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=============
Fetching data
=============
Overview of a fetch
===================
Inside bzr, a typical fetch happens like this:
* a user runs a command like ``bzr branch`` or ``bzr pull``
* ``Repository.fetch`` is called (by a higher-level method such as
``ControlDir.sprout``, ``Branch.fetch``, etc).
* An ``InterRepository`` object is created. The exact implementation of
``InterRepository`` chosen depends on the format/capabilities of the
source and target repos.
* The source and target repositories are compared to determine which data
needs to be transferred.
* The repository data is copied. Often this is done by creating a
``StreamSource`` and ``StreamSink`` from the source and target
repositories and feeding the stream from the source into the sink, but
some ``InterRepository`` implementations do differently.
How objects to be transferred are determined
============================================
See ``InterRepository._walk_to_common_revisions``. The basic idea is to
do a breadth-first search in the source repository's revision graph
(starting from the head or heads the caller asked for), and look in the
target repository to see if those revisions are already present.
Eventually this will find the common ancestors in both graphs, and thus
the set of revisions to be copied has been identified.
All inventories for the copied revisions need to be present (and all
parent inventories at the stacking boundary too, to support stacking).
All texts versions introduced by those inventories need to be transferred
(but see also stacking constraints).
Fetch specs
===========
The most ``fetch`` methods accept a ``fetch_spec`` parameter. This is how
the caller controls what is fetched: e.g. all revisions for a given head
(that aren't already present in the target), the full ancestry for one or
more heads, or even the full contents of the source repository.
The ``fetch_spec`` parameter is an object that implements the interface
defined by ``AbstractSearchResult`` in ``bzrlib.graph``. It describes
which keys should be fetched. Current implementations are
``SearchResult``, ``PendingAncestryResult``, ``EmptySearchResult``, and
``EverythingResult``. Some have options controlling if missing revisions
cause errors or not, etc.
There are also some “search” objects, which can be used to conveniently
construct a search result for common cases: ``EverythingNotInOther`` and
``NotInOtherForRevs``. They provide an ``execute`` method that performs
the search and returns a search result.
Also, ``Graph._make_breadth_first_searcher`` returns an object with a
``get_result`` method that returns a search result.
Streams
=======
A **stream** is an iterable of (substream type, substream) pairs.
The **substream type** is a ``str`` that will be one of ``texts``,
``inventories``, ``inventory-deltas``, ``chk_bytes``, ``revisions`` or
``signatures``. A **substream** is a record stream. The format of those
records depends on the repository format being streamed, except for
``inventory-deltas`` records which are format-independent.
A stream source can be constructed with ``repo._get_source(to_format)``,
and it provides a ``get_stream(search)`` method (among others). A stream
sink can be constructed with ``repo._get_sink()``, and provides an
``insert_stream(stream, src_format, resume_tokens)`` method (among
others).
..
vim: ft=rst tw=74 ai
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