Re: [BuildStream] CAS server resource names & instance names

[Jürg Billeter <j bitron ch>]

On Mon, 2018-12-10 at 09:08 +0000, Daniel Silverstone via BuildStream-list wrote:
> On Fri, Dec 07, 2018 at 17:32:37 +0000, Jim MacArthur via BuildStream-list wrote:
> > So, with the default instance_name of a blank string, we should have a
> > leading slash on the resource name. If we add this in the client, the server
> > will reject it at the moment. It's a similar situation for writing blobs.
> >
> > It looks like the CAS server in BuildGrid already handles both cases. Is
> > there anything important I'm missing here, or can I go ahead and correct the
> > resource names in BuildStream?
>
> Slightly earlier in the same proto, there is:
>
> // A single server MAY support multiple instances of the execution system, each 
> // with their own workers, storage, cache, etc. The exact relationship between 
> // instances is up to the server. If the server does, then the `instance_name` 
> // is an identifier, possibly containing multiple path segments, used to 
> // distinguish between the various instances on the server, in a manner defined 
> // by the server. For servers which do not support multiple instances, then the 
> // `instance_name` is the empty path and the leading slash is omitted, so that 
> // the `resource_name` becomes `uploads/{uuid}/blobs/{hash}/{size}`.
>
> Which I would project to "if `instance_name` is the empty path, the blob URI
> does not need to start with a `/`" as well.

This matches my interpretation of the spec. It also matches what the
Bazel client does [1].

> I imagine BuildGrid is handling both no leading slash, and a leading slash,
> purely as a convenience for how this specification *could* be read both ways.
>
> From my PoV, I'd probably go for a combination approach and update the CAS server
> in BuildStream to support both, but not change the client since the way I read the
> spec, the client is correct currently.

I would not change anything, client or server side.

Cheers,
Jürg

[1] 
https://github.com/bazelbuild/bazel/blob/4392ba43bb49414feb4cbf6bdb32b27e7f1e7fde/src/main/java/com/google/devtools/build/lib/remote/ByteStreamUploader.java#L412