path: root/Documentation/git-fast-import.txt

git-fast-import(1)
==================

NAME
----
git-fast-import - Backend for fast Git data importers


SYNOPSIS
--------
frontend | 'git-fast-import' [options]

DESCRIPTION
-----------
This program is usually not what the end user wants to run directly.
Most end users want to use one of the existing frontend programs,
which parses a specific type of foreign source and feeds the contents
stored there to git-fast-import.

fast-import reads a mixed command/data stream from standard input and
writes one or more packfiles directly into the current repository.
When EOF is received on standard input, fast import writes out
updated branch and tag refs, fully updating the current repository
with the newly imported data.

The fast-import backend itself can import into an empty repository (one that
has already been initialized by gitlink:git-init[1]) or incrementally
update an existing populated repository.  Whether or not incremental
imports are supported from a particular foreign source depends on
the frontend program in use.


OPTIONS
-------
--date-format=<fmt>::
	Specify the type of dates the frontend will supply to
	fast-import within `author`, `committer` and `tagger` commands.
	See ``Date Formats'' below for details about which formats
	are supported, and their syntax.

--force::
	Force updating modified existing branches, even if doing
	so would cause commits to be lost (as the new commit does
	not contain the old commit).

--max-pack-size=<n>::
	Maximum size of each output packfile, expressed in MiB.
	The default is 4096 (4 GiB) as that is the maximum allowed
	packfile size (due to file format limitations). Some
	importers may wish to lower this, such as to ensure the
	resulting packfiles fit on CDs.

--depth=<n>::
	Maximum delta depth, for blob and tree deltification.
	Default is 10.

--active-branches=<n>::
	Maximum number of branches to maintain active at once.
	See ``Memory Utilization'' below for details.  Default is 5.

--export-marks=<file>::
	Dumps the internal marks table to <file> when complete.
	Marks are written one per line as `:markid SHA-1`.
	Frontends can use this file to validate imports after they
	have been completed, or to save the marks table across
	incremental runs.  As <file> is only opened and truncated
	at checkpoint (or completion) the same path can also be
	safely given to \--import-marks.

--import-marks=<file>::
	Before processing any input, load the marks specified in
	<file>.  The input file must exist, must be readable, and
	must use the same format as produced by \--export-marks.
	Multiple options may be supplied to import more than one
	set of marks.  If a mark is defined to different values,
	the last file wins.

--export-pack-edges=<file>::
	After creating a packfile, print a line of data to
	<file> listing the filename of the packfile and the last
	commit on each branch that was written to that packfile.
	This information may be useful after importing projects
	whose total object set exceeds the 4 GiB packfile limit,
	as these commits can be used as edge points during calls
	to gitlink:git-pack-objects[1].

--quiet::
	Disable all non-fatal output, making fast-import silent when it
	is successful.	This option disables the output shown by
	\--stats.

--stats::
	Display some basic statistics about the objects fast-import has
	created, the packfiles they were stored into, and the
	memory used by fast-import during this run.  Showing this output
	is currently the default, but can be disabled with \--quiet.


Performance
-----------
The design of fast-import allows it to import large projects in a minimum
amount of memory usage and processing time.  Assuming the frontend
is able to keep up with fast-import and feed it a constant stream of data,
import times for projects holding 10+ years of history and containing
100,000+ individual commits are generally completed in just 1-2
hours on quite modest (~$2,000 USD) hardware.

Most bottlenecks appear to be in foreign source data access (the
source just cannot extract revisions fast enough) or disk IO (fast-import
writes as fast as the disk will take the data).  Imports will run
faster if the source data is stored on a different drive than the
destination Git repository (due to less IO contention).


Development Cost
----------------
A typical frontend for fast-import tends to weigh in at approximately 200
lines of Perl/Python/Ruby code.  Most developers have been able to
create working importers in just a couple of hours, even though it
is their first exposure to fast-import, and sometimes even to Git.  This is
an ideal situation, given that most conversion tools are throw-away
(use once, and never look back).


Parallel Operation
------------------
Like `git-push` or `git-fetch`, imports handled by fast-import are safe to
run alongside parallel `git repack -a -d` or `git gc` invocations,
or any other Git operation (including `git prune`, as loose objects
are never used by fast-import).

fast-import does not lock the branch or tag refs it is actively importing.
After the import, during its ref update phase, fast-import tests each
existing branch ref to verify the update will be a fast-forward
update (the commit stored in the ref is contained in the new
history of the commit to be written).  If the update is not a
fast-forward update, fast-import will skip updating that ref and instead
prints a warning message.  fast-import will always attempt to update all
branch refs, and does not stop on the first failure.

Branch updates can be forced with \--force, but its recommended that
this only be used on an otherwise quiet repository.  Using \--force
is not necessary for an initial import into an empty repository.


Technical Discussion
--------------------
fast-import tracks a set of branches in memory.  Any branch can be created
or modified at any point during the import process by sending a
`commit` command on the input stream.  This design allows a frontend
program to process an unlimited number of branches simultaneously,
generating commits in the order they are available from the source
data.  It also simplifies the frontend programs considerably.

fast-import does not use or alter the current working directory, or any
file within it.  (It does however update the current Git repository,
as referenced by `GIT_DIR`.)  Therefore an import frontend may use
the working directory for its own purposes, such as extracting file
revisions from the foreign source.  This ignorance of the working
directory also allows fast-import to run very quickly, as it does not
need to perform any costly file update operations when switching
between branches.

Input Format
------------
With the exception of raw file data (which Git does not interpret)
the fast-import input format is text (ASCII) based.  This text based
format simplifies development and debugging of frontend programs,
especially when a higher level language such as Perl, Python or
Ruby is being used.

fast-import is very strict about its input.  Where we say SP below we mean
*exactly* one space.  Likewise LF means one (and only one) linefeed.
Supplying additional whitespace characters will cause unexpected
results, such as branch names or file names with leading or trailing
spaces in their name, or early termination of fast-import when it encounters
unexpected input.

Date Formats
~~~~~~~~~~~~
The following date formats are supported.  A frontend should select
the format it will use for this import by passing the format name
in the \--date-format=<fmt> command line option.

`raw`::
	This is the Git native format and is `<time> SP <offutc>`.
	It is also fast-import's default format, if \--date-format was
	not specified.
+
The time of the event is specified by `<time>` as the number of
seconds since the UNIX epoch (midnight, Jan 1, 1970, UTC) and is
written as an ASCII decimal integer.
+
The local offset is specified by `<offutc>` as a positive or negative
offset from UTC.  For example EST (which is 5 hours behind UTC)
would be expressed in `<tz>` by ``-0500'' while UTC is ``+0000''.
The local offset does not affect `<time>`; it is used only as an
advisement to help formatting routines display the timestamp.
+
If the local offset is not available in the source material, use
``+0000'', or the most common local offset.  For example many
organizations have a CVS repository which has only ever been accessed
by users who are located in the same location and timezone.  In this
case a reasonable offset from UTC could be assumed.
+
Unlike the `rfc2822` format, this format is very strict.  Any
variation in formatting will cause fast-import to reject the value.

`rfc2822`::
	This is the standard email format as described by RFC 2822.
+
An example value is ``Tue Feb 6 11:22:18 2007 -0500''.  The Git
parser is accurate, but a little on the lenient side.  It is the
same parser used by gitlink:git-am[1] when applying patches
received from email.
+
Some malformed strings may be accepted as valid dates.  In some of
these cases Git will still be able to obtain the correct date from
the malformed string.  There are also some types of malformed
strings which Git will parse wrong, and yet consider valid.
Seriously malformed strings will be rejected.
+
Unlike the `raw` format above, the timezone/UTC offset information
contained in an RFC 2822 date string is used to adjust the date
value to UTC prior to storage.  Therefore it is important that
this information be as accurate as possible.
+
If the source material uses RFC 2822 style dates,
the frontend should let fast-import handle the parsing and conversion
(rather than attempting to do it itself) as the Git parser has
been well tested in the wild.
+
Frontends should prefer the `raw` format if the source material
already uses UNIX-epoch format, can be coaxed to give dates in that
format, or its format is easiliy convertible to it, as there is no
ambiguity in parsing.

`now`::
	Always use the current time and timezone.  The literal
	`now` must always be supplied for `<when>`.
+
This is a toy format.  The current time and timezone of this system
is always copied into the identity string at the time it is being
created by fast-import.  There is no way to specify a different time or
timezone.
+
This particular format is supplied as its short to implement and
may be useful to a process that wants to create a new commit
right now, without needing to use a working directory or
gitlink:git-update-index[1].
+
If separate `author` and `committer` commands are used in a `commit`
the timestamps may not match, as the system clock will be polled
twice (once for each command).  The only way to ensure that both
author and committer identity information has the same timestamp
is to omit `author` (thus copying from `committer`) or to use a
date format other than `now`.

Commands
~~~~~~~~
fast-import accepts several commands to update the current repository
and control the current import process.  More detailed discussion
(with examples) of each command follows later.

`commit`::
	Creates a new branch or updates an existing branch by
	creating a new commit and updating the branch to point at
	the newly created commit.

`tag`::
	Creates an annotated tag object from an existing commit or
	branch.  Lightweight tags are not supported by this command,
	as they are not recommended for recording meaningful points
	in time.

`reset`::
	Reset an existing branch (or a new branch) to a specific
	revision.  This command must be used to change a branch to
	a specific revision without making a commit on it.

`blob`::
	Convert raw file data into a blob, for future use in a
	`commit` command.  This command is optional and is not
	needed to perform an import.

`checkpoint`::
	Forces fast-import to close the current packfile, generate its
	unique SHA-1 checksum and index, and start a new packfile.
	This command is optional and is not needed to perform
	an import.

`commit`
~~~~~~~~
Create or update a branch with a new commit, recording one logical
change to the project.

....
	'commit' SP <ref> LF
	mark?
	('author' SP <name> SP LT <email> GT SP <when> LF)?
	'committer' SP <name> SP LT <email> GT SP <when> LF
	data
	('from' SP <committish> LF)?
	('merge' SP <committish> LF)?
	(filemodify | filedelete | filedeleteall)*
	LF
....

where `<ref>` is the name of the branch to make the commit on.
Typically branch names are prefixed with `refs/heads/` in
Git, so importing the CVS branch symbol `RELENG-1_0` would use
`refs/heads/RELENG-1_0` for the value of `<ref>`.  The value of
`<ref>` must be a valid refname in Git.  As `LF` is not valid in
a Git refname, no quoting or escaping syntax is supported here.

A `mark` command may optionally appear, requesting fast-import to save a
reference to the newly created commit for future use by the frontend
(see below for format).  It is very common for frontends to mark
every commit they create, thereby allowing future branch creation
from any imported commit.

The `data` command following `committer` must supply the commit
message (see below for `data` command syntax).  To import an empty
commit message use a 0 length data.  Commit messages are free-form
and are not interpreted by Git.  Currently they must be encoded in
UTF-8, as fast-import does not permit other encodings to be specified.

Zero or more `filemodify`, `filedelete` and `filedeleteall` commands
may be included to update the contents of the branch prior to
creating the commit.  These commands may be supplied in any order.
However it is recommended that a `filedeleteall` command preceed
all `filemodify` commands in the same commit, as `filedeleteall`
wipes the branch clean (see below).

`author`
^^^^^^^^
An `author` command may optionally appear, if the author information
might differ from the committer information.  If `author` is omitted
then fast-import will automatically use the committer's information for
the author portion of the commit.  See below for a description of
the fields in `author`, as they are identical to `committer`.

`committer`
^^^^^^^^^^^
The `committer` command indicates who made this commit, and when
they made it.

Here `<name>` is the person's display name (for example
``Com M Itter'') and `<email>` is the person's email address
(``cm@example.com'').  `LT` and `GT` are the literal less-than (\x3c)
and greater-than (\x3e)