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Commit 6ad6d3d3 authored by Linus Torvalds's avatar Linus Torvalds
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Update README to reflect the hierarchical tree objects, 

and other newfangled things like merging.

Also, talk more about the actual operations, and give some
rough examples of what you can do.
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GIT - the stupid content tracker
 
"git" can mean anything, depending on your mood.
 
- random three-letter combination that is pronounceable, and not
@@ -17,127 +20,196 @@ doesn't do a whole lot, but what it _does_ do is track directory
contents efficiently.
 
There are two object abstractions: the "object database", and the
"current directory cache".
"current directory cache" aka "index".
 
The Object Database (SHA1_FILE_DIRECTORY)
 
The object database is literally just a content-addressable collection
of objects. All objects are named by their content, which is
approximated by the SHA1 hash of the object itself. Objects may refer
to other objects (by referencing their SHA1 hash), and so you can build
up a hierarchy of objects.
 
There are several kinds of objects in the content-addressable collection
database. They are all in deflated with zlib, and start off with a tag
of their type, and size information about the data. The SHA1 hash is
always the hash of the _compressed_ object, not the original one.
In particular, the consistency of an object can always be tested
All objects have a statically determined "type" aka "tag", which is
determined at object creation time, and which identifies the format of
the object (ie how it is used, and how it can refer to other objects).
There are currently three different object types: "blob", "tree" and
"commit".
A "blob" object cannot refer to any other object, and is, like the tag
implies, a pure storage object containing some user data. It is used to
actually store the file data, ie a blob object is associated with some
particular version of some file.
A "tree" object is an object that ties one or more "blob" objects into a
directory structure. In addition, a tree object can refer to other tree
objects, thus creating a directory hierarchy.
Finally, a "commit" object ties such directory hierarchies together into
a DAG of revisions - each "commit" is associated with exactly one tree
(the directory hierarchy at the time of the commit). In addition, a
"commit" refers to one or more "parent" commit objects that describe the
history of how we arrived at that directory hierarchy.
As a special case, a commit object with no parents is called the "root"
object, and is the point of an initial project commit. Each project
must have at least one root, and while you can tie several different
root objects together into one project by creating a commit object which
has two or more separate roots as its ultimate parents, that's probably
just going to confuse people. So aim for the notion of "one root object
per project", even if git itself does not enforce that.
Regardless of object type, all objects are share the following
characteristics: they are all in deflated with zlib, and have a header
that not only specifies their tag, but also size information about the
data in the object. It's worth noting that the SHA1 hash that is used
to name the object is always the hash of this _compressed_ object, not
the original data.
As a result, the general consistency of an object can always be tested
independently of the contents or the type of the object: all objects can
be validated by verifying that (a) their hashes match the content of the
file and (b) the object successfully inflates to a stream of bytes that
forms a sequence of <ascii tag without space> + <space> + <ascii decimal
size> + <byte\0> + <binary object data>.
 
BLOB: A "blob" object is nothing but a binary blob of data, and doesn't
refer to anything else. There is no signature or any other verification
of the data, so while the object is consistent (it _is_ indexed by its
sha1 hash, so the data itself is certainly correct), it has absolutely
no other attributes. No name associations, no permissions. It is
purely a blob of data (ie normally "file contents").
TREE: The next hierarchical object type is the "tree" object. A tree
object is a list of permission/name/blob data, sorted by name. In other
words the tree object is uniquely determined by the set contents, and so
two separate but identical trees will always share the exact same
object.
Again, a "tree" object is just a pure data abstraction: it has no
history, no signatures, no verification of validity, except that the
contents are again protected by the hash itself. So you can trust the
contents of a tree, the same way you can trust the contents of a blob,
but you don't know where those contents _came_ from.
Side note on trees: since a "tree" object is a sorted list of
"filename+content", you can create a diff between two trees without
actually having to unpack two trees. Just ignore all common parts, and
your diff will look right. In other words, you can effectively (and
efficiently) tell the difference between any two random trees by O(n)
where "n" is the size of the difference, rather than the size of the
tree.
Side note 2 on trees: since the name of a "blob" depends entirely and
exclusively on its contents (ie there are no names or permissions
involved), you can see trivial renames or permission changes by noticing
that the blob stayed the same. However, renames with data changes need
a smarter "diff" implementation.
The structured objects can further have their structure and connectivity
to other objects verified. This is generally done with the "fsck-cache"
program, which generates a full dependency graph of all objects, and
verifies their internal consistency (in addition to just verifying their
superficial consistency through the hash).
The object types in some more detail:
BLOB: A "blob" object is nothing but a binary blob of data, and
doesn't refer to anything else. There is no signature or any
other verification of the data, so while the object is
consistent (it _is_ indexed by its sha1 hash, so the data itself
is certainly correct), it has absolutely no other attributes.
No name associations, no permissions. It is purely a blob of
data (ie normally "file contents").
In particular, since the blob is entirely defined by its data,
if two files in a directory tree (or in multiple different
versions of the repository) have the same contents, they will
share the same blob object. The object is toally independent
of it's location in the directory tree, and renaming a file does
not change the object that file is associated with in any way.
TREE: The next hierarchical object type is the "tree" object. A tree
object is a list of mode/name/blob data, sorted by name.
Alternatively, the mode data may specify a directory mode, in
which case instead of naming a blob, that name is associated
with another TREE object.
Like the "blob" object, a tree object is uniquely determined by
the set contents, and so two separate but identical trees will
always share the exact same object. This is true at all levels,
ie it's true for a "leaf" tree (which does not refer to any
other trees, only blobs) as well as for a whole subdirectory.
For that reason a "tree" object is just a pure data abstraction:
it has no history, no signatures, no verification of validity,
except that since the contents are again protected by the hash
itself, we can trust that the tree is immutable and its contents
never change.
So you can trust the contents of a tree to be valid, the same
way you can trust the contents of a blob, but you don't know
where those contents _came_ from.
Side note on trees: since a "tree" object is a sorted list of
"filename+content", you can create a diff between two trees
without actually having to unpack two trees. Just ignore all
common parts, and your diff will look right. In other words,
you can effectively (and efficiently) tell the difference
between any two random trees by O(n) where "n" is the size of
the difference, rather than the size of the tree.
Side note 2 on trees: since the name of a "blob" depends
entirely and exclusively on its contents (ie there are no names
or permissions involved), you can see trivial renames or
permission changes by noticing that the blob stayed the same.
However, renames with data changes need a smarter "diff" implementation.
 
CHANGESET: The "changeset" object is an object that introduces the
notion of history into the picture. In contrast to the other objects,
it doesn't just describe the physical state of a tree, it describes how
we got there, and why.
A "changeset" is defined by the tree-object that it results in, the
parent changesets (zero, one or more) that led up to that point, and a
comment on what happened. Again, a changeset is not trusted per se:
the contents are well-defined and "safe" due to the cryptographically
strong signatures at all levels, but there is no reason to believe that
the tree is "good" or that the merge information makes sense. The
parents do not have to actually have any relationship with the result,
for example.
Note on changesets: unlike real SCM's, changesets do not contain rename
information or file mode chane information. All of that is implicit in
the trees involved (the result tree, and the result trees of the
parents), and describing that makes no sense in this idiotic file
manager.
notion of history into the picture. In contrast to the other
objects, it doesn't just describe the physical state of a tree,
it describes how we got there, and why.
A "changeset" is defined by the tree-object that it results in,
the parent changesets (zero, one or more) that led up to that
point, and a comment on what happened. Again, a changeset is
not trusted per se: the contents are well-defined and "safe" due
to the cryptographically strong signatures at all levels, but
there is no reason to believe that the tree is "good" or that
the merge information makes sense. The parents do not have to
actually have any relationship with the result, for example.
Note on changesets: unlike real SCM's, changesets do not contain
rename information or file mode chane information. All of that
is implicit in the trees involved (the result tree, and the
result trees of the parents), and describing that makes no sense
in this idiotic file manager.
 
TRUST: The notion of "trust" is really outside the scope of "git", but
it's worth noting a few things. First off, since everything is hashed
with SHA1, you _can_ trust that an object is intact and has not been
messed with by external sources. So the name of an object uniquely
identifies a known state - just not a state that you may want to trust.
Furthermore, since the SHA1 signature of a changeset refers to the
SHA1 signatures of the tree it is associated with and the signatures
of the parent, a single named changeset specifies uniquely a whole
set of history, with full contents. You can't later fake any step of
the way once you have the name of a changeset.
So to introduce some real trust in the system, the only thing you need
to do is to digitally sign just _one_ special note, which includes the
name of a top-level changeset. Your digital signature shows others that
you trust that changeset, and the immutability of the history of
changesets tells others that they can trust the whole history.
In other words, you can easily validate a whole archive by just sending
out a single email that tells the people the name (SHA1 hash) of the top
changeset, and digitally sign that email using something like GPG/PGP.
In particular, you can also have a separate archive of "trust points" or
tags, which document your (and other peoples) trust. You may, of
course, archive these "certificates of trust" using "git" itself, but
it's not something "git" does for you.
Another way of saying the same thing: "git" itself only handles content
it's worth noting a few things. First off, since everything is
hashed with SHA1, you _can_ trust that an object is intact and
has not been messed with by external sources. So the name of an
object uniquely identifies a known state - just not a state that
you may want to trust.
Furthermore, since the SHA1 signature of a changeset refers to
the SHA1 signatures of the tree it is associated with and the
signatures of the parent, a single named changeset specifies
uniquely a whole set of history, with full contents. You can't
later fake any step of the way once you have the name of a
changeset.
So to introduce some real trust in the system, the only thing
you need to do is to digitally sign just _one_ special note,
which includes the name of a top-level changeset. Your digital
signature shows others that you trust that changeset, and the
immutability of the history of changesets tells others that they
can trust the whole history.
In other words, you can easily validate a whole archive by just
sending out a single email that tells the people the name (SHA1
hash) of the top changeset, and digitally sign that email using
something like GPG/PGP.
In particular, you can also have a separate archive of "trust
points" or tags, which document your (and other peoples) trust.
You may, of course, archive these "certificates of trust" using
"git" itself, but it's not something "git" does for you.
Another way of saying the last point: "git" itself only handles content
integrity, the trust has to come from outside.
 
Current Directory Cache (".git/index")
 
The "current directory cache" is a simple binary file, which contains an
efficient representation of a virtual directory content at some random
time. It does so by a simple array that associates a set of names,
dates, permissions and content (aka "blob") objects together. The cache
is always kept ordered by name, and names are unique at any point in
time, but the cache has no long-term meaning, and can be partially
updated at any time.
 
In particular, the "current directory cache" certainly does not need to
be consistent with the current directory contents, but it has two very
important attributes:
The "index" aka "Current Directory Cache" (".git/index")
The index is a simple binary file, which contains an efficient
representation of a virtual directory content at some random time. It
does so by a simple array that associates a set of names, dates,
permissions and content (aka "blob") objects together. The cache is
always kept ordered by name, and names are unique (with a few very
specific rules) at any point in time, but the cache has no long-term
meaning, and can be partially updated at any time.
In particular, the index certainly does not need to be consistent with
the current directory contents (in fact, most operations will depend on
different ways to make the index _not_ be consistent with the directory
hierarchy), but it has three very important attributes:
 
(a) it can re-generate the full state it caches (not just the directory
structure: through the "blob" object it can regenerate the data too)
structure: it contains pointers to the "blob" objects so that it
can regenerate the data too)
 
As a special case, there is a clear and unambiguous one-way mapping
from a current directory cache to a "tree object", which can be
@@ -146,23 +218,243 @@ important attributes:
one time uniquely specifies one and only one "tree" object (but
has additional data to make it easy to match up that tree object
with what has happened in the directory)
and
 
(b) it has efficient methods for finding inconsistencies between that
cached state ("tree object waiting to be instantiated") and the
current state.
 
Those are the two ONLY things that the directory cache does. It's a
(c) it can additionally efficiently represent information about merge
conflicts between different tree objects, allowing each pathname to
be associated with sufficient information about the trees involved
that you can create a three-way merge between them.
Those are the three ONLY things that the directory cache does. It's a
cache, and the normal operation is to re-generate it completely from a
known tree object, or update/compare it with a live tree that is being
developed. If you blow the directory cache away entirely, you haven't
lost any information as long as you have the name of the tree that it
described.
(But directory caches can also have real information in them: in
particular, they can have the representation of an intermediate tree
that has not yet been instantiated. So they do have meaning and usage
outside of caching - in one sense you can think of the current directory
cache as being the "work in progress" towards a tree commit).
developed. If you blow the directory cache away entirely, you generally
haven't lost any information as long as you have the name of the tree
that it described.
At the same time, the directory index is at the same time also the
staging area for creating new trees, and creating a new tree always
involves a controlled modification of the index file. In particular,
the index file can have the representation of an intermediate tree that
has not yet been instantiated. So the index can be thought of as a
write-back cache, which can contain dirty information that has not yet
been written back to the backing store.
The Workflow
Generally, all "git" operations work on the index file. Some operations
work _purely_ on the index file (showing the current state of the
index), but most operations move data to and from the index file. Either
from the database or from the working directory. Thus there are four
main combinations:
1) working directory -> index
You update the index with information from the working directory
with the "update-cache" command. You generally update the index
information by just specifying the filename you want to update,
like so:
update-cache filename
but to avoid common mistakes with filename globbing etc, the
command will not normally add totally new entries or remove old
entries, ie it will normally just update existing cache entryes.
To tell git that yes, you really do realize that certain files
no longer exist in the archive, or that new files should be
added, you should use the "--remove" and "--add" flags
respectively.
NOTE! A "--remove" flag does _not_ mean that subsequent
filenames will necessarily be removed: if the files still exist
in your directory structure, the index will be updated with
their new status, not removed. The only thing "--remove" means
is that update-cache will be considering a removed file to be a
valid thing, and if the file really does not exist any more, it
will update the index accordingly.
As a special case, you can also do "update-cache --refresh",
which will refresh the "stat" information of each index to match
the current stat information. It will _not_ update the object
status itself, and it wil only update the fields that are used
to quickly test whether an object still matches its old backing
store object.
2) index -> object database
You write your current index file to a "tree" object with the
program
write-tree
that doesn't come with any options - it will just write out the
current index into the set of tree objects that describe that
state, and it will return the name of the resulting top-level
tree. You can use that tree to re-generate the index at any time
by going in the other direction:
3) object database -> index
You read a "tree" file from the object database, and use that to
populate (and overwrite - don't do this if your index contains
any unsaved state that you might want to restore later!) your
current index. Normal operation is just
read-tree <sha1 of tree>
and your index file will now be equivalent to the tree that you
saved earlier. However, that is only your _index_ file: your
working directory contents have not been modified.
4) index -> working directory
You update your working directory from the index by "checking
out" files. This is not a very common operation, since normally
you'd just keep your files updated, and rather than write to
your working directory, you'd tell the index files about the
changes in your working directory (ie "update-cache").
However, if you decide to jump to a new version, or check out
somebody elses version, or just restore a previous tree, you'd
populate your index file with read-tree, and then you need to
check out the result with
checkout-cache filename
or, if you want to check out all of the index, use "-a".
NOTE! checkout-cache normally refuses to overwrite old files, so
if you have an old version of the tree already checked out, you
will need to use the "-f" flag (_before_ the "-a" flag or the
filename) to _force_ the checkout.
Finally, there are a few odds and ends which are not purely moving from
one representation to the other:
5) Tying it all together
To commit a tree you have instantiated with "write-tree", you'd
create a "commit" object that refers to that tree and the
history behind it - most notably the "parent" commits that
preceded it in history.
Normally a "commit" has one parent: the previous state of the
tree before a certain change was made. However, sometimes it can
have two or more parent commits, in which case we call it a
"merge", due to the fact that such a commit brings together
("merges") two or more previous states represented by other
commits.
In other words, while a "tree" represents a particular directory
state of a working directory, a "commit" represents that state
in "time", and explains how we got there.
You create a commit object by giving it the tree that describes
the state at the time of the commit, and a list of parents:
commit-tree <tree> -p <parent> [-p <parent2> ..]
and then giving the reason for the commit on stdin (either
through redirection from a pipe or file, or by just typing it at
the tty).
commit-tree will return the name of the object that represents
that commit, and you should save it away for later use.
Normally, you'd commit a new "HEAD" state, and while git doesn't
care where you save the note about that state, in practice we
tend to just write the result to the file ".git/HEAD", so that
we can always see what the last committed state was.
6) Examining the data
You can examine the data represented in the object database and
the index with various helper tools. For every object, you can
use "cat-file" to examine details about the object:
cat-file -t <objectname>
shows the type of the object, and once you have the type (which
is usually implicit in where you find the object), you can use
cat-file blob|tree|commit <objectname>
to show its contents. NOTE! Trees have binary content, and as a
result there is a special helper for showing that content,
called "ls-tree", which turns the binary content into a more
easily readable form.
It's especially instructive to look at "commit" objects, since
those tend to be small and fairly self-explanatory. In
particular, if you follow the convention of having the top
commit name in ".git/HEAD", you can do
cat-file commit $(cat .git/HEAD)
to see what the top commit was.
7) Merging multiple trees
Git helps you do a three-way merge, which you can expand to
n-way by repeating the merge procedure arbitrary times until you
finally "commit" the state. The normal situation is that you'd
only do one three-way merge (two parents), and commit it, but if
you like to, you can do multiple parents in one go.
To do a three-way merge, you need the two sets of "commit"
objects that you want to merge, use those to find the closest
common parent (a third "commit" object), and then use those
commit objects to find the state of the directory ("tree"
object) at these points.
To get the "base" for the merge, you first look up the common
parent of two commits with
merge-base <commit1> <commit2>
which will return you the commit they are both based on. You
should now look up the "tree" objects of those commits, which
you can easily do with (for example)
cat-file commit <commitname> | head -1
since the tree object information is always the first line in a
commit object.
Once you know the three trees you are going to merge (the one
"original" tree, aka the common case, and the two "result" trees,
aka the branches you want to merge), you do a "merge" read into
the index. This will throw away your old index contents, so you
should make sure that you've committed those - in fact you would
normally always do a merge against your last commit (which
should thus match what you have in your current index anyway).
To do the merge, do
read-tree -m <origtree> <target1tree> <target2tree>
which will do all trivial merge operations for you directly in
the index file, and you can just write the result out with
"write-tree".
NOTE! Because the merge is done in the index file, and not in
your working directory, your working directory will no longer
match your index. You can use "checkout-cache -f -a" to make the
effect of the merge be seen in your working directory.
NOTE2! Sadly, many merges aren't trivial. If there are files
that have been added.moved or removed, or if both branches have
modified the same file, you will be left with an index tree that
contains "merge entries" in it. Such an index tree can _NOT_ be
written out to a tree object, and you will have to resolve any
such merge clashes using other tools before you can write out
the result.
[ fixme: talk about resolving merges here ]
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