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

[Daniel Silverstone <daniel silverstone codethink co uk>]

On Fri, Dec 07, 2018 at 17:32:37 +0000, Jim MacArthur via BuildStream-list wrote:
> However, the spec says:
>
> // For downloading blobs, the client must use the
> // [Read method][google.bytestream.ByteStream.Read] of the ByteStream API,
> with
> // a `resource_name` of `"{instance_name}/blobs/{hash}/{size}"`, where
> // `instance_name` is the instance name (see above), and `hash` and `size`
> are
> // the [Digest][build.bazel.remote.execution.v2.Digest] of the blob.
>
> 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.

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.

D.

-- 
Daniel Silverstone                          https://www.codethink.co.uk/
Solutions Architect               GPG 4096/R Key Id: 3CCE BABE 206C 3B69