psarna

single executable with its own frontend is the way; I followed the pattern with https://worb.cloud . Nice for users but also extremely easy to have a short debugging feedback loop

related: https://printed.games/gates/

First of all, let me start by reiterating the first sentence from the bottomless repo - it's work in heavy progress (and we'll move it under the sqld/ repo soon, to keep everything in one place). Now, answers:

> - Looks like bottomless does automatic restoring of the database? Litestream seems to avoid this, I assume because of concerns about accidentally creating multiple writers on a rolling-deploy (or similar mistake). Any concerns about this possible footgun?

It's a valid concern, but what always happens on boot is starting a new generation (a generation is basically a snapshot of the main file + its continuously replicated WAL), distinguished by a uuid v7, the timestamped one. So even if a collision happens, it would be recoverable - e.g. one of the stray generations should be deleted.

> - Bottomless is a sqlite extension, not a separate process? Pros and cons compared to litestream?

The only con I see is that a bug in the extension could interfere with the database. As for pros: way less maintenance work, because everything is already embedded, we're also hooked into the database via a virtual WAL interface (libSQL-only), so we have full control over when to replicate, without having to observe the .wal file and reason about it from a separate process.

> - Similar questions for sqld. How does sqld handle the lifecycle of deploys and multiple readers/writers talking to the database? Anything a new user should be concerned about?

There are going to be multiple flavors of sqld, but the rough idea would be to trust the users to only launch a single primary. In the current state of the code, replicas contact the primary in order to register themselves, so the model is centralized. Once we build something on top of a consensus algorithm, leader election will be pushed to the algorithm itself.

In case somebody stumbles upon this old thread, here's the recording of the meetup: https://youtu.be/pgzjPeIQ3Us

maybe_yield may or may not be the right solution here, but I think it may be useful in general - e.g. when you have long I/O-less computations. In such a case, I'd like to be able to say "yield here if my budget is drained, but continue otherwise and don't put my task at the end of the queue". Although for that the only thing I really need is a way to peek at your budget - with that, open-coding maybe_yield is trivial

Now that I think of it, it would probably be beneficial even outside of the unconstrained scope, especially for long computations. When iterating over millions of elements, it would be great to have a mechanism for maybe yielding if we're past the budget, but we don't really want to force-yield on every X iterations and put the task at the back of the queue. If the maybe_yield API is potentially controversial, a sufficient building block would be a function that allows peeking into the state of your budget - and then, if you're out of it, you just explicitly call yield_now().

wrt. preemption - in Seastar we have maybe_yield(), which gives up the cpu, but only if the task quota (more or less a semantic equivalent for Tokio's budget) has passed. Wouldn't it make sense to have a similar capability in Tokio? Then, if somebody is not a big fan of the default preemption, they could run their tasks under tokio::task::unconstrained and only check the budget in very specific, explicitly chosen places - where they call maybe_yield(). That could of course also be open-coded by trying to implement maybe_yield on top of yield_now and some time measurements, but since the whole budgeting code is already there... Do you think it's feasible?

Indeed - 1ms was a nice round number, but it was also not that far from the real, sub-1ms number. But note that by "latency" here I mean not only the round trip, but the total measured time of executing a single request, including the time its task spends in Tokio (and its queues), and until its response is fully processed as well. Since the program sent requests with configurable concurrency set to ~1024 or more, the overall throughput was still satisfying.

On the one hand it was (and still is) concerning that the observed latency per simple request was that high, on the other - it never really came up in our distributed tests, since there the network latency imposes a few milliseconds anyway. When we figure out the real source of this behavior, I'll be happy to describe the investigation process in a blog post (:

> In my experience so far, highly proficient Cpp programmers tend to find rust constraints overly restrictive

For me it's the exact opposite - I find Rust righteously restrictive exactly in places I wish the C++ compiler would complain. For instance, the fact that variables are legal to use after move is an obvious C++ footgun - you almost never want it to actually happen in your code. And then, if one really knows better, Rust has `unsafe`, which I actually never used yet except for providing C/C++ bindings to link a Rust project with Scylla.

So, to sum up, I think that Rust has much better defaults (e.g. move the value by default, everything is const by default, etc.), but still lets you bend the rules explicitly, while in C++ the rules are already slightly bent for you, just in case you need it, which is instead a common source of bugs.

And the adoption of Rust is going great at ScyllaDB. More of it is hopefully coming soon, including a rewrite of user-defined functions support in Rust, which would allow us to fully utilize wasmtime (a very neat Rust project) as the WebAssembly engine.

That's great news! Especially that the observed performance of the test program based on FuturesUnordered, even though it stopped being quadratic, it was still considerably slower than the task::unconstrained one, which suggests there's room for improvement. Probably due to the fact that you still pay with a constant number of polls (32) each time you go out of budget.

Hi, author of the post here. I'll also be talking about this issue in a little more detail at an online Rust Warsaw meetup tomorrow, feel free to join, especially if you're up for a live discussion later: https://www.meetup.com/Rust-Warsaw/events/282879405/