bfc2a36ff2
Clarify that, when the packfile-uri feature is used, the client should not assume that the extra packfiles downloaded would only contain a single blob, but support packfiles containing multiple objects of all types. Signed-off-by: Jonathan Tan <jonathantanmy@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
82 lines
3.6 KiB
Plaintext
82 lines
3.6 KiB
Plaintext
Packfile URIs
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=============
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This feature allows servers to serve part of their packfile response as URIs.
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This allows server designs that improve scalability in bandwidth and CPU usage
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(for example, by serving some data through a CDN), and (in the future) provides
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some measure of resumability to clients.
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This feature is available only in protocol version 2.
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Protocol
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--------
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The server advertises the `packfile-uris` capability.
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If the client then communicates which protocols (HTTPS, etc.) it supports with
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a `packfile-uris` argument, the server MAY send a `packfile-uris` section
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directly before the `packfile` section (right after `wanted-refs` if it is
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sent) containing URIs of any of the given protocols. The URIs point to
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packfiles that use only features that the client has declared that it supports
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(e.g. ofs-delta and thin-pack). See protocol-v2.txt for the documentation of
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this section.
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Clients should then download and index all the given URIs (in addition to
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downloading and indexing the packfile given in the `packfile` section of the
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response) before performing the connectivity check.
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Server design
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-------------
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The server can be trivially made compatible with the proposed protocol by
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having it advertise `packfile-uris`, tolerating the client sending
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`packfile-uris`, and never sending any `packfile-uris` section. But we should
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include some sort of non-trivial implementation in the Minimum Viable Product,
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at least so that we can test the client.
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This is the implementation: a feature, marked experimental, that allows the
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server to be configured by one or more `uploadpack.blobPackfileUri=<sha1>
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<uri>` entries. Whenever the list of objects to be sent is assembled, all such
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blobs are excluded, replaced with URIs. As noted in "Future work" below, the
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server can evolve in the future to support excluding other objects (or other
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implementations of servers could be made that support excluding other objects)
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without needing a protocol change, so clients should not expect that packfiles
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downloaded in this way only contain single blobs.
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Client design
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-------------
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The client has a config variable `fetch.uriprotocols` that determines which
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protocols the end user is willing to use. By default, this is empty.
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When the client downloads the given URIs, it should store them with "keep"
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files, just like it does with the packfile in the `packfile` section. These
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additional "keep" files can only be removed after the refs have been updated -
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just like the "keep" file for the packfile in the `packfile` section.
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The division of work (initial fetch + additional URIs) introduces convenient
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points for resumption of an interrupted clone - such resumption can be done
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after the Minimum Viable Product (see "Future work").
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Future work
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-----------
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The protocol design allows some evolution of the server and client without any
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need for protocol changes, so only a small-scoped design is included here to
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form the MVP. For example, the following can be done:
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* On the server, more sophisticated means of excluding objects (e.g. by
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specifying a commit to represent that commit and all objects that it
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references).
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* On the client, resumption of clone. If a clone is interrupted, information
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could be recorded in the repository's config and a "clone-resume" command
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can resume the clone in progress. (Resumption of subsequent fetches is more
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difficult because that must deal with the user wanting to use the repository
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even after the fetch was interrupted.)
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There are some possible features that will require a change in protocol:
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* Additional HTTP headers (e.g. authentication)
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* Byte range support
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* Different file formats referenced by URIs (e.g. raw object)
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