git-annex/doc/internals.mdwn

In the world of git, we're not scared about internal implementation
details, and sometimes we like to dive in and tweak things by hand. Here's
some documentation to that end.

## `.git/annex/objects/aa/bb/*/*`

This is where locally available file contents are actually stored.
Files added to the annex get a symlink checked into git that points
to the file content.

First there are two levels of directories used for hashing, to prevent
too many things ending up in any one directory.
See [[hashing]] for details.

Each subdirectory has the [[name_of_a_key|key_format]] in one of the
[[key-value_backends|backends]]. The file inside also has the name of the key.
This two-level structure is used because it allows the write bit to be removed
from the subdirectories as well as from the files. That prevents accidentially
deleting or changing the file contents.

In [[direct_mode]], file contents are not stored in here, and instead
are stored directly in the file. However, the same symlinks are still
committed to git, internally.

Also in [[direct_mode]], some additional data is stored in these directories.
`.cache` files contain cached file stats used in detecting when a file has
changed, and `.map` files contain a list of file(s) in the work directory
that contain the key.

## The git-annex branch

This branch is managed by git-annex, with the contents listed below.

The file `.git/annex/index` is a separate git index file it uses
to accumulate changes for the git-annex branch.
Also, `.git/annex/journal/` is used to record changes before they
are added to git.

### `uuid.log`

Records the UUIDs of known repositories, and associates them with a
description of the repository. This allows git-annex to display something
more useful than a UUID when it refers to a repository that does not have
a configured git remote pointing at it.

The file format is simply one line per repository, with the uuid followed by a
space and then the description, followed by a timestamp. Example:

	e605dca6-446a-11e0-8b2a-002170d25c55 laptop timestamp=1317929189.157237s
	26339d22-446b-11e0-9101-002170d25c55 usb disk timestamp=1317929330.769997s

If there are multiple lines for the same uuid, the one with the most recent
timestamp wins. git-annex union merges this and other files.

## `remote.log`

Holds persistent configuration settings for [[special_remotes]] such as
Amazon S3.

The file format is one line per remote, starting with the uuid of the
remote, followed by a space, and then a series of var=value pairs,
each separated by whitespace, and finally a timestamp.

Encrypted special remotes store their encryption key here,
in the "cipher" value. It is base64 encoded, and unless shared [[encryption]]
is used, is encrypted to one or more gpg keys. The first 256 bytes of
the cipher is used as the HMAC SHA1 encryption key, to encrypt filenames
stored on the special remote. The remainder of the cipher is used as a gpg
symmetric encryption key, to encrypt the content of files stored on the special
remote.

## `trust.log`

Records the [[trust]] information for repositories. Does not exist unless
[[trust]] values are configured.

The file format is one line per repository, with the uuid followed by a
space, and then either `1` (trusted), `0` (untrusted), `?` (semi-trusted),
`X` (dead) and finally a timestamp.

Example:

	e605dca6-446a-11e0-8b2a-002170d25c55 1 timestamp=1317929189.157237s
	26339d22-446b-11e0-9101-002170d25c55 ? timestamp=1317929330.769997s

Repositories not listed are semi-trusted.

## `group.log`

Used to group repositories together.

The file format is one line per repository, with the uuid followed by a space,
and then a space-separated list of groups this repository is part of,
and finally a timestamp.

## `preferred-content.log`

Used to indicate which repositories prefer to contain which file contents.

The file format is one line per repository, with the uuid followed by a space,
then a boolean expression, and finally a timestamp.

Files matching the expression are preferred to be retained in the
repository, while files not matching it are preferred to be stored
somewhere else.

## `aaa/bbb/*.log`

These log files record [[location_tracking]] information
for file contents. Again these are placed in two levels of subdirectories
for hashing. See [[hashing]] for details.

The name of the key is the filename, and the content
consists of a timestamp, either 1 (present) or 0 (not present), and
the UUID of the repository that has or lacks the file content.

Example:

	1287290776.765152s 1 e605dca6-446a-11e0-8b2a-002170d25c55
	1287290767.478634s 0 26339d22-446b-11e0-9101-002170d25c55

These files are designed to be auto-merged using git's [[union merge driver|git-union-merge]].
The timestamps allow the most recent information to be identified.

## `aaa/bbb/*.log.web`

These log files record urls used by the
[[web_special_remote|special_remotes/web]]. Their format is similar
to the location tracking files, but with urls rather than UUIDs.