From: J. Bruce Fields Date: Sun, 10 Jun 2007 19:15:08 +0000 (-0400) Subject: user-manual: rewrite object database discussion X-Git-Tag: v1.5.3.2~15^2~5 X-Git-Url: https://git.tokkee.org/?a=commitdiff_plain;h=1bbf1c7900337a646b6ca65c8b3505e784012f39;p=git.git user-manual: rewrite object database discussion Rewrite the introduction. Rewrite each section completely to make them work in the new order, to add some examples, and to move plumbing commands (like git-commit-tree) to the following chapter. Signed-off-by: J. Bruce Fields --- diff --git a/Documentation/user-manual.txt b/Documentation/user-manual.txt index 223ec7591..4fb2f30ef 100644 --- a/Documentation/user-manual.txt +++ b/Documentation/user-manual.txt @@ -2723,46 +2723,44 @@ database>> and the <>. The Object Database ------------------- -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. - -All objects have a statically determined "type" which is -determined at object creation time, and which identifies the format of -the object (i.e. how it is used, and how it can refer to other -objects). There are currently four different object types: "blob", -"tree", "commit", and "tag". -A <> cannot refer to any other object, -and is, as the name implies, a pure storage object containing some -user data. It is used to actually store the file data, i.e. a blob -object is associated with some particular version of some file. - -A <> 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. - -A <> ties such directory hierarchies -together into a <> 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" -commit, 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. - -A <> symbolically identifies and can be -used to sign other objects. It contains the identifier and type of -another object, a symbolic name (of course!) and, optionally, a -signature. +We already saw in <> that all commits are stored +under a 40-digit "object name". In fact, all the information needed to +represent the history of a project is stored in objects with such names. +In each case the name is calculated by taking the SHA1 hash of the +contents of the object. The SHA1 hash is a cryptographic hash function. +What that means to us is that it is impossible to find two different +objects with the same name. This has a number of advantages; among +others: + +- Git can quickly determine whether two objects are identical or not, + just by comparing names. +- Since object names are computed the same way in ever repository, the + same content stored in two repositories will always be stored under + the same name. +- Git can detect errors when it reads an object, by checking that the + object's name is still the SHA1 hash of its contents. + +(See <> for the details of the object formatting and +SHA1 calculation.) + +There are four different types of objects: "blob", "tree", "commit", and +"tag". + +- A <> is used to store file data. +- A <> 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. +- A <> ties such directory hierarchies + together into a <> of revisions - each + commit contains the object name of exactly one tree designating the + directory hierarchy at the time of the commit. In addition, a commit + refers to "parent" commit objects that describe the history of how we + arrived at that directory hierarchy. +- A <> symbolically identifies and can be + used to sign other objects. It contains the object name and type of + another object, a symbolic name (of course!) and, optionally, a + signature. The object types in some more detail: @@ -2770,109 +2768,142 @@ The object types in some more detail: Commit Object ~~~~~~~~~~~~~ -The "commit" 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 "commit" is defined by the tree-object that it results in, the -parent commits (zero, one or more) that led up to that point, and a -comment on what happened. Again, a commit 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 commits: unlike some SCM's, commits do not contain -rename information or file mode change 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. - -A commit is created with gitlink:git-commit-tree[1] and -its data can be accessed by gitlink:git-cat-file[1]. +The "commit" object links a physical state of a tree with a description +of how we got there and why. Use the --pretty=raw option to +gitlink:git-show[1] or gitlink:git-log[1] to examine your favorite +commit: + +------------------------------------------------ +$ git show -s --pretty=raw 2be7fcb476 +commit 2be7fcb4764f2dbcee52635b91fedb1b3dcf7ab4 +tree fb3a8bdd0ceddd019615af4d57a53f43d8cee2bf +parent 257a84d9d02e90447b149af58b271c19405edb6a +author Dave Watson 1187576872 -0400 +committer Junio C Hamano 1187591163 -0700 + + Fix misspelling of 'suppress' in docs + + Signed-off-by: Junio C Hamano +------------------------------------------------ + +As you can see, a commit is defined by: + +- a tree: The SHA1 name of a tree object (as defined below), representing + the contents of a directory at a certain point in time. +- parent(s): The SHA1 name of some number of commits which represent the + immediately prevoius step(s) in the history of the project. The + example above has one parent; merge commits may have more than + one. A commit with no parents is called a "root" commit, and + represents the initial revision of a project. Each project must have + at least one root. A project can also have multiple roots, though + that isn't common (or necessarily a good idea). +- an author: The name of the person responsible for this change, together + with its date. +- a committer: The name of the person who actually created the commit, + with the date it was done. This may be different from the author, for + example, if the author was someone who wrote a patch and emailed it + to the person who used it to create the commit. +- a comment describing this commit. + +Note that a commit does not itself contain any information about what +actually changed; all changes are calculated by comparing the contents +of the tree referred to by this commit with the trees associated with +its parents. In particular, git does not attempt to record file renames +explicitly, though it can identify cases where the existence of the same +file data at changing paths suggests a rename. (See, for example, the +-M option to gitlink:git-diff[1]). + +A commit is usually created by gitlink:git-commit[1], which creates a +commit whose parent is normally the current HEAD, and whose tree is +taken from the content currently stored in the index. [[tree-object]] Tree Object ~~~~~~~~~~~ -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, i.e. 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 (i.e. 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. - -A tree is created with gitlink:git-write-tree[1] and -its data can be accessed by gitlink:git-ls-tree[1]. -Two trees can be compared with gitlink:git-diff-tree[1]. +The ever-versatile gitlink:git-show[1] command can also be used to +examine tree objects, but gitlink:git-ls-tree[1] will give you more +details: + +------------------------------------------------ +$ git ls-tree fb3a8bdd0ce +100644 blob 63c918c667fa005ff12ad89437f2fdc80926e21c .gitignore +100644 blob 5529b198e8d14decbe4ad99db3f7fb632de0439d .mailmap +100644 blob 6ff87c4664981e4397625791c8ea3bbb5f2279a3 COPYING +040000 tree 2fb783e477100ce076f6bf57e4a6f026013dc745 Documentation +100755 blob 3c0032cec592a765692234f1cba47dfdcc3a9200 GIT-VERSION-GEN +100644 blob 289b046a443c0647624607d471289b2c7dcd470b INSTALL +100644 blob 4eb463797adc693dc168b926b6932ff53f17d0b1 Makefile +100644 blob 548142c327a6790ff8821d67c2ee1eff7a656b52 README +... +------------------------------------------------ + +As you can see, a tree object contains a list of entries, each with a +mode, object type, SHA1 name, and name, sorted by name. It represents +the contents of a single directory tree. + +The object type may be a blob, representing the contents of a file, or +another tree, representing the contents of a subdirectory. Since trees +and blobs, like all other objects, are named by the SHA1 hash of their +contents, two trees have the same SHA1 name if and only if their +contents (including, recursively, the contents of all subdirectories) +are identical. This allows git to quickly determine the differences +between two related tree objects, since it can ignore any entries with +identical object names. + +(Note: in the presence of submodules, trees may also have commits as +entries. See gitlink:git-submodule[1] and gitlink:gitmodules.txt[1] +for partial documentation.) + +Note that the files all have mode 644 or 755: git actually only pays +attention to the executable bit. [[blob-object]] Blob Object ~~~~~~~~~~~ -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 (i.e. normally "file -contents"). +You can use gitlink:git-show[1] to examine the contents of a blob; take, +for example, the blob in the entry for "COPYING" from the tree above: + +------------------------------------------------ +$ git show 6ff87c4664 + + Note that the only valid version of the GPL as far as this project + is concerned is _this_ particular version of the license (ie v2, not + v2.2 or v3.x or whatever), unless explicitly otherwise stated. +... +------------------------------------------------ -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 totally independent of its location in the -directory tree, and renaming a file does not change the object that -file is associated with in any way. +A "blob" object is nothing but a binary blob of data. It doesn't refer +to anything else or have attributes of any kind. -A blob is typically created when gitlink:git-update-index[1] -is run, and its data can be accessed by gitlink:git-cat-file[1]. +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 totally independent of its location in the directory tree, and +renaming a file does not change the object that file is associated with. + +Note that any tree or blob object can be examined using +gitlink:git-show[1] with the : syntax. This can +sometimes be useful for browsing the contents of a tree that is not +currently checked out. [[trust]] Trust ~~~~~ -An aside on the notion of "trust". 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. +If you receive the SHA1 name of a blob from one source, and its contents +from another (possibly untrusted) source, you can still trust that those +contents are correct as long as the SHA1 name agrees. This is because +the SHA1 is designed so that it is infeasible to find different contents +that produce the same hash. -Furthermore, since the SHA1 signature of a commit refers to the -SHA1 signatures of the tree it is associated with and the signatures -of the parent, a single named commit 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 commit. +Similarly, you need only trust the SHA1 name of a top-level tree object +to trust the contents of the entire directory that it refers to, and if +you receive the SHA1 name of a commit from a trusted source, then you +can easily verify the entire history of commits reachable through +parents of that commit, and all of those contents of the trees referred +to by those commits. 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 @@ -2891,23 +2922,31 @@ To assist in this, git also provides the tag object... Tag Object ~~~~~~~~~~ -Git provides the "tag" object to simplify creating, managing and -exchanging symbolic and signed tokens. The "tag" object at its -simplest simply symbolically identifies another object by containing -the sha1, type and symbolic name. - -However it can optionally contain additional signature information -(which git doesn't care about as long as there's less than 8k of -it). This can then be verified externally to git. +A tag object contains an object, object type, tag name, the name of the +person ("tagger") who created the tag, and a message, which may contain +a signature, as can be seen using the gitlink:git-cat-file[1]: -Note that despite the tag features, "git" itself only handles content -integrity; the trust framework (and signature provision and -verification) has to come from outside. +------------------------------------------------ +$ git cat-file tag v1.5.0 +object 437b1b20df4b356c9342dac8d38849f24ef44f27 +type commit +tag v1.5.0 +tagger Junio C Hamano 1171411200 +0000 + +GIT 1.5.0 +-----BEGIN PGP SIGNATURE----- +Version: GnuPG v1.4.6 (GNU/Linux) + +iD8DBQBF0lGqwMbZpPMRm5oRAuRiAJ9ohBLd7s2kqjkKlq1qqC57SbnmzQCdG4ui +nLE/L9aUXdWeTFPron96DLA= +=2E+0 +-----END PGP SIGNATURE----- +------------------------------------------------ -A tag is created with gitlink:git-mktag[1], -its data can be accessed by gitlink:git-cat-file[1], -and the signature can be verified by -gitlink:git-verify-tag[1]. +See the gitlink:git-tag[1] command to learn how to create and verify tag +objects. (Note that gitlink:git-tag[1] can also be used to create +"lightweight tags", which are not tag objects at all, but just simple +references in .git/refs/tags/). [[the-index]] @@ -2978,6 +3017,24 @@ scripts using a smaller core of low-level git commands. These can still be useful when doing unusual things with git, or just as a way to understand its inner workings. +[[object-manipulation]] +Object access and manipulation +------------------------------ + +The gitlink:git-cat-file[1] command can show the contents of any object, +though the higher-level gitlink:git-show[1] is usually more useful. + +The gitlink:git-commit-tree[1] command allows constructing commits with +arbitrary parents and trees. + +A tree can be created with gitlink:git-write-tree[1] and its data can be +accessed by gitlink:git-ls-tree[1]. Two trees can be compared with +gitlink:git-diff-tree[1]. + +A tag is created with gitlink:git-mktag[1], and the signature can be +verified by gitlink:git-verify-tag[1], though it is normally simpler to +use gitlink:git-tag[1] for both. + [[the-workflow]] The Workflow ------------