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- may massively reduce the latency involved.
Those sharing Cap'n Proto-encoded data may have particular interest. Cap'n Proto (https://capnproto.org) is fantastic at its core task - in-place serialization with zero-copy - and we wanted to make the IPC (inter-process communication) involving capnp-serialized messages be zero-copy, end-to-end.
That said, we paid equal attention to other varieties of payload; it's not limited to capnp-encoded messages. For example there is painless (<-- I hope!) zero-copy transmission of arbitrary combinations of STL-compliant native C++ data structures.
To help determine whether Flow-IPC is relevant to you we wrote an intro blog post. It works through an example, summarizes the available features, and has some performance results. https://www.linode.com/blog/open-source/flow-ipc-introductio...
Of course there's nothing wrong with going straight to the GitHub link and getting into the README and docs.
Currently Flow-IPC is for Linux. (macOS/ARM64 and Windows support could follow soon, depending on demand/contributions.)
- Split-up a certain important C++ service into several parts, for various reasons, without adding latency to the request path.
The latter task meant, among other things, communicating large amounts of user data from server application to server application. capnp-encoded structures (sometimes big - but not necessarily) would also need to be transmitted; as would FDs.
The technical answers to these challenges are not necessarily rocket science. FDs can be transmitted via Unix domain socket as "ancillary data"; the POSIX `sendmsg()` API is hairy but usable. Small messages can be transmitted via Unix domain socket, or pipe, or POSIX MQ (etc.). Large blobs of data it would not be okay to transmit via those transports, as too much copying into and out of kernel buffers is involved and would add major latency, so we'd have to use shared memory (SHM). Certainly a hairy technology... but again, doable. And as for capnp - well - you "just" code a `MessageBuilder` implementation that allocates segments in SHM instead of regular heap like `capnp::MallocMessageBuilder` does.
Thing is, I noticed that various parts of the company had similar needs. I've observed some variation of each of the aforementioned tasks custom-implemented - again, and again, and again. None of these implementations could really be reused anywhere else. Most of them ran into the same problems - none of which is that big a deal on its own, but together (and across projects) it more than adds up. To coders it's annoying. And to the business, it's expensive!
Plus, at least one thing actually proved to be technically quite hard. Sharing (via SHM) a native C++ structure involving STL containers and/or raw pointers: downright tough to achieve in a general way. At least with Boost.interprocess (https://www.boost.org/doc/libs/1_84_0/doc/html/interprocess....) - which is really quite thoughtful - one can accomplish a lot... but even then, there are key limitations, in terms of safety and ease of use/reusability. (I'm being a bit vague here... trying to keep the length under control.)
So, I decided to not just design/code an "IPC thing" for that original key C++ service I was being asked to split... but rather one that could be used as a general toolkit, for any C++ applications. Originally we named it Akamai-IPC, then renamed it Flow-IPC.
As a result of that origin story, Flow-IPC is... hmmm... meat-and-potatoes, pragmatic. It is not a "framework." It does not replace or compete with gRPC. (It can, instead, speed RPC frameworks up by providing the zero-copy transmission substrate.) I hope that it is neither niche nor high-maintenance.
To wit: If you merely want to send some binary-blob messages and/or FDs, it'll do that - and make it easier by letting you set-up a single session between the 2 processes, instead of making you worry about socket names and cleanup. (But, that's optional! If you simply want to set up a Unix domain socket yourself, you can.) If you want to add structured messaging, it supports Cap'n Proto - as noted - and right out of the box it'll be zero-copy end-to-end. That is, it'll do all the SHM stuff without a single `shm_open()` or `mmap()` or `ftruncate()` on your part. And if you want to customize how that all works, those layers and concepts are formally available to you. (No need to modify Flow-IPC yourself: just implement certain concepts and plug them in, at compile-time.)
Lastly, for those who want to work with native C++ data directly in SHM, it'll simplify setup/cleanup considerably compared to what's typical. For the original Akamai service in question, we needed to use SHM as intensively as one typically uses the regular heap. So in particular Boost.interprocess's built-in 2 SHM-allocation algorithms were not sufficient. We needed something more industrial-strength. So we adapted jemalloc (https://jemalloc.net/) to work in SHM, and worked that into Flow-IPC as a standard available feature. (jemalloc powers FreeBSD and big parts of Meta.) So jemalloc's anti-fragmentation algorithms, thread caching - all that stuff - will work for our SHM allocations.
Having accepted this basic plan - develop a reusable IPC library that handled the above oft-repeated needs - Eddy Chan joined and especially heavily contributed on the jemalloc aspects. A couple years later we had it ready for internal Akamai use. All throughout we kept it general - not Akamai-specific (and certainly not specific to that original C++ service that started it all off) - and personally I felt it was a very natural candidate for open-source.
To my delight, once I announced it internally, the immediate reaction from higher-up was, "you should open-source it." Not only that, we were given the resources and goodwill to actually do it. I have learned that it's not easy to make something like this presentable publicly, even having developed it with that in mind. (BTW it is about 69k lines of code, 92k lines of comments, excluding the Manual.)
So, that's what happened. We wrote a thing useful for various teams internally at Akamai - and then Akamai decided we should share it with the world. That's how open-source thrives, we figured.
On a personal level, of course it would be gratifying if others found it useful and/or themselves contributed. What a cool feeling that would be! After working with exemplary open-source stuff like capnp, it'd be amazing to offer even a fraction of that usefulness. But, we don't gain from "market share." It really is just there to be useful. So we hope it is!