Cap’n Proto 1.0

kentonv
on 28 Jul 2023

It’s been a little over ten years since the first release of Cap’n Proto, on April 1, 2013. Today I’m releasing version 1.0 of Cap’n Proto’s C++ reference implementation.

Don’t get too excited! There’s not actually much new. Frankly, I should have declared 1.0 a long time ago – probably around version 0.6 (in 2017) or maybe even 0.5 (in 2014). I didn’t mostly because there were a few advanced features (like three-party handoff, or shared-memory RPC) that I always felt like I wanted to finish before 1.0, but they just kept not reaching the top of my priority list. But the reality is that Cap’n Proto has been relied upon in production for a long time. In fact, you are using Cap’n Proto right now, to view this site, which is served by Cloudflare, which uses Cap’n Proto extensively (and is also my employer, although they used Cap’n Proto before they hired me). Cap’n Proto is used to encode millions (maybe billions) of messages and gigabits (maybe terabits) of data every single second of every day. As for those still-missing features, the real world has seemingly proven that they aren’t actually that important. (I still do want to complete them though.)

Ironically, the thing that finally motivated the 1.0 release is so that we can start working on 2.0. But again here, don’t get too excited! Cap’n Proto 2.0 is not slated to be a revolutionary change. Rather, there are a number of changes we (the Cloudflare Workers team) would like to make to Cap’n Proto’s C++ API, and its companion, the KJ C++ toolkit library. Over the ten years these libraries have been available, I have kept their APIs pretty stable, despite being 0.x versioned. But for 2.0, we want to make some sweeping backwards-incompatible changes, in order to fix some footguns and improve developer experience for those on our team.

Some users probably won’t want to keep up with these changes. Hence, I’m releasing 1.0 now as a sort of “long-term support” release. We’ll backport bugfixes as appropriate to the 1.0 branch for the long term, so that people who aren’t interested in changes can just stick with it.

What’s actually new in 1.0?

Again, not a whole lot has changed since the last version, 0.10. But there are a few things worth mentioning:

• A number of optimizations were made to improve performance of Cap’n Proto RPC. These include reducing the amount of memory allocation done by the RPC implementation and KJ I/O framework, adding the ability to elide certain messages from the RPC protocol to reduce traffic, and doing better buffering of small messages that are sent and received together to reduce syscalls. These are incremental improvements.

• Breaking change: Previously, servers could opt into allowing RPC cancellation by calling context.allowCancellation() after a call was delivered. In 1.0, opting into cancellation is instead accomplished using an annotation on the schema (the allowCancellation annotation defined in c++.capnp). We made this change aft