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* dVCS by way of git
Sharing complete repositories is a simple concept which involves a
subtle paradigm shift, which in turn opens up interesting new
pastures.
In this talk I will demonstrate some of these constraints
and their solutions, as implemented in git.
* A look back at SVN
** Linear history is normal, all graphs are trees
In other words, any given commit can have many children, but only
one parent.
** Merging is painful and error prone
Most solutions to this problem involve writing appropriate commit
logs or writing out to files so merges can be traced. Screwing this
up can be bad, and as a result it is avoided as much as possible.
** Sharing changes consists of mailing patches
Obviously this was all workable, but it didn't exactly engender
itself to lazy people like myself. The existance and popularity of
CVSup in spite of being written in Modula 3 shows the value of
repository sharing. You can think of git as CVSup done right.
* Constraints
** Repositories are collections of interwoven histories
So:
*** Offline operation means history is frequently not linear
*** Merging must be easy
*** Sharing changes must be easy
* How git satisfies dVCS constraints
** History is no longer linear
Time is no longer a useful identifier when comparing the history of
disparate repositories, and thus can't be used for commit
identifiers. Something new must be found.
*** git uses SHA hashes to identify repository objects
SHA-1 hashes are the basic identifier of every object in the git
system, which yields a bunch of nice properties we'll get into
later.
** Merging is elevated to a first class operation
Git makes merging easy(ier). It will probably never be trivial, but
git at least automates the grunt work of tracking down common
ancestors to reduce conflicts and ease merging.
** Branching is trivial and encouraged
Creating a branch is just creating another ref pointing to an
existing commit. It's very fast and efficient. It's very easy to
move things between branches, and they are encouraged for any
non-trivial work. It doesn't even mess up your history graph a lot
of the time, and when it does you can often alter it so it does not.
*** What is the object store?
**** blobs
Blobs are blobs of binary data.
**** trees
Trees point to blobs or other trees.
**** commits
Git commits contain a tree, its parent commits, and a tree object,
along with meta-data: message, author, commiter, and so forth.
In git, a commit can have many parents, as opposed to SVN where a
commit can have only one parent. All commits contain a tree, so when
you had to resolve conflicts from a merge, those will be contained
in the commit's tree object.
**** tags
Tag objects contain a commit id and an optional message and
cryptographic signature. If neither are present, a tag is merely a
symbolic ref.
*** All objects are identified by SHA hashes.
The hash table has a number of advantages:
# since type and length are part of the object you can use one
namespace.
# good entropy properties for building hash tables
# finding tree changes is very simple, since trees effectively
contain their subtrees and files.
# system and its history is trivially verifiable. commits are
effectively signed by all their parents.
*** Investigating the object store
**** TODO Show perl code and output of commit/tree/blob from .git/objects
**** There is no delta concept in the object store
Deltas are generated by `git gc' when it creates pack files.
** SHA hashes are a pain to type
Git has a concept of `refs' which are typically symbolic references
to commits. At the end of the day, every ref ends up as a SHA hash.
*** SHA hashes can typically be shortened to a few characters
*** tags are fixed refs
Tags always refer to a commit, but can also contain a cryptographic
signature and message, in which case the ref points to a tag object,
which, in turn, points to a commit. For almost any use of tags, you
don't need to care about this, since git is fairly smart about it.
*** branches and HEAD are symbolic refs
Branches are moving refs and always reference their tips. HEAD is a
pointer to the tip of the current branch.
*** $ref^ and $ref~$n
You can follow parents by using caret or tilde notation. Merge
commits are followed in their order in the commit blob.
# ^ is the parent, ^^ is the paren't parent, and so on
e.g: HEAD^ (The next most-recent commit on the current branch)
# ~2 is shorthand for ^^
e.g: HEAD~2 (The third most-recent commit on the current branch)
*** $ref@{N}
Where $ref was N moves ago. You can also specify by date.
** Sharing commits
*** Remotes
Remotes are named repositories. They're useful when you push or pull
from the same repository repeatedly. The `origin' remote is used as
the default remote with many commands.
*** Implicit read-only "vendor" branches.
When you fetch a remote you get all its objects, so you can always
look at any point of its history. This duplicates `vendor branch'
functionality.
*** Push and Pull
You fetch changes via `fetch,' but frequently use `pull' instead,
which does a fetch and merge of any remote branches being
tracked. To publish changes to a remote, use `push.'
*** Space efficient
Because the object store identifies by the SHA of object contents,
it won't store duplicates. In effect, this means the cost of an
additional remote is only the cost of its differences.
*** Transports
# HTTP
# GIT
# SSH
# Rsync
# Files
# Others
*** Example
* Merge strategies
*** Fast forward
When the merge target is an ancestor of the other branch, this just
points the target's HEAD at the other branch.
*** Recursive
Used when more than one common ancestor exists. Builds the merge
base revision by recursively merging common ancestors.
*** And others
See git-merge(1)
* A brief note on the index
The index stores the tree object of the commit-to-be.
# adding to the index cache: git add
# removing: git rm --cached
** git reset
Can be used to reset the index, or certain files in the index, to a
given commit, which is HEAD by default.
* Problems git solves
** Mixed two patches together
# git reset $filename
# git add --patch
# git commit [--amend]
** In combination with git rebase, entire histories can be manipulated
# git rebase -i $ref
# git reset HEAD^
# git add --patch
# git commit -c ORIG_HEAD
# git add -u
# git commit
# git rebase --continue
* My seekrit agenda
I like git. It's the first version control system that allows me to
use it the way I want to. I commit like crazy, I like to use
branches, and I make a lot of mistakes. Git encourages this, or at
least gives you the tools to recover. It's a security blanket.
* Additional Resources
# Git - SVN Crash Course
<http://git.or.cz/course/svn.html>
# GitWiki
<http://git.or.cz/gitwiki/FrontPage>
# Git User's Manual
<http://www.kernel.org/pub/software/scm/git/docs/user-manual.html>
# Extensive Man Pages
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