Re: [PATCH v3 12/35] serve: introduce git-serve
flat view
From: Jeff King <hidden>
Date: 2018-02-22 09:33:35
On Tue, Feb 06, 2018 at 05:12:49PM -0800, Brandon Williams wrote:
quoted hunk ↗ jump to hunk
+In protocol v2 communication is command oriented. When first contacting a +server a list of capabilities will advertised. Some of these capabilities +will be commands which a client can request be executed. Once a command +has completed, a client can reuse the connection and request that other +commands be executed.
If I understand this correctly, we'll potentially have a lot more round-trips between the client and server (one per "command"). And for git-over-http, each one will be its own HTTP request? We've traditionally tried to minimize HTTP requests, but I guess it's not too bad if we can keep the connection open in most cases. Then we just suffer some extra framing bytes, but we don't have to re-establish the TCP connection each time. I do wonder if the extra round trips will be noticeable in high-latency conditions. E.g., if I'm 200ms away, converting the current ref-advertisement spew to "capabilities, then the client asks for refs, then we spew the refs" is going to cost an extra 200ms, even if the fetch just ends up being a noop. I'm not sure how bad that is in the grand scheme of things (after all, the TCP handshake involves some round-trips, too).
quoted hunk ↗ jump to hunk
+ Capability Advertisement +-------------------------- + +A server which decides to communicate (based on a request from a client) +using protocol version 2, notifies the client by sending a version string +in its initial response followed by an advertisement of its capabilities. +Each capability is a key with an optional value. Clients must ignore all +unknown keys. Semantics of unknown values are left to the definition of +each key. Some capabilities will describe commands which can be requested +to be executed by the client. + + capability-advertisement = protocol-version + capability-list + flush-pkt + + protocol-version = PKT-LINE("version 2" LF) + capability-list = *capability + capability = PKT-LINE(key[=value] LF) + + key = 1*CHAR + value = 1*CHAR + CHAR = 1*(ALPHA / DIGIT / "-" / "_") + +A client then responds to select the command it wants with any particular +capabilities or arguments. There is then an optional section where the +client can provide any command specific parameters or queries. + + command-request = command + capability-list + (command-args) + flush-pkt + command = PKT-LINE("command=" key LF) + command-args = delim-pkt + *arg + arg = 1*CHAR
For a single stateful TCP connection like git:// or git-over-ssh, the client would get the capabilities once and then issue a series of commands. For git-over-http, how does it work? The client speaks first in HTTP, so we'd first make a request to get just the capabilities from the server? And then proceed from there with a series of requests, assuming that the capabilities for each server we subsequently contact are the same? That's probably reasonable (and certainly the existing http protocol makes that capabilities assumption). I don't see any documentation on how this all works with http. But reading patch 34, it looks like we just do the usual service=git-upload-pack request (with the magic request for v2), and then the server would send us capabilities. Which follows my line of thinking in the paragraph above. -Peff