Sqlite3 WebAssembly

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)

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

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.

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.

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

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...

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.

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.

Of course. The only thing LLMs are good for is...

https://hachyderm.io/@inthehands/112006855076082650

> You might be surprised to learn that I actually think LLMs have the potential to be not only fun but genuinely useful. “Show me some bullshit that would be typical in this context” can be a genuinely helpful question to have answered, in code and in natural language — for brainstorming, for seeing common conventions in an unfamiliar context, for having something crappy to react to.

> Alas, that does not remotely resemble how people are pitching this technology.

Emscripten's default file system provider is memfs, in memory. Maybe there would be some challenges, some spec limitations using that, but I strongly expect it closer to a weekend or two of hacking to get some special weird mystic quirkiness that WAL relies on than some long ordeal that keeps going on endlessly (to get sqlite running with WAL). https://emscripten.org/docs/api_reference/Filesystem-API.htm...

OPFS is interesting tech but again a red herring misdirecting from what had been raised, using an in-memory filesystem like the default thing that emscripten (the default toolchain) does.

I believe that streaming changes from SQLite is what https://sqlsync.dev/ is

Upvote for that book.

I read it a few months ago and was really impressed with how easy it was to read.

It starts out with simple stuff, like serialising data as JSON vs XML. But it moves into complex areas - like how replication and WALs work, including different ways of handling consensus when using leader-leader replication and how Spanner needs atomic clocks to handle it.

But even the complex stuff was explained in a way that I understood, which is an immense achievement.

I… really don't get this.

The people on SQLite, employed to work on this full time for over a year, have this to say (on the link I posted above):

“Because the WASM build does not have shared memory APIs, activating WAL requires that a client specifically activate exclusive-locking mode for a db handle immediately after opening it, before doing anything else with it…

“WAL mode does not provide any concurrency benefits in this environment. On the contrary, the requirement for exclusive locking eliminates all concurrency support…”

I personally worked to implement shared memory WAL for a server side Wasm port of SQLite. But random internet poster decides to “strongly expect it closer to a weekend or two of hacking.”

Please, do me a favor and do spend that weekend or two for the benefit of the rest of us all. It'll sincerely be much appreciated.

PS: it was a random internet poster¹ (who's been posting in this thread) who helped me figure out how to implement shared memory WAL for my port. It still took way more than “a weekend or two.” So if you do figure out how to crack this, I'm sure that people who've been trying for the past year² will definitely appreciate it.

1: https://github.com/ncruces/go-sqlite3/discussions/69

2: https://github.com/rhashimoto/wa-sqlite

Yep, that doesn't use WAL mode at all, and it's a decent amount of work.

It uses a custom VFS, memory journal mode, and bypasses SQLite for optimistic concurrency handling. I dunno how it handles crash safety, or if it even uses the same on disk file format.

My point here is not that this is impossible, it's that SQlite's WAL implementation isn't meant to be pluggable, and Litestream is a very clever hack, that requires a very specific setup to work well.

It takes a fair amount of work to replicate that in other environments.

Yep, this is my number 1 "I wish I'd read this X years ago" book.

I'm someone who has been doing this stuff for almost two decades without really knowing this is what I'm doing. I used to think what I was going to do was systems level programming like operating systems and maybe the database systems themselves (e.g. postgres, datomic etc.). But for whatever reason my entire career (so far, but I don't see it changing) has been building data systems for businesses and users.

I read the book from cover to cover and half of it was like "ohh... that's how that works, that's what I'm doing wrong" and the other half was "shit, this is something I kinda knew after trying and failing for years, and someone has just written it down in a way I never could".

> WAL mode does not provide any concurrency benefits in this environment.

Except we aren't interested in concurrency or performance benefits of WAL; we want it for something else entirely (replication).

O'Reilly shows a 2nd edition slated for December 2025 but it seems like you can access it early with Safari Books.

https://www.oreilly.com/library/view/designing-data-intensiv...

Then you don't need WAL. I didn't say replication isn't possible, I said "that's not how it works."

A bunch of different people solved this already (though none in a couple of weekends); common to all of them: they don't use WAL mode.

> The shadow WAL is a directory next to your SQLite database where WAL files are effectively recreated as a sequence. . . These WAL files contain the original WAL frames & checksums to ensure consistency.

https://litestream.io/how-it-works/

> (though none in a couple of weekends)

As I said, getting sqlite in wasm to run in WAL mode such that we could start to implement replication, like for example how litestream does. Not this strawman you build, of implementing replication.

"The general idea of streaming changes from SQLite would work, but it's a lot of work"

Seems easy to me, just store/stream all update sql statements. That should have all information needed and don't have to mess with WAL data format etc.

Seriously?

Litestream works out-of-process. It needs shared memory WAL and file locking to work exactly like on "desktop" SQLite for it to even function and produce this "shadow WAL." No ifs, no buts, no inbetweens.

You just don't build Litestream without shared memory WAL. That's my entire point. These pieces you seem to think can be made to simply work together can't, in fact, work together at all, much less simply.

Exclusive locking mode WAL doesn't work with Litestream, and can't be made to work with Litestream, even with a little elbow grease.

Yeah, I wonder why no one thought of that.

The big problem with statement-based replication is that many queries are non-deterministic. e.g. Inserting a row with current_timestamp or random()

Litestreams how-we-did-this to says they take a lock on the whole db during checkpoints to work, so I don't see why shared would be needed.

Seriously bro, take a massive chill pill. You have been way overcommitted to the "it's all impossible!!! How stupid to imagine! If only you were smart like me you'd see everything is impossible!" bit way too hard. Take a breather from your possessed absolute self certainty & derogatory belittling of others.

There's also lots of different fs impls for wasm! Just because some limitations apply to emscriptens impl doesn't mean another impl has the same limitations, or that we can't improve or work around. Use some of your hard working loves-to-tax-itself big brain to find avenues of possibility, rather than just pissing on every possibility.

I'm chill bro. Still waiting for your productive couple of weekends.

But that'll require more than looking at docs reading the words "lock" and "shared" and assuming you know what those mean.

You've been wrong about how some of these replications work, you've been wrong about why the WAL doesn't work, and you still insist on being a pompous rude brat here? You have not been chill.

Litestream is out of process, requires shared memory WAL, doesn't work with exclusive mode WAL, wouldn't work work on Wasm.

LiteFS didn't support WAL mode initially, now does. The Wasm version doesn't support WAL mode at all.

mvSQLite doesn't support shared memory WAL. SQLSync doesn't support WAL at all.

Browser Wasm builds either don't have WAL enabled, or don't support shared memory WAL.

These replications are VFSes or FUSE drivers, and for those, it's much easier to work with rollback mode than WAL, especially if you don't want concurrency (or intend to provide concurrency through other means, like optimistic concurrency).

In open source, only Turso actually did a VFS to replicate the WAL. They had to break SQLite so much they call it libSQL. It also requires shared memory as is, would be hard (if possible) to port to Wasm.

Please, do take the challenge and build this. It will be much appreciated.

I've been asking the Turso guys to document their thing for a year, with little success. They have the interface (which just opens up undocumented internal SQLite interfaces), with no open source samples of how to use it (except a logger), and zero additional docs.

TBF, to this bystander ncruces isn't the one of the two of you who is coming off as more of "a pompous rude brat".