Cap’n Proto is an insanely fast data interchange format and capability-based RPC system. Think
JSON, except binary. Or think Protocol Buffers, except faster.
In fact, in benchmarks, Cap’n Proto is INFINITY TIMES faster than Protocol Buffers.

This benchmark is, of course, unfair. It is only measuring the time to encode and decode a message
in memory. Cap’n Proto gets a perfect score because there is no encoding/decoding step. The Cap’n
Proto encoding is appropriate both as a data interchange format and an in-memory representation, so
once your structure is built, you can simply write the bytes straight out to disk!

But doesn’t that mean the encoding is platform-specific?

NO! The encoding is defined byte-for-byte independent of any platform. However, it is designed to
be efficiently manipulated on common modern CPUs. Data is arranged like a compiler would arrange a
struct – with fixed widths, fixed offsets, and proper alignment. Variable-sized elements are
embedded as pointers. Pointers are offset-based rather than absolute so that messages are
position-independent. Integers use little-endian byte order because most CPUs are little-endian,
and even big-endian CPUs usually have instructions for reading little-endian data.

Doesn’t that make backwards-compatibility hard?

Not at all! New fields are always added to the end of a struct (or replace padding space), so
existing field positions are unchanged. The recipient simply needs to do a bounds check when
reading each field. Fields are numbered in the order in which they were added, so Cap’n Proto
always knows how to arrange them for backwards-compatibility.

Won’t fixed-width integers, unset optional fields, and padding waste space on the wire?

Yes. However, since all these extra bytes are zeros, when bandwidth matters, we can apply