partial-clone.txt

Partial Clone Design Notes
==========================

The "Partial Clone" feature is a performance optimization for Git that
allows Git to function without having a complete copy of the repository.
The goal of this work is to allow Git better handle extremely large
repositories.

During clone and fetch operations, Git downloads the complete contents
and history of the repository.  This includes all commits, trees, and
blobs for the complete life of the repository.  For extremely large
repositories, clones can take hours (or days) and consume 100+GiB of disk
space.

Often in these repositories there are many blobs and trees that the user
does not need such as:

  1. files outside of the user's work area in the tree.  For example, in
     a repository with 500K directories and 3.5M files in every commit,
     we can avoid downloading many objects if the user only needs a
     narrow "cone" of the source tree.

  2. large binary assets.  For example, in a repository where large build
     artifacts are checked into the tree, we can avoid downloading all
     previous versions of these non-mergeable binary assets and only
     download versions that are actually referenced.

Partial clone allows us to avoid downloading such unneeded objects *in
advance* during clone and fetch operations and thereby reduce download
times and disk usage.  Missing objects can later be "demand fetched"
if/when needed.

A remote that can later provide the missing objects is called a
promisor remote, as it promises to send the objects when
requested. Initially Git supported only one promisor remote, the origin
remote from which the user cloned and that was configured in the
"extensions.partialClone" config option. Later support for more than
one promisor remote has been implemented.

Use of partial clone requires that the user be online and the origin
remote or other promisor remotes be available for on-demand fetching
of missing objects.  This may or may not be problematic for the user.
For example, if the user can stay within the pre-selected subset of
the source tree, they may not encounter any missing objects.
Alternatively, the user could try to pre-fetch various objects if they
know that they are going offline.


Non-Goals
---------

Partial clone is a mechanism to limit the number of blobs and trees downloaded
*within* a given range of commits -- and is therefore independent of and not
intended to conflict with existing DAG-level mechanisms to limit the set of
requested commits (i.e. shallow clone, single branch, or fetch '<refspec>').


Design Overview
---------------

Partial clone logically consists of the following parts:

- A mechanism for the client to describe unneeded or unwanted objects to
  the server.

- A mechanism for the server to omit such unwanted objects from packfiles
  sent to the client.

- A mechanism for the client to gracefully handle missing objects (that
  were previously omitted by the server).

- A mechanism for the client to backfill missing objects as needed.


Design Details
--------------

- A new pack-protocol capability "filter" is added to the fetch-pack and
  upload-pack negotiation.
+
This uses the existing capability discovery mechanism.
See "filter" in Documentation/technical/pack-protocol.txt.

- Clients pass a "filter-spec" to clone and fetch which is passed to the
  server to request filtering during packfile construction.
+
There are various filters available to accommodate different situations.
See "--filter=<filter-spec>" in Documentation/rev-list-options.txt.

- On the server pack-objects applies the requested filter-spec as it
  creates "filtered" packfiles for the client.
+
These filtered packfiles are *incomplete* in the traditional sense because
they may contain objects that reference objects not contained in the
packfile and that the client doesn't already have.  For example, the
filtered packfile may contain trees or tags that reference missing blobs
or commits that reference missing trees.

- On the client these incomplete packfiles are marked as "promisor packfiles"
  and treated differently by various commands.

- On the client a repository extension is added to the local config to
  prevent older versions of git from failing mid-operation because of
  missing objects that they cannot handle.
  See "extensions.partialClone" in Documentation/technical/repository-version.txt"


Handling Missing Objects
------------------------

- An object may be missing due to a partial clone or fetch, or missing
  due to repository corruption.  To differentiate these cases, the
  local repository specially indicates such filtered packfiles
  obtained from promisor remotes as "promisor packfiles".
+
These promisor packfiles consist of a "<name>.promisor" file with
arbitrary contents (like the "<name>.keep" files), in addition to
their "<name>.pack" and "<name>.idx" files.

- The local repository considers a "promisor object" to be an object that
  it knows (to the best of its ability) that promisor remotes have promised
  that they have, either because the local repository has that object in one of
  its promisor packfiles, or because another promisor object refers to it.
+
When Git encounters a missing object, Git can see if it is a promisor object
and handle it appropriately.  If not, Git can report a corruption.
+
This means that there is no need for the client to explicitly maintain an
expensive-to-modify list of missing objects.[a]

- Since almost all Git code currently expects any referenced object to be
  present locally and because we do not want to force every command to do
  a dry-run first, a fallback mechanism is added to allow Git to attempt
  to dynamically fetch missing objects from promisor remotes.
+
When the normal object lookup fails to find an object, Git invokes
promisor_remote_get_direct() to try to get the object from a promisor
remote and then retry the object lookup.  This allows objects to be
"faulted in" without complicated prediction algorithms.
+
For efficiency reasons, no check as to whether the missing object is
actually a promisor object is performed.
+
Dynamic object fetching tends to be slow as objects are fetched one at
a time.

- `checkout` (and any other command using `unpack-trees`) has been taught
  to bulk pre-fetch all required missing blobs in a single batch.

- `rev-list` has been taught to print missing objects.
+
This can be used by other commands to bulk prefetch objects.
For example, a "git log -p A..B" may internally want to first do
something like "git rev-list --objects --quiet --missing=print A..B"
and prefetch those objects in bulk.

- `fsck` has been updated to be fully aware of promisor objects.

- `repack` in GC has been updated to not touch promisor packfiles at all,
  and to only repack other objects.

- The global variable "fetch_if_missing" is used to control whether an
  object lookup will attempt to dynamically fetch a missing object or
  report an error.
+
We are not happy with this global variable and would like to remove it,
but that requires significant refactoring of the object code to pass an
additional flag.


Fetching Missing Objects
------------------------

- Fetching of objects is done by invoking a "git fetch" subprocess.

- The local repository sends a request with the hashes of all requested
  objects, and does not perform any packfile negotiation.
  It then receives a packfile.

- Because we are reusing the existing fetch mechanism, fetching
  currently fetches all objects referred to by the requested objects, even
  though they are not necessary.


Using many promisor remotes
---------------------------

Many promisor remotes can be configured and used.

This allows for example a user to have multiple geographically-close
cache servers for fetching missing blobs while continuing to do
filtered `git-fetch` commands from the central server.

When fetching objects, promisor remotes are tried one after the other
until all the objects have been fetched.

Remotes that are considered "promisor" remotes are those specified by
the following configuration variables:

- `extensions.partialClone = <name>`

- `remote.<name>.promisor = true`

- `remote.<name>.partialCloneFilter = ...`

Only one promisor remote can be configured using the
`extensions.partialClone` config variable. This promisor remote will
be the last one tried when fetching objects.

We decided to make it the last one we try, because it is likely that
someone using many promisor remotes is doing so because the other
promisor remotes are better for some reason (maybe they are closer or
faster for some kind of objects) than the origin, and the origin is
likely to be the remote specified by extensions.partialClone.

This justification is not very strong, but one choice had to be made,
and anyway the long term plan should be to make the order somehow
fully configurable.

For now though the other promisor remotes will be tried in the order
they appear in the config file.

Current Limitations
-------------------

- It is not possible to specify the order in which the promisor
  remotes are tried in other ways than the order in which they appear
  in the config file.
+
It is also not possible to specify an order to be used when fetching
from one remote and a different order when fetching from another
remote.

- It is not possible to push only specific objects to a promisor
  remote.
+
It is not possible to push at the same time to multiple promisor
remote in a specific order.

- Dynamic object fetching will only ask promisor remotes for missing
  objects.  We assume that promisor remotes have a complete view of the
  repository and can satisfy all such requests.

- Repack essentially treats promisor and non-promisor packfiles as 2
  distinct partitions and does not mix them.  Repack currently only works
  on non-promisor packfiles and loose objects.

- Dynamic object fetching invokes fetch-pack once *for each item*
  because most algorithms stumble upon a missing object and need to have
  it resolved before continuing their work.  This may incur significant
  overhead -- and multiple authentication requests -- if many objects are
  needed.

- Dynamic object fetching currently uses the existing pack protocol V0
  which means that each object is requested via fetch-pack.  The server
  will send a full set of info/refs when the connection is established.
  If there are large number of refs, this may incur significant overhead.


Future Work
-----------

- Improve the way to specify the order in which promisor remotes are
  tried.
+
For example this could allow to specify explicitly something like:
"When fetching from this remote, I want to use these promisor remotes
in this order, though, when pushing or fetching to that remote, I want
to use those promisor remotes in that order."

- Allow pushing to promisor remotes.
+
The user might want to work in a triangular work flow with multiple
promisor remotes that each have an incomplete view of the repository.

- Allow repack to work on promisor packfiles (while keeping them distinct
  from non-promisor packfiles).

- Allow non-pathname-based filters to make use of packfile bitmaps (when
  present).  This was just an omission during the initial implementation.

- Investigate use of a long-running process to dynamically fetch a series
  of objects, such as proposed in [5,6] to reduce process startup and
  overhead costs.
+
It would be nice if pack protocol V2 could allow that long-running
process to make a series of requests over a single long-running
connection.

- Investigate pack protocol V2 to avoid the info/refs broadcast on
  each connection with the server to dynamically fetch missing objects.

- Investigate the need to handle loose promisor objects.
+
Objects in promisor packfiles are allowed to reference missing objects
that can be dynamically fetched from the server.  An assumption was
made that loose objects are only created locally and therefore should
not reference a missing object.  We may need to revisit that assumption
if, for example, we dynamically fetch a missing tree and store it as a
loose object rather than a single object packfile.
+
This does not necessarily mean we need to mark loose objects as promisor;
it may be sufficient to relax the object lookup or is-promisor functions.


Non-Tasks
---------

- Every time the subject of "demand loading blobs" comes up it seems
  that someone suggests that the server be allowed to "guess" and send
  additional objects that may be related to the requested objects.
+
No work has gone into actually doing that; we're just documenting that
it is a common suggestion.  We're not sure how it would work and have
no plans to work on it.
+
It is valid for the server to send more objects than requested (even
for a dynamic object fetch), but we are not building on that.


Footnotes
---------

[a] expensive-to-modify list of missing objects:  Earlier in the design of
    partial clone we discussed the need for a single list of missing objects.
    This would essentially be a sorted linear list of OIDs that the were
    omitted by the server during a clone or subsequent fetches.

This file would need to be loaded into memory on every object lookup.
It would need to be read, updated, and re-written (like the .git/index)
on every explicit "git fetch" command *and* on any dynamic object fetch.

The cost to read, update, and write this file could add significant
overhead to every command if there are many missing objects.  For example,
if there are 100M missing blobs, this file would be at least 2GiB on disk.

With the "promisor" concept, we *infer* a missing object based upon the
type of packfile that references it.


Related Links
-------------
[0] https://crbug.com/git/2
    Bug#2: Partial Clone

[1] https://lore.kernel.org/git/20170113155253.1644-1-benpeart@microsoft.com/ +
    Subject: [RFC] Add support for downloading blobs on demand +
    Date: Fri, 13 Jan 2017 10:52:53 -0500

[2] https://lore.kernel.org/git/cover.1506714999.git.jonathantanmy@google.com/ +
    Subject: [PATCH 00/18] Partial clone (from clone to lazy fetch in 18 patches) +
    Date: Fri, 29 Sep 2017 13:11:36 -0700

[3] https://lore.kernel.org/git/20170426221346.25337-1-jonathantanmy@google.com/ +
    Subject: Proposal for missing blob support in Git repos +
    Date: Wed, 26 Apr 2017 15:13:46 -0700

[4] https://lore.kernel.org/git/1488999039-37631-1-git-send-email-git@jeffhostetler.com/ +
    Subject: [PATCH 00/10] RFC Partial Clone and Fetch +
    Date: Wed,  8 Mar 2017 18:50:29 +0000

[5] https://lore.kernel.org/git/20170505152802.6724-1-benpeart@microsoft.com/ +
    Subject: [PATCH v7 00/10] refactor the filter process code into a reusable module +
    Date: Fri,  5 May 2017 11:27:52 -0400

[6] https://lore.kernel.org/git/20170714132651.170708-1-benpeart@microsoft.com/ +
    Subject: [RFC/PATCH v2 0/1] Add support for downloading blobs on demand +
    Date: Fri, 14 Jul 2017 09:26:50 -0400