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********
Revfiles
********
The unit for compressed storage in bzr is a *revfile*, whose design
was suggested by Matt Mackall.
Requirements
============
Compressed storage is a tradeoff between several goals:
* Reasonably compact storage of long histories.
* Robustness and simplicity.
* Fast extraction of versions and addition of new versions (preferably
without rewriting the whole file, or reading the whole history.)
* Fast and precise annotations.
* Storage of files of at least a few hundred MB.
Design
======
revfiles store the history of a single logical file, which is
identified in bzr by its file-id. In this sense they are similar to
an RCS or CVS ``,v`` file or an SCCS sfile.
Each state of the file is called a *text*.
Renaming, adding and deleting this file is handled at a higher level
by the inventory system, and is outside the scope of the revfile. The
revfile name is typically based on the file id which is itself
typically based on the name the file had when it was first added. But
this is purely cosmetic.
For example a file now called ``frob.c`` may have the id
``frobber.c-12873`` because it was originally called
``frobber.c``. Its texts are kept in the revfile
``.bzr/revfiles/frobber.c-12873.revs``.
When the file is deleted from the inventory the revfile does not
change. It's just not used in reproducing trees from that point
onwards.
The revfile does not record the date when the text was added, a commit
message, properties, or any other metadata. That is handled in the
higher-level revision history.
Inventories and other metadata files that vary from one version to the
next can themselves be stored in revfiles.
revfiles store files as simple byte streams, with no consideration of
translating character sets, line endings, or keywords. Those are also
handled at a higher level. However, the revfile may make use of
knowledge that a file is line-based in generating a diff.
(The Python builtin difflib is too slow when generating a purely
byte-by-byte delta so we always make a line-by-line diff; when this
is fixed it may be feasible to use line-by-line diffs for all
files.)
Files whose text does not change from one revision to the next are
stored as just a single text in the revfile. This can happen even if
the file was renamed or other properties were changed in the
inventory.
Skip-deltas
-----------
Because the basis of a delta does not need to be the text's logical
predecessor, we can adjust the deltas
Annotations
-----------
Storing
Open issues
===========
* revfiles use unsigned 32-bit integers both in diffs and the index.
This should be more than enough for any reasonable source file but
perhaps not enough for large binaries that are frequently committed.
Perhaps for those files there should be an option to continue to use
the text-store. There is unlikely to be any benefit in holding
deltas between them, and deltas will anyhow be hard to calculate.
* The append-only design does not allow for destroying committed data,
as when confidential information is accidentally added. That could
be fixed by creating the fixed repository as a separate branch, into
which only the preserved revisions are exported.
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