Ayende @ Rahien

Oren Eini aka Ayende Rahien CEO of Hibernating Rhinos LTD, which develops RavenDB, a NoSQL Open Source Document Database.

You can reach me by:

oren@ravendb.net

+972 52-548-6969

, @ Q j

Posts: 6,878 | Comments: 49,254

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time to read 2 min | 345 words

I’m pretty much done with my Rust protocol impl. The last thing that I wanted to try was to see how it would look like when I allow for messages to be handled out of band.

Right now, my code consuming the protocol library looks like this:

This is pretty simple, but note that the function definition forces us to return a value immediately, and that we don’t have a way to handle a command asynchronously.

What I wanted to do is to change things around so I could do that. I decided to implemented the command:

remind 15 Nap

Which should help me remember to nap. In order to handle this scenario, I need to provide a way to do async work and to keep sending messages to the client. Here was the first change I made:

image

Instead of returning a value from the function, we are going to give it the sender (which will render the value to the client) and can return an error if the command is invalid in some form.

That said, it means that the echo implementation is a bit more complex.

There is… a lot of ceremony here, even for something so small. Let’s see what happens when we do something bigger, shall we? Here is the implementation of the reminder handler:

Admittedly, a lot of that is error handling, but there is a lot of code here to do something that simple.  Compare that to something like C#, where the same thing could be written as:

I’m not sure that the amount of complexity that is brought about by the tokio model, even with the async/await macros is worth it at this point. I believe that it needs at least a few more iterations before it is going to be usable for the general public.

There is way too much ceremony and work to be done, and a single miss and you are faced with a very pissed off compiler.

time to read 3 min | 410 words

After a lot of trouble, I’m really happy that I was able to build an async I/O implementation of my protocol. However, for real code, I think that I would probably recommend using with the sync API instead, since at least that is straightforward and doesn’t incur so much overhead at development time. The async stuff is still very much a “use at your own risk” kind of deal from my perspective. And I can’t imagine trying to use it in a large project and no suffering from the complexity.

As a good example, take a look at the following bit of code:

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It doesn’t seem to be doing much, right? And it is clear what the intent of the code is.

However, if you try to compile this code you’ll get:

image

Now, it took me a long while to figure out what is going on.  The issue is that the code I’m seeing isn’t the actual code, because of macro expansions.

So let’s resolve this and see what the expanded code looks like:

This is after formatting, of course, but it certainly looks scary. Glancing at this code doesn’t tell me what the problem was, so I tried replacing the method with the expanded result, and I got the same error, but this time I got it on a line that helped me figure it out. Here is the issue:

image

We use the ? to return early from the poll method, and the Receiver I’m using in this case is defined to have a Result<String, ()>, so this is the cause of the problem.

I returned my own error type as a result, giving me the ability to convert from (), but that was a really hard thing to resolve.

It might be better to have Rust also offer to show the error on the expanded code by default, because it was somewhat of a chore to actually get to this.

What made this oh so confusing is that I had the exact same code, but using a Stream<String, io:Error> that worked, obviously. But it was decidedly non obvious to see what was the difference between two identical pieces of code.

time to read 3 min | 588 words

On my last post, I got really frustrated with tokio’s complexity and wanted to move to use mio directly. The advantages are that the programming model is pretty simple, even if actually working with is is hard. Event loops can cause your logic to spread over many different locations and make it hard to follow. I started to go that path until I figure out just how much work it would take. I decided to give tokio a second change, and at this point, I looked into attempts to provide async/await functionality to Rust.

It seems that at least some work is already available for this, using futures + some Rust macros. That let me write code that is much more natural looking, and I actually managed to make it work.

Before I get to the code, I want to point out some concerns that I have right now. The futures-await crate (and indeed, all of tokio) seems to be in a state of flux. There is an await in tokio, and I think that there is some merging around of all of those libraries into a single whole. What I don’t know, and can’t find any information about, is what I should actually be using, and how all the pieces come together. I have to note that even with async/await, the programming model is still somewhat awkward, but it is at a level that I can live with. Here is how I built it.

First, we need to accept connections, which is done like so:

Note that I have two #[async[ annotations. One for the method as a whole and one for the for loop. This just accept the connection and spawn a task to handle that, the most interesting tidbits are in the actual processing of the connection:

You can see that this is fairly straightforward code. We first do the TLS handshake, then we validate the certificate. If there is an auth error, we send it to the user and back off. If we are successful, however, things get interesting.

I create a channel, which allow me to  split off the read and write portions of the task. This means that I can send results out of order, if I wanted to, which is great for the actual protocol handling. The first thing to do is to send the OK string to the client, so they know that we successfully connected, then we spawn the read/write tasks. The write task is pretty simple, overall:

You can see the funny .0 references, which is an artifact of the fact that the write_all() function consumes the writer we pass to it and return (a potentially different) writer in the result.  This is pretty common for functional languages.

I’m pretty sure that I can avoid the two calls to write_all for the postfix, but that is easier for now.

Processing the commands is simple as well:

For each command we support, we have an entry on the server configuration and we fetch and invoke it. The result of the command will be written to the client by the write task. Right now we have a 1:1 association between them, but this is now easily broken.

And finally, having an actually command run and running the server itself:

This is pretty simple now, and it give us a nice model to program commands and responses.

I pushed the whole code to this branch, if you care to look at it.

I have some more comments about this code, but I’ll reserve them for another post.

time to read 2 min | 346 words

I kept going with tokio for a while, I even got something that I think would eventually work. The whole concept is around streams, so I create a way to generate them. This is basically taking this code and making it async.

I gave up well into the second hour. Here is where I stopped:

image

I gave up when I realized that the reader I’m using (which is SslStream) didn’t seem to have poll_read. The way I’m reading the code, it is supposed to, but I just threw up my hands at disgust at this time. If it this hard, it ain’t going to happen.

I wrote significant amount of async code in C# at the time when events and callbacks were the only option and then when the TPL and ContinueWith was the way to go. That was hard, and async/await is a welcome relief, but the level of frustration and “is this wrong, or am I really this stupid?” that I got midway through is far too much.

Note that this isn’t even about Rust. Some number of issues that I run into were because of Rust, but the major issue that I have here is that I’m trying to write a function that can be expressed in a sync manner in less than 15 lines of code and took me about 10 minutes to write the first time. And after spending more hours than I’m comfortable admitting, I couldn’t get it to work. The programming model you have here, even if everything did work, means that you have to either decompose your behavior to streams and interact with them in this manner or you put everything as nested lambdas.

Either option doesn’t make for a nice model to work with. I believe that there is another async I/O model for Rust, the MIO crate, which is based on the event loop model. I’ve already implemented that in C, so that might be a more natural way to do things.

time to read 5 min | 941 words

Now that we have a secured and authentication connection, the next stage in making a proper library is to make it run more than a single connection at time. I could have use a thread per connection, of course, or even use a thread pool, but neither of those options is valid for the kind of work that I want to see, so I’m going to jump directly into async I/O in Rust and see how that goes.

The sad thing about this is that I expect that this will make me lose some / all of the nice API that I get for OpenSSL in the sync mode.

Async in Rust is handled by a crate called tokio, and there seems to be active work to bring async/await to the language itself. In the meantime, we have to make do with the usual facilities, which ought to make this interesting.

It actually looks like there is a crate that gives pretty nice handling of tokio async I/O and OpenSSL so that is encouraging. However, as part of trying to re-write everything in tokio style, I got the compiler very upset with me. Here is (partial) error message:

image

Last time I had to parse such errors, I was working in C++ templated code and the year was 1999.

And here is the piece of code it so dislikes:

image

I googled around and there is this detailed answer on a similar topic that frankly, frightened me. I shouldn’t have to dig this deeply and have to start drawing diagrams on so many disparate pieces of the code just to figure out a compiler error.

Let’s try to break it to its component parts and see if that make sense, I reduce the code in question to just:

image

Got another big scary error message. Okay, let’s try it without the OpenSSL stuff?

image

This produce the same error, but in a much less scary tone:

image

Okay, now this looks about as simple as it can be. And now the fix is pretty obvious:

image

The key to understand here, I believe (I haven’t tested it yet) that the write_all call will either perform its work or schedule it, so any future work based on it should go in a nested and_then call. So the result of the single for_each invocation is not the direct continuation of the previous call.

That is fine, I’ll deal with that, I guess.

Cue here about six hours of programming montage.

I have been programming over 20 years, I like to think that I have been around the block a few times. And the simple task of reading a message from TCP using async I/O took me far too long. Here is what I eventually ended up with:

image

This is after fighting with the borrow checker (a lot, it ended up winning), trying to grok my head around the model that tokio has. It is like they took the worst parts of async programming, married it to stream programming’s ugly second cousin and then decided to see if any of the wedding guests is open for adoption.

And if the last sentence doesn’t make sense to you, you are welcome, that is how I felt at certain points. Here is one of the errors that I run into:

image

What is this string, where did it come from and why do we have a unit “()” there? Let me see if I can explain what is going on here. Here is a very simple bit of code that would explain things.

image

And here is the error it generates:

image

The problem is that spawn is expecting a future that results a result that has no meaning, something like: Future<Result<(), ()>>. This make sense, since there isn’t really anything that it can do with whatever the result is. But the error can be really confusing. I spent a lot of time trying to actually parse this, then I had to go and check the signatures of the method involved, and then I had to reconstruct what are the generic parameters that are required, etc.

The fix, btw, is this:

image

Ask yourself how long it would take you to figure what the changes between these versions of the code are without the marker.

Anyway, although I’m happy that I got something done, this approach is really not sustainable. I’m pretty sure that I’m either doing something wrong or missing something. It shouldn’t be this hard. I got some ideas that I want to try, which I’ll talk about in the next post.

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RECENT SERIES

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    01 Aug 2019 - Complex data aggregation with RavenDB
  2. Reviewing mimalloc (2):
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  3. Production postmortem (26):
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  4. Reviewing Sled (3):
    23 Apr 2019 - Part III
  5. RavenDB 4.2 Features (5):
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