Expand description
Replies out: wire bytes, written once.
This is Y18 and it is a locked decision rather than a preference. A reply is
built directly as the bytes that go on the socket. There is no intermediate
value, no enum to match on later, no boxed trait object, and no second pass
that turns a structure into bytes. Redis 8.8 gained forty percent on SCAN
by fixing exactly this in its own reply path, which is the best available
evidence that the shape matters more than the constant factors inside it.
The other half of Y18 is presizing. A reply whose size is known should
reserve once, before the first byte, from whichever side of the operation is
smaller. Out::reserve and the *_len helpers are there so that a command
can do that arithmetic without writing anything twice.
§The protocol lives here
A command writes a map. Whether that map goes out as RESP3’s % or as
RESP2’s flattened array is this module’s problem and not the command’s. Every
command in the engine is written once, against the richer protocol, and the
downgrade happens in one place where it can be tested. The alternative is a
protocol check in three hundred command implementations, and the failure
mode of that is a command that works on one protocol and not the other.
Structs§
- Out
- A reply buffer for one connection.