Hello Maxim! On 8/24/22 18:00, Maxim Dounin wrote: [...]
For having nginx as a reverse proxy in front of a server, such as unit, it's faster to communicate through Unix sockets (UDS), rather than TCP (localhost).I would rather say "it's a common misconception that it's faster to communicate through Unix sockets". While it Unix sockets can be beneficial for some microbenchmarks,
Yeah, I don't know how much of an improvement it will really bring. I'm going to develop some benchmark soon, as a side effect of another task, but it will still be a bit artificial, and not a real load. But it will help see how much of an improvement UDS have in the context of Unit.
in most production setups it makes no difference, yet used to introduce various issues.
The only one I can think of is security, since you don't have file permissions, but one could also argue that the security can't be lower than that of TCP sockets.
Nginx already supports UDS. But UDS has a problem: the kernel creates a file in the fs, and it's not always trivial to clean up those files. Then, if the application is restarted, there's no SO_REUSEADDR to allow reusing the socket file, so the application will just fail. This happens in nginx Unit, which creates listener sockets from a privileged thread, and then uses them from unprivileged threads. When the unprivileged thread stops using the socket, it can't remove the file, and doing so would require huge complexity to implement. It's easier to just tell the kernel we want an abstract UDS (AUDS), so that there's no file at all. Then if the user restarts Unit, it'll be able to recreate the AUDS. A user reported this problem with normal UDS and we concluded that the easiest solution would be to add support for AUDS. His set-up is a kubernetes pod, where a container uses nginx and another container uses Unit. Communicating through an AUDS would be trivial and fast.So, you are trying to implement abstract namespace sockets as a bandaid for Unit bug, which is not able to properly remove Unixsockets in some cases and fails to restart, correct?
Yeah, it helps avoid fixing that bug. But that's why the Linux kernel implemented that feature in the first place: so that programs don't need to cleanup the filesystem in these cases, recognizing that it may not be always trivial.
This does not look like a valid reason to me, especially given that a) abstract namespace sockets are Linux-only, and
Since other systems can use localhost, I don't think it's so important to support abstract sockets in them (it's not losing a big feature, but rather a minor improvement). What Unit (and some other web servers do) is just report an error in the configuration.
b) there are multiple approaches to filing the abstract socket address.
I don't understand this last point. Do you mean the NUL and @ alternatives? NUL is the one supported by the kernel, and so the most obvious one. Since NUL is not usable by most tools, most programs use the alternative syntax @. Some programs only allow @; but since it doesn't hurt to allow NUL, and it makes for a very easy implementation, I'd do it that way.
Cheers, Alex -- Alejandro Colomar <http://www.alejandro-colomar.es/>
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