~bzr-pqm/bzr/bzr.dev

*****************

Bazaar-NG formats

*****************

.. contents::

Since branches are working directories there is just a single

directory format.

There is one metadata directory called ``.bzr`` at the top of each

tree.  Control files inside ``.bzr`` are never touched by patches and

should not normally be edited by the user.

These files are designed so that repository-level operations are ACID

without depending on atomic operations spanning multiple files.  There

are two particular cases: aborting a transaction in the middle, and

contention from multiple processes.  We also need to be careful to

flush files to disk at appropriate points; even this may not be

totally safe if the filesystem does not guarantee ordering between

multiple file changes, so we need to be sure to roll back.

The design must also be such that the directory can simply be copied

and that hardlinked directories will work.  (So we must always replace

files, never just append.)

A cache is kept under here of easily-accessible information about

previous revisions.  This should be under a single directory so that

it can be easily identified, excluded from backups, removed, etc.

This might contain pristine tree from previous revisions, manifests

and inventories, etc.  It might also contain working directories when

building a commit, etc.  Call this maybe ``cache`` or ``tmp``.

I wonder if we should use .zip files for revisions and cacherevs

rather than tar files so that random access is easier/more efficient.

There is a Python library ``zipfile``.

Signing XML files

*****************

bzr relies on storing hashes or GPG signatures of various XML files.

There can be multiple equivalent representations of the same XML tree,

but these will have different byte-by-byte hashes.

Once signed files are written out, they must be stored byte-for-byte

and never re-encoded or renormalized, because that would break their

hash or signature.

Branch metadata

***************

All inside ``.bzr``

``README``

  Tells people not to touch anything here.

``branch-format``

  Identifies the parent as a Bazaar-NG branch; contains the overall

  branch metadata format as a string.

``pristine-directory``

  Identifies that this is a pristine directory and may not be

  committed to.

``patches/``

  Directory containing all patches applied to this branch, one per

  file.  Patches are stored as compressed deltas.  We also store the

  hash of the delta, hash of the before and after manifests, and

  optionally a GPG signature.

``cache/``

  Contains various cached data that can be destroyed and will be

  recreated.  (It should not be modified.)

``cache/pristine/``

  Contains cached full trees for selected previous revisions, used

  when generating diffs, etc.

``cache/inventory/``

  Contains cached inventories of previous revisions.

``cache/snapshot/``

  Contains tarballs of cached revisions of the tree, named by their

  revision id.  These can also be removed, but 

``patch-history``

  File containing the UUIDs of all patches taken in this branch,

  in the order they were taken.

  Each commit adds exactly one line to this file; lines are

  never removed or reordered.

``merged-patches``

  List of foreign patches that have been merged into this branch.

  Must have no entries in common with ``patch-history``.  Commits that

  include merges add to this file; lines are never removed or

  reordered.

``pending-merge-patches``

  List of foreign patches that have been merged and are waiting to be

  committed. 

``branch-name``

  User-qualified name of the branch, for the purpose of describing the

  origin of patches, e.g. ``mbp@sourcefrog.net/distcc--main``.

``friends``

  List of branches from which we have pulled; file containing a list

  of pairs of branch-name and location.

``parent``

  Default pull/push target.

``pending-inventory``

  Mapping from UUIDs to file name in the current working directory.  

``branch-lock``

  Lock held while modifying the branch, to protect against clashing

  updates.

Locking

*******

Is locking a good strategy?  Perhaps somekind of read-copy-update or

seq-lock based mechanism would work better?

If we do use a locking algorithm, is it OK to rely on filesystem

locking or do we need our own mechanism?  I think most hosts should

have reasonable ``flock()`` or equivalent, even on NFS.  One risk is

that on NFS it is easy to have broken locking and not know it, so it

might be better to have something that will fail safe.

Filesystem locks go away if the machine crashes or the process is

terminated; this can be a feature in that we do not need to deal with

stale locks but also a feature in that the lock itself does not

indicate cleanup may be needed.

robertc points out that tla converged on renaming a directory as a

mechanism: this is one thing which is known to be atomic on almost all

filesystems.  Apparently renaming files, creating directories, making

symlinks etc are not good enough.

Delta

*****

XML document plus a bag of patches, expressing the difference between

two revisions.  May be a partial delta.

* list of entries

  * entry

    * parent directory (if any)

    * before-name or null if new

    * after-name or null if deleted

    * uuid

    * type (dir, file, symlink, ...)

    * patch type (patch, full-text, xdelta, ...)

    * patch filename (?)

Inventory

*********

XML document; series of entries.  (Quite similar to the svn

``entries`` file; perhaps should even have that name.)

Stored identified by its hash.

An inventory is stored for recorded revisions, also a

``pending-inventory`` for a working directory.

Revision

********

XML document.  Stored identified by its hash.

committer

  RFC-2822-style name of the committer.  Should match the key used to

  sign the revision.

comment

  multi-line free-form text; whitespace and line breaks preserved

timestamp

  As floating-point seconds since epoch.

precursor

  ID of the previous revision on this branch.  May be absent (null) if

  this is the start of a new branch.

branch name

  Name of the branch to which this was originally committed.    

  (I'm not totally satisfied that this is the right way to do it; the

  results will be a bit weird when a series of revisions pass through

  variously named branches.)

inventory_hash

  Acts as a pointer to the inventory for this revision.

merged-branches

  Revision ids of complete branches merged into this revision.  If a

  revision is listed, that revision and transitively its predecessor

  and all other merged-branches are merged.  This is empty except

  where cherry-picks have occurred.

merged-patches

  Revision ids of cherry-picked patches.  Patches whose branches are

  merged need not be listed here.  Listing a revision ID implies that

  only the change of that particular revision from its predecessor has

  been merged in.   This is empty except where cherry-picks have

  occurred.

The transitive closure avoids Arch's problem of needing to list a

large number of previous revisions.  As ddaa writes:

    Continuation revisions (created by tla tag or baz branch) are associated

    to a patchlog whose New-patches header lists the revisions associated to

    all the patchlogs present in the tree. That was introduced as an

    optimisation so the set of patchlogs in any revision could be determined

    solely by examining the patchlogs of ancestor revisions in the same

    branch. This behaves well as long as the total count of patchlog is

    reasonably small or new branches are not very frequent.

    A continuation revision on $tree currently creates a patchlog of

    about 500K. This patchlog is present in all descendent of the revision,

    and all revisions that merges it.

It may be useful at some times to keep a cache of all the branches, or

all the revisions, present in the history of a branch, so that we do

need to walk the whole history of the branch to build this list.

----

Proposed changes

****************

* Don't store parent-id in all revisions, but rather have <DIRECTORY>

  nodes that contain entries for children?

* Assign an id to the root of the tree, perhaps listed in the top of

  the inventory?