Julius B. Lucks/Projects/Python Articles/Using Git

From OpenWetWare
Jump to: navigation, search

Introduction

This document aims to describe how to set git repositories in a distributed mode of development. As such, it draws heavily on [1], with extra notes for project-specific configuration. Specifically, we want to create a repository structure/flow cycle modeled after [2], where there are two developers, each with a personal (private) repo, and a public (shared) repo.

For more information about working with git, including viewing logs, diffs, making patches. etc. - see [1].

Distributed SCM Structure with 2 developers

Unlike centralized SCM (cvs,svn) in distributed SCM, there are more than one repository, each no more special than another. In centralized SCM, there is one master repository which people sync with, ofter only permitting write access to special users. Below we will outline how to work with a distributed system shared by two developers, each with a local and public repo

you  (Developer A): local repo, public repo
them (Developer B): local repo, public repo

The general practice is that the developers work with their local repositories, much like working with a locally checkout out copy in cvs. When changes are worth reporting, they sync this local repository with their public repository. The public repository is accessible by the other developer which then updates their local repository with changes from your public repository.

The main difference is that you are dealing with whole repositories, not just checked out copies. This lets you have, locally, all the power of committing, branching, merging, etc., without having to be in communication with a central server. This is not only extremely beneficial when developing without an internet connection, it also allows you to try out code and make progress without conflicting with others. You can also perform many incremental commits as we will see later, without worrying about corrupting the main repository.

The distributed nature of the repositories also means that the repo is in many places causing increased speed and automatic data redundancy.

Distributed SCM Project Flow with 2 developers

Below we describe the steps necessary to setup and interact with this repo structure. A typical project flow consists of you making changes, pushing (updating) them to your public repo, the other developer pulling those changes from your public repo, and merging them into their local repo, according to the following diagram [1]:

                      you push

your local repo ----------------------> your public repo

      ^                                     |

      |                                     |

      | you pull                            | they pull

      |                                     |

      |                                     |

      |               they push             V

their public repo <------------------- their local repo

We also discuss why this particular setup makes a lot of sense.

Repository setup

Each developer is assumed to have

  • git installed on a local development machine
  • access to a machine accessible via http and ssh with git installed

Below we outline the steps necessary to setup the configuration of repos noted in 1.1 above.

Setup the first local git repo

At this point, Developer B is not involved. Developer A follows the standard git repo setup on their machine [3]:

$ mkdir dev_a
$ cd dev_a
$ git init

Now either files are imported or created into dev_a, and the initial commit is created

$ #add some files
$ git add .  # Add the new files to the git index
$ git commit # Commit the changes to the index to the object database
             # Specify a commmit message (usually "initial import" at 
             # this point)

If you execute

$ git branch

You should see only one branch (* master - the star indicating that you are in the master branch). Even though there is minimal content in the repo, we have enough to set up Developer A's public repo.

Setup the public version of the first repo

Now we need to make a public version of this repo to put on a web-accessible server. Outside of the dev_a repo, make a cloned, bare repo [4] (a bare repo is just the contents of the .git directory, with none of the project files checked out):

$ git clone --bare dev_a/ dev_a.git
$ touch dev_a.git/git-daemon-export-ok

Now package this up and put in on the server. We will use tar and scp to do this. To make things easier, now and for development, put your id_rsa.pub public key inside of .ssh/authorized_keys on the server so you don't have to type your ssh password each time you login. Also make sure that 'PermitUserEnvironment yes' is enabled in /etc/sshd_config, and you have the correct PATH variable set in .ssh/environment (with PATH=$PATH:path/to/git) so that git will be found when ssh is used. Once this is setup

$ tar czvf repo.tar.gz dev_a.git/
$ scp repo.tar.gz you@server.domain:

Now put the repo in a web-accessible location

$ ssh you@server.domain
$ cd ~/public_html #(~/Sites on a Mac)
$ mkdir git
$ cd git
$ tar xzvf ~/repo.tar.gz

You should now see the repo dev_a.git unpacked. Now tell git about it

$ cd dev_a.git
$ git --bare update-server-info
$ chmod a+x hooks/post-update

You are now ready to begin using the public repo.

Developer B clones the public repo for their local repo

Now Developer B enters the story. We want Developer B to clone the public repository of Developer A initially so they are on the same page. Developer B should then execute

$ git clone http://server.domain/~developer_a/git/dev_a.git dev_b
$ cd dev_b
$ ls

Developer b should now see the full repo. Doing

$ git branch

Should show one branch (* master) as before.

Developer B creates a public repo

Developer B creates a public repo in the exact same way as in 2.2. We should now have the repo structure as depicted in 1.1.

Project Workflow

To carry out the workflow depicted in 1.2., we will need a few more git commands. To start out, we will play around with our local git repo, assuming we are just a solo developer. From there it will be a small step to having a the workflow between Developer A and Developer B.

Using Git as a Solo Developer

Git makes creating and merging branches really simple, so the recommended practice is to create a new local branch for any work you want to do. To see the branches we have, we do

$ git branch
 * master

There is only one line of output indicating that we only have the single (default) branch, master. The * beside it indicates that we are in the master branch. Let's create a branch called add-new-feature (remember this is a local branch, so we don't need to worry about other people understanding the branch names, or branch name conflicts. No one can see them but us!)

$ git branch add-new-feature
$ git branch
 * master
   add-new-feature

We now see another branch, but we are still in the master branch. To move into the other branch we do

$ git checkout add-new-feature
$ git branch
   master
 * add-new-feature

We can now work in this local branch. Let's say we want to add a file called TODO.txt. We first create the file, then add it to git's index using git add

$ echo 'Feature TODO file' > TODO.txt
$ git add TODO.txt

To commit this change, we do

$ git commit

And add a commit message [5]. Suppose we now want to add a line to TODO.txt:

$ echo 'Write documentation' >> TODO.txt

We need to tell git about this by using git add again.

$ git add TODO.txt

Unlike cvs or svn, the add command is used to add ANY changes, not just new files. This is because git does not track files, it tracks content. By using git add, we are telling git we have added content. Now we need to commit this content in the same way

$ git commit

If this seems like a lot of commands, there are many shortcuts which we will go over later. Now suppose we are done with our new feature, and it works, so we are ready to merge it into our master branch. Once all changes are committed, we can switch to the master branch, and merge in our changes from the add-new-feature branch

$ git checkout master
$ git merge add-new-feature

If all went well, there will be no conflicts. If there were conflics, we can resolve them and commit - see [6]. We can be done with the add-new-feature branch, so we delete it with

$ git branch -d add-new-feature
$ git branch
 * master

Branches are so easy and convenient that we can create a bunch of them for all the aspects we are working on - sub projects, documentation, etc. In this way, git's fine-grained control over commits and branches allows us to keep relevant work in a series of commits with many things going on at once.

Communicating your changes with Developer B with git push

Suppose we want Developer B to see this new feature. We need to push our new master branch into our public repo. We can do this with the git push command

$ git push ssh://server.domain/~developer_a/git/dev_a.git master:master

The url after git push gives the location of our public repo. The last part indicates that we want to push changes in our master branch, onto the master branch of our public repo.

Now Developer B needs a way to get those changes.

Getting changes from Developer B with git fetch

This step is very similar to that in 3.1., except instead of creating a branch from the HEAD of our master branch, we will create a branch from the public repo of Developer B

$ git fetch http://devb_machine.domain/~developer_b/git/dev_b.git master:dev_b_changes
$ git branch
 * master
   dev_b_changes

The last part of git fetch tells us to create a branch called dev_b_changes from the master branch of Developer b's public git repo. We can then checkout dev_b_changes in the same way to see what they have been up to, or we can merge the changes with

$ git merge dev_b_changes

Once we are done, we can remove this new branch with git branch -d

The 2 Developer Development Cycle

The development cycle outlined in 1.2. then consists of

  • making local changes with git branch, checkout, add, commit, merge
  • publishing changes to the public repo with git push
  • fetching changes from Developer B with git fetch
    • merging these changes with git merge

Multi-Developer Distributed SCM

Adding another developer just means that developer then has local and public repos. When changes are ready to be fetched, Developer C just notifies Developer's A and B that they should check out the updates. Alternatively, a developer can periodically check out what the other two are doing with repeated git fetch calls. This can be done by greating branches for the two other devolopers. Suppose we are Developer A

$ git fetch http://dev_b_repo_path master:dev_b_master
$ git fetch http://dev_c_repo_path master:dev_r_master

After sometime, to check for updates, Developer A can do

$ git fetch --all

Which will update these branches. We can then git checkout and git diff to see what has been going on. Of course you don't need to keep track of what everyone is doing. You can make git work like svn or cvs by having some acknowledged master copy with only certain developers having write (push) access. The cool thing about git is that in principle no repo is more special than any other - it is up to the community to decide which repo to trust. This could mean that someone is considered the project maintainer in which case their repo might be the one to trust. In this case, the maintainer pulls from developers when they want to submit changes to the main line of code.

The distributed structure of git allows teams to work on different aspects of the project without interfering and coordinating with eachother. For example, one team might be bug fixing for a release, while others are experimenting with new features. Each team pushes and pulls within eachother so that they don't have to worry about conflicts. The maintainer can decide when larger project coordination should happen.

Git Shortcuts

It is recommended that you use the long form of the commmands as stated above until you understand what git is doing, and how it is different from an SCM system you may be used to. Once you are comfortable, you can use the command short forms below to save some typing.


References

^1 http://www.kernel.org/pub/software/scm/git/docs/user-manual.html

^2 http://www.kernel.org/pub/software/scm/git/docs/user-manual.html#public-repositories

^3 http://www.kernel.org/pub/software/scm/git/docs/user-manual.html#creating-a-new-repository

^4 http://www.kernel.org/pub/software/scm/git/docs/user-manual.html#setting-up-a-public-repository

^5 http://www.kernel.org/pub/software/scm/git/docs/user-manual.html#creating-good-commit-messages

^6 http://www.kernel.org/pub/software/scm/git/docs/user-manual.html#resolving-a-merge