[Git] Mô tả về GIT của Linus Torvalds
Đây là mô tả về GIT mà chủ nhân của nó - ông Linus Torvalds đã viết khi công khai mã nguồn. Cụ thể bài này được copy lại từ Github.
GIT - the stupid content tracker
“git” can mean anything, depending on your mood.
- random three-letter combination that is pronounceable, and not actually used by any common UNIX command. The fact that it is a mispronounciation of “get” may or may not be relevant.
- stupid. contemptible and despicable. simple. Take your pick from the dictionary of slang.
- “global information tracker”: you’re in a good mood, and it actually works for you. Angels sing, and a light suddenly fills the room.
- “goddamn idiotic truckload of sh*t”: when it breaks
This is a stupid (but extremely fast) directory content manager. It 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”.
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
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>.
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”).
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 “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.
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 integrity, the trust has to come from outside.
Current Directory Cache (“.dircache/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:
(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)
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 efficiently created from just the current directory cache without actually looking at any other data. So a directory cache at any 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)
(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 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).