory" could be a SharedArrayBuffer, or just a normal buffer if you only have one database connection open at a time (for example in a SharedWorker, which is already common). It may not be simple to implement, but definitely possible.

The biggest actual issue is that it will capture block-level changes, not row-level changes. This means it can work to replicate a complete database, but partial sync (e.g. sharing some rows with other users) won't be feasible.

To get row-level changes for partial sync, you need to use something like triggers or the SQLite session extension [1]. For PowerSync we just use triggers on the client side. I find that works really well, and I haven't found any real downsides to that except perhaps for the work of maintaining the triggers.

[1]: https://sqlite.org/sessionintro.html

Not really, or it depends. That complexity is dealt with by SQLite.

The complexity for you comes from trying to reuse their (battle tested) implementation when (as I wrote above) the primitives they depend upon were not meant to make porting to a browser sandbox easy.

And the problems there are the specific concurrency model they depend upon: communicate by sharing memory.

Then, you're either working at the wrong abstraction level (and it shows), or you're patching and gutting SQLite.

SQLite is meant to work with files, file locks, shared memory, fsync and mmap.

It also doesn't work out great if you try to persist to an object store, to a KV store, or…

I'm repeating myself, but yeah. You can make it work. Others have made it work. But it's still a lot of work, and you're throwing away a lot of what makes SQLite… SQLite.