Git - Revision Control Perfected
In 2005, after just two weeks, Linus Torvalds completed the first version of Git, an open-source version control system. Unlike typical centralized systems, Git is based on a distributed model. It is extremely flexible and guarantees data integrity while being powerful, fast and efficient. With widespread and growing rates of adoption, and the increasing popularity of services like GitHub, many consider Git to be the best version control tool ever created.
Surprisingly, Linus had little interest in writing a version control tool before this endeavor. He created Git out of necessity and frustration. The Linux Kernel Project needed an open-source tool to manage its massively distributed development effectively, and no existing tools were up to the task.
Many aspects of Git's design are radical departures from the approach of tools like CVS and Subversion, and they even differ significantly from more modern tools like Mercurial. This is one of the reasons Git is intimidating to many prospective users. But, if you throw away your assumptions of how version control should work, you'll find that Git is actually simpler than most systems, but capable of more.
In this article, I cover some of the fundamentals of how Git works and stores data before moving on to discuss basic usage and work flow. I found that knowing what is going on behind the scenes makes it much easier to understand Git's many features and capabilities. Certain parts of Git that I previously had found complicated suddenly were easy and straightforward after spending a little time learning how it worked.
I find Git's design to be fascinating in and of itself. I peered behind the curtain, expecting to find a massively complex machine, and instead saw only a little hamster running in a wheel. Then I realized a complicated design not only wasn't needed, but also wouldn't add any value.
Git Object Repository
Git, at its core, is a simple indexed name/value database. It stores pieces of data (values) in "objects" with unique names. But, it does this somewhat differently from most systems. Git operates on the principle of "content-addressed storage", which means the names are derived from the values. An object's name is chosen automatically by its content's SHA1 checksum—a 40-character string like this:
SHA1 is cryptographically strong, which guarantees a different checksum for different data (the actual risk of two different pieces of data sharing the same SHA1 checksum is infinitesimally small). The same chunk of data always will have the same SHA1 checksum, which always will identify only that chunk of data. Because object names are SHA1 checksums, they identify the object's content while being truly globally unique—not just to one repository, but to all repositories everywhere, forever.
To put this into perspective, the example SHA1 listed above happens to be the ID of the first commit of the Linux kernel into a Git repository by Linus Torvalds in 2005 (2.6.12-rc2). This is a lot more useful than some arbitrary revision number with no real meaning. Nothing except that commit ever will have the same ID, and you can use those 40 characters to verify the data in every file throughout that version of Linux. Pretty cool, huh?
Git stores all the data for a repository in four types of objects: blobs, trees, commits and tags. They are all just objects with an SHA1 name and some content. The only difference between them is the type of information they contain.
Blobs and Trees
A blob stores the raw data content of a file. This is the simplest of the four object types.
A tree stores the contents of a directory. This is a flat list of file/directory names, each with a corresponding SHA1 representing its content. These SHA1s are the names of other objects in the repository. This referencing technique is used throughout Git to link all kinds of information together. For file entries, the referenced object is a blob. For directory entries, the referenced object is a tree that can contain more directory entries, in turn referencing more trees to define a complete and potentially unlimited hierarchy.
It's important to recognize that blobs and trees are not themselves files and directories; they are just the contents of files and directories. They don't know about anything outside their own content, including the existence of any references in other objects that point to them. References are one-way only.
Figure 1. An example directory structure and how it might be stored in Git as tree and blob objects (I truncated the SHA1 names to six characters for readability).
In the example shown in Figure 1, I'm assuming that the files MyApp.pm and MyApp1.pm have the same contents, and so by definition, they must reference the same blob object. This behavior is implicit in Git because of its content-addressable design and works equally well for directories with the same content.
As you can see, directory structures are defined by chains of references stored in trees. A tree is able to represent all of the data in the files and directories under it even though it contains only one level of names and references. Because SHA1s of the referenced objects are within its content, a tree's SHA1 exactly identifies and verifies the data throughout the structure; a checksum resulting from a series of checksums verifies all the underlying data regardless of the number of levels.
Consider storing a change to the file README illustrated in Figure 1. When committed, this would create a new blob (with a new SHA1), which would require a new tree to represent "foo" (with a new SHA1), which would require a new tree for the top directory (with a new SHA1).
While creating three new objects to store one change might seem inefficient, keep in mind that aside from the critical path of tree objects from changed file to root, every other object in the hierarchy remains identical. If you have a gigantic hierarchy of 10,000 files and you change the text of one file ten directories deep, 11 new objects allow you to describe both the old and the new state of the tree.
One potential problem of the content-addressed design is that two large files with minor differences must be stored as different objects. However, Git optimizes these cases by using deltas to eliminate duplicate data between objects wherever possible. The size-reduced data is stored in a highly efficient manner in "pack files", which also are further compressed. This operates transparently underneath the object repository layer.
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