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This document describes the key classes and concepts within Bazaar. It is
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intended to be useful to people working on the Bazaar codebase, or to
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people writing plugins.
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people writing plugins. People writing plugins may also like to read the
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guide to `Integrating with Bazaar <integration.html>`_ for some specific recipes.
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There's some overlap between this and the `Core Concepts`_ section of the
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user guide, but this document is targetted to people interested in the
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internals. In particular the user guide doesn't go any deeper than
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"revision", because regular users don't care about lower-level details
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like inventories, but this guide does.
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If you have any questions, or if something seems to be incorrect, unclear
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or missing, please talk to us in ``irc://irc.freenode.net/#bzr``, or write
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to the Bazaar mailing list. To propose a correction or addition to this
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document, send a merge request or new text to the mailing list.
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The current version of this document is available in the file
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``doc/developers/overview.txt`` in the source tree, and available online
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within the developer documentation, <http://doc.bazaar-vcs.org/developers/>.
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Essential Domain Classes
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########################
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The core domain objects within the bazaar model are:
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Transports are explained below. See http://bazaar-vcs.org/Classes/
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for an introduction to the other key classes.
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or missing, please talk to us in ``irc://irc.freenode.net/#bzr``, write to
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the Bazaar mailing list, or simply file a bug report.
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All IDs are globally unique identifiers. Inside bzrlib they are almost
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always represented as UTF-8 encoded bytestrings (i.e. ``str`` objects).
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:Revision IDs: The unique identifier of a single revision, such as
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``pqm@pqm.ubuntu.com-20110201161347-ao76mv267gc1b5v2``
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:File IDs: The unique identifier of a single file.
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By convention, in the bzrlib API, parameters of methods that are expected
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to be IDs (as opposed to keys, revision numbers, or some other handle)
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will end in ``id``, e.g. ``revid`` or ``file_id``.
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Ids may be stored directly or they can be inferred from other
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data. Native Bazaar formats store ids directly; foreign VCS
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support usually generates them somehow. For example, the
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Git commit with SHA ``fb235a3be6372e40ff7f7ebbcd7905a08cb04444``
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is referred to with the revision ID
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``git-v1:fb235a3be6372e40ff7f7ebbcd7905a08cb04444``. IDs are expected
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File ids are unique identifiers for files. There are three slightly different
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categories of file ids.
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Tree file ids are used in the ``Tree`` API and can either be UTF-8 encoded
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bytestrings or tuples of UTF-8 encoded bytestrings. Plain bytestrings
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are considered to be the equivalent of a 1-tuple containing that
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Tree file ids should be considered valid only for a specific tree context.
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Note that this is a stricter interpretation than what the current bzr
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format implementation provides - its file ids are persistent across runs
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For some formats (most notably bzr's own formats) it's possible for the
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implementation to specify the file id to use. In other case the tree
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mandates a specific file id.
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Inventories are specific to the bzr native format and are the main layer
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below the ``Tree`` implementation of bzr. File ids in inventories can
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only be UTF-8 encoded bytestrings. A single Tree object can be associated
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with multiple inventories if there are nested trees.
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Tree file ids for bzr formats are a tuple of inventory file ids for the file
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in question. Each non-last item in the tuple refers to the tree
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reference of an inner tree. The last item in the tuple refers to the
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actual file. This means that lookups of file ids doesn't scale with
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the number of nested trees.
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Inventory file ids are only relevant for native Bazaar formats; foreign
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formats don't use inventories.
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Transform ids are used during tree transform operations (used by e.g. merge).
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The same transform id is expected to be used for two instances of the
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same file. At the moment transform ids are directly derived from file
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ids, but in the future they could be based on other data too (e.g.
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automatic rename detection or format-specific rules).
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A composite of one or more ID elements. E.g. a (file-id, revision-id)
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pair is the key to the "texts" store, but a single element key of
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(revision-id) is the key to the "revisions" store.
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The ``Transport`` layer handles access to local or remote directories.
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Each Transport object acts as a logical connection to a particular
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elsewhere. Information that Transports return, such as from ``list_dir``,
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is also in the form of URL components.
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* `Developer guide to bzrlib transports <transports.html>`_
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* API docs for ``bzrlib.transport.Transport``
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Control directories are responsible for storing versioning data. Each
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control directory (such as ".bzr/") can contain zero or one repositories,
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zero or one working trees and zero or more branches.
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The ``BzrDir`` class is the ``ControlDir`` implementation that is
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responsible for the ".bzr/" directory and its implementation. Plugins
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that provide support for other version control systems can provide
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other subclasses of ``ControlDir``.
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A representation of a directory of files (and other directories and
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symlinks etc). The most important kinds of Tree are:
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:WorkingTree: the files on disk editable by the user
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:RevisionTree: a tree as recorded at some point in the past
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Trees can map file paths to file-ids and vice versa (although trees such
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as WorkingTree may have unversioned files not described in that mapping).
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Trees have an inventory and parents (an ordered list of zero or more
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The implementation of ``Tree`` for Bazaar's own formats is based around
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``Inventory`` objects which describe the shape of the tree. Each tree has
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at least one inventory associated with it, which is available as the
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``root_inventory`` attribute on tree. The tree can have more inventories
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associated with it if there are references to other trees in it. These
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references are indicated with ``tree-reference`` inventory entry at the
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point where the other tree is nested. The tree reference entry contains
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sufficient information for looking up the inventory associated with the
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nested tree. There can be multiple layers of nesting.
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Not each ``Tree`` implementation will necessarily have an associated
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``root_inventory``, as not all implementations of ``Tree`` are based
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around inventories (most notably, implementations of foreign VCS file
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A workingtree is a special type of Tree that's associated with a working
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directory on disk, where the user can directly modify the files.
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* Maintaining a WorkingTree on disk at a file path.
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* Maintaining the basis inventory (the inventory of the last commit done)
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* Maintaining the working inventory.
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* Maintaining the pending merges list.
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* Maintaining the stat cache.
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* Maintaining the last revision the working tree was updated to.
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* Knows where its Branch is located.
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* local access to the working tree
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A Branch is a key user concept - its a single line of history that one or
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more people have been committing to.
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A Branch is responsible for:
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* Holding user preferences that are set in a Branch.
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* Holding the 'tip': the last revision to be committed to this Branch.
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(And the revno of that revision.)
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* Knowing how to open the Repository that holds its history.
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* Allowing write locks to be taken out to prevent concurrent alterations to the branch.
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* URL access to its base directory.
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* A Transport to access its files.
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* A Repository to hold its history.
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Repositories store committed history: file texts, revisions, inventories,
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and graph relationships between them.
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and graph relationships between them. A repository holds a bag of
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revision data that can be pointed to by various branches:
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* Maintains storage of various history data at a URL:
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* Revisions (Must have a matching inventory)
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* Inventories for each Revision. (Must have all the file texts available).
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* Synchronizes concurrent access to the repository by different
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processes. (Most repository implementations use a physical mutex only
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for a short period, and effectively support multiple readers and
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Stacked Repositories
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A repository can be configured to refer to a list of "fallback"
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repositories. If a particular revision is not present in the original
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server exposes the stacked-on URL and the client can open that.
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This section describes the model for how bzr stores its data. The
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representation of that data on disk varies considerable depending on the
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format of the repository (and to a lesser extent the format of the branch
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and working tree), but ultimately the set of objects being represented is
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A branch directly contains:
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* the ID of the current revision that branch (a.k.a. the “tip”)
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* some settings for that branch (the values in “branch.conf”)
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* the set of tags for that branch (not supported in all formats)
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A branch implicitly references:
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* A repository. The repository might be colocated in the same directory
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as the branch, or it might be somewhere else entirely.
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A repository contains:
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A store is a key-value mapping. This says nothing about the layout on
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disk, just that conceptually there are distinct stores, each with a
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separate namespace for the keys. Internally the repository may serialize
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stores in the same file, and/or e.g. apply compression algorithms that
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combine records from separate stores in one block, etc.
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You can consider the repository as a single key space, with keys that look
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like *(store-name, ...)*. For example, *('revisions',
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revision-id)* or *('texts', revision-id, file-id)*.
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Stores revision objects. The keys are GUIDs. The value is a revision
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object (the exact representation on disk depends on the repository
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As described in `Core Concepts`_ a revision describes a snapshot of the
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tree of files and some metadata about them.
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* parent revisions (an ordered sequence of zero or more revision IDs)
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* (and all other revision properties)
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* an inventory ID (that inventory describes the tree contents). Is often
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the same as the revision ID, but doesn't have to be (e.g. if no files
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were changed between two revisions then both revisions will refer to
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Stores inventory objects. The keys are GUIDs. (Footnote: there will
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usually be a revision with the same key in the revision store, but there
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are rare cases where this is not true.)
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An inventory object contains:
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* a set of inventory entries
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An inventory entry has the following attributes
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* a file-id (a GUID, or the special value TREE_ROOT for the root entry of
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inventories created by older versions of bzr)
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* a revision-id, a GUID (generally corresponding to the ID of a
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revision). The combination of (file-id, revision-id) is a key into the
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* a kind: one of file, directory, symlink, tree-reference (tree-reference
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is only supported in unsupported developer formats)
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* parent-id: the file-id of the directory that contains this entry (this
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value is unset for the root of the tree).
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* name: the name of the file/directory/etc in that parent directory
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* executable: a flag indicating if the executable bit is set for that
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An inventory entry will have other attributes, depending on the kind:
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For some more details see `Inventories <inventory.html>`_.
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Stores the contents of individual versions of files. The keys are pairs
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of (file-id, revision-id), and the values are the full content (or
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"text") of a version of a file.
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For consistency/simplicity text records exist for all inventory entries,
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but in general only entries with of kind "file" have interesting records.
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Stores cryptographic signatures of revision contents. The keys match
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those of the revision store.
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.. _Core Concepts: http://doc.bazaar.canonical.com/latest/en/user-guide/core_concepts.html
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