Documentation/technical/packfile-uri.txt

   1 Packfile URIs
   2 =============
   3
   4 This feature allows servers to serve part of their packfile response as URIs.
   5 This allows server designs that improve scalability in bandwidth and CPU usage
   6 (for example, by serving some data through a CDN), and (in the future) provides
   7 some measure of resumability to clients.
   8
   9 This feature is available only in protocol version 2.
  10
  11 Protocol
  12 --------
  13
  14 The server advertises the `packfile-uris` capability.
  15
  16 If the client then communicates which protocols (HTTPS, etc.) it supports with
  17 a `packfile-uris` argument, the server MAY send a `packfile-uris` section
  18 directly before the `packfile` section (right after `wanted-refs` if it is
  19 sent) containing URIs of any of the given protocols. The URIs point to
  20 packfiles that use only features that the client has declared that it supports
  21 (e.g. ofs-delta and thin-pack). See protocol-v2.txt for the documentation of
  22 this section.
  23
  24 Clients should then download and index all the given URIs (in addition to
  25 downloading and indexing the packfile given in the `packfile` section of the
  26 response) before performing the connectivity check.
  27
  28 Server design
  29 -------------
  30
  31 The server can be trivially made compatible with the proposed protocol by
  32 having it advertise `packfile-uris`, tolerating the client sending
  33 `packfile-uris`, and never sending any `packfile-uris` section. But we should
  34 include some sort of non-trivial implementation in the Minimum Viable Product,
  35 at least so that we can test the client.
  36
  37 This is the implementation: a feature, marked experimental, that allows the
  38 server to be configured by one or more `uploadpack.blobPackfileUri=<sha1>
  39 <uri>` entries. Whenever the list of objects to be sent is assembled, all such
  40 blobs are excluded, replaced with URIs. The client will download those URIs,
  41 expecting them to each point to packfiles containing single blobs.
  42
  43 Client design
  44 -------------
  45
  46 The client has a config variable `fetch.uriprotocols` that determines which
  47 protocols the end user is willing to use. By default, this is empty.
  48
  49 When the client downloads the given URIs, it should store them with "keep"
  50 files, just like it does with the packfile in the `packfile` section. These
  51 additional "keep" files can only be removed after the refs have been updated -
  52 just like the "keep" file for the packfile in the `packfile` section.
  53
  54 The division of work (initial fetch + additional URIs) introduces convenient
  55 points for resumption of an interrupted clone - such resumption can be done
  56 after the Minimum Viable Product (see "Future work").
  57
  58 Future work
  59 -----------
  60
  61 The protocol design allows some evolution of the server and client without any
  62 need for protocol changes, so only a small-scoped design is included here to
  63 form the MVP. For example, the following can be done:
  64
  65  * On the server, more sophisticated means of excluding objects (e.g. by
  66    specifying a commit to represent that commit and all objects that it
  67    references).
  68  * On the client, resumption of clone. If a clone is interrupted, information
  69    could be recorded in the repository's config and a "clone-resume" command
  70    can resume the clone in progress. (Resumption of subsequent fetches is more
  71    difficult because that must deal with the user wanting to use the repository
  72    even after the fetch was interrupted.)
  73
  74 There are some possible features that will require a change in protocol:
  75
  76  * Additional HTTP headers (e.g. authentication)
  77  * Byte range support
  78  * Different file formats referenced by URIs (e.g. raw object)