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Using Git

The following is primarily designed for the core OME developers who are contributing to our code base using Git. It should contain all the useful commands and configuration you need for doing most Git tasks.

Note

This section assumes that “gh” is your own personal GitHub repository, and “origin” is one of the official openmicroscopy repositories.

Installing Git

In general, see the Git downloads page for installation options.

Linux

Most flavors of Linux have git available through the package manager. For example, on Debian or Ubuntu:

sudo apt-get install git

Mac OS X

You can install Git using Homebrew:

brew install git

Or you can use the binary installer.

Windows

We recommend using either msysGit for a basic Git installation, or Cygwin for a full-featured Unix-style environment that includes Git. You can also use TortoiseGit for Git shell integration. You may also want to consider installing VirtualBox with a Linux guest OS to make your life easier. Lastly, when using Git on Windows, please be aware of the CRLF conversion issue.

Git configuration

If you are looking to get started as quickly as possible, the minimum you will need is to have Git installed and then:

git config --global user.name "Full name"
git config --global user.email YOUR_EMAIL
git clone --recursive https://github.com/openmicroscopy/REPOSITORY_NAME
cd REPOSITORY_NAME

You will not be able to push back to this repository, but you will at least have something you can start looking at.

Git provides a number of options which can make working with it considerably more pleasant. These configuration options are available either globally in $HOME/.gitconfig or in the .git directory of your repository. The file is in INI-format, but can also be modified using the git config command, as illustrated in the examples following.

The most important thing is to update your ‘global’ credentials that are used in your commits. These values are saved in ~/.gitconfig:

git config --global user.name "Full name"
git config --global user.email YOUR_EMAIL

If you have a PGP key for signing commits and tags, you may want to add it as well:

git config --global user.signingkey YOUR_PGP_KEY_ID

Color and display options make log and diff output much more friendly:

git config --global color.ui true
git config --global color.diff auto
git config --global color.graph auto
git config --global color.status auto
git config --global color.branch auto

git config --global core.ui always
git config --global core.editor mate_wait

Aliases provide a way to make shortcuts for longer Git commands. One that is often used among the OME team is graph:

git config --global alias.graph "log --date-order --graph --decorate --oneline"

See Helpful command aliases for more examples.

Interacting with GitHub

Cloning the repositories

You can fork any of the openmicroscopy repositories you will be working on by clicking the fork icon in the top righthand corner of each repo’s homepage on GitHub. This will give you your own copy of the repo on GitHub. To set this up from the command line so you can push to it and open PRs, you need to clone the repo. The following example uses the documentation repo:

git clone https://github.com/openmicroscopy/ome-documentation
cd ome-documentation
git remote add gh git@github.com:YOUR_USERNAME/ome-documentation.git

To clone private repositories you need to use the SSH protocol:

git clone git@github.com:openmicroscopy/REPO_NAME.git

GitHub remotes

You can add the other members of the OME network as remotes, so you can follow what they are doing:

git remote add SOMEUSER git://github.com/SOMEUSER/openmicroscopy.git
git fetch SOMEUSER

If you would like to work more closely with someone, via pushing directly to their branch or they from yours, then you can have them add you as a collaborator on their repository or do the same for them on yours. This is done under https://github.com/account/repositories

If you have not made such a repository yet as a remote, then you should do so using the SSH protocol:

git remote add SOMEUSER git@github.com:SOMEUSER/openmicroscopy.git

Otherwise, you will need to modify its URL

git remote set-url SOMEUSER git@github.com:SOMEUSER/openmicroscopy.git

If you would like to be kept up-to-date on what several users are doing on GitHub, you can set the “default remotes” value to the list of people you would like to check in .git/config:

git config remotes.default "ome team origin gh official chris ola will jm colin"

Now, git remote update will check the above list of repositories.

Pushing to GitHub

When you have work which you want to share with the rest of the team, it is vital that you push it to your GitHub fork.

git push gh your-branch

This will create a new branch, and the same command can be used to subsequently update that branch.

If you NEED to use a different name for the branch on GitHub, you can do:

git push gh your-branch:refs/heads/branch-name-on-gh

As mentioned elsewhere, the “refs/heads/” prefix only needs to be used to create a new branch, and can be dropped for subsequent pushes.

Tracking others’ branches

The flip-side of pushing your own branches is being aware that other OME developers will also be pushing theirs. GitHub provides a number of ways of monitoring either a user or a repository. Notifications about what watched users and repositories are doing can be seen in your GitHub inbox or via RSS feeds. See Be social for more information.

Even if you do not feel able to watch the everyone’s repository, you will likely want to periodically check in on the current Pull Requests (PRs). These will contain screenshots and other updates about what the team is working on. When the PRs have been sufficiently reviewed, they will be merged into the develop branch so that others’ work will start to be based on it.

Cleaning up your GitHub branches

Once your branches have been merged into the mainline (“develop” of openmicroscopy/openmicroscopy) you should delete them from your repository.

git branch -d your-branch
git push gh :your-branch

This way, anyone looking at your fork clearly sees what is currently in progress or upcoming.

Common Git Commands

Although everyone has a slightly different way of working, the following command examples should show you much of what you will want to do on a daily or weekly basis when working with OME via Git.

See if you have any changes that you might need to commit. This also displays some useful tips on how to add and remove files:

git status

Create a branch from the “develop” branch:

git checkout -b feature/foo origin/develop

At this point, you are ready to do some work:

git checkout my-work  # Just to be sure.
vim README.txt        # edit files
git merge anotheruser/some-work
git status            # See what you have done

You can also add files or directories to the ‘cache’ with interactive choice of which ‘chunks’ to accept or decline (useful for checking that you are not adding any unintended changes, print statements etc.):

git add -p path/to/dir/or/file

Check the status again - to see summary of what you are about to commit:

git status

Any remaining changes that you want to discard can be reverted by:

git checkout -- path/to/file.txt

When committing the code you have just modified/merged your commit message should refer to related tickets. E.g. “See #1111” will link the commit to the ticket on trac, and “Fixes #2222“” will link and close the ticket on trac.

git commit -m "Add message here and refer to the ticket number. See #1234. Fixes #5678"

Note

If you want to add more than a short one-line message, you can omit the -m “message” and Git will open your specified editor, where you should add a single line summary followed by line space and then a paragraph of more text. See Commit messages for more discussion.

After you have committed, you can keep working and committing as above - the changes are only saved to your local git.

For example, you can move to another branch to continue work on a different feature. To see a list branches:

git branch

Add the -a to list remote branches too.

To simply move between branches, use git checkout. All the files on your file-system will be updated to the new branch:

git checkout dev_4_2

Note

Make sure they are refreshed if you have files open in an editor or IDE

If you have forgotten what you did on a particular branch, you can use git log. Add the -p flag to see the actual diff for each commit.

You can use the first 5 characters of a commit’s hash key to begin the log at a certain commit. E.g. show diff for commit 83dad:

git log 83dad -p

Or to display a nice graph:

git log --graph --decorate --oneline

If an alias has been set-up as described in the configuration section above, you can just do:

git graph

This is most useful when showing how two branches are related:

git graph origin/develop develop

When you are ready, you will need to push your local changes to your own forked repository in order to share with others. If the branch does not yet exist in your repository, you will need to prefix the push command with refs/heads:

git push gh my_fix_123:refs/heads/my_fix_123

After that initial push, the following will suffice as long as you are on the my_fix_123 branch:

git push gh my_fix_123

You will find it easier if you name remote branches the same as local branch though it is not a requirement:

git push gh name/of/branch:refs/heads/name/of/branch
# E.g:
git push gh feature/export:refs/heads/feature/export

Once you have pushed, you can open a “Pull Request” to inform the team about the changes. More on that below.

You can also create a local branch from a remote branch, whether it is your own or belongs to someone else on the team. These will be ‘tracked’ so that commits you push automatically go to the corresponding remote branch:

git fetch SOMEUSER && git checkout -b name/of/branch SOMEUSER/name/of/branch
# work on the branch then:
git push SOMEUSER name/of/branch

Collaborating via git rebase

If you have been permitted write access to someone else’s forked repository, or you have granted someone else write access to your repository, then there is a further aspect that you need to be aware of.

If both of you are working on the my_fix_123 branch from above, then when it is time to push, your version may not represent the latest state. To prevent losing any commits or introducing unnecessary merge messages, you will first need to access the latest remote changes:

git fetch gh

To see the differences between your local changes (‘my_fix_123’) and the remote changes (‘gh/my_fix_123’), you can use the log command:

git log --graph --date-order gh/my_fix_123 my_fix_123

If the remote branch (‘gh/my_fix_123’) have moved ahead of yours, then you will want to rebase your work on top of this new work:

git rebase gh/my_fix_123

Now your local changes will follow the remote changes in the log. You can check how this looks by viewing the graph again:

git log --graph --date-order gh/my_fix_123 my_fix_123

Now you can push your changes on the ‘my_fix_123’ branch to the remote repository:

git push gh my_fix_123

Rebasing allows you to update the ‘base’ point at which you branched from another branch (as described above). You can also use ‘rebase’ to organize your commits before merging.

It can strip whitespace, since it is good practice not to commit extra whitespace at the end of lines or files. Git allows you to remove all extra whitespace during rebase e.g. to origin/develop branch

git rebase --whitespace=strip origin/develop

Rebase “interactive” using the -i flag allows you to remove, edit, combine etc commits. Git will open an editor to allow you to edit the commit summary along with instructions on how to omit, modify commits. For example, to rebase onto origin/develop branch:

git rebase -i origin/develop

Working with submodules

Since submodules are git repositories, all the tools described previously (add remotes, edit/merge, commit…) can be used within each submodule repository:

$ cd components/bioformats
$ git remote add melissalinkert git@github.com:melissalinkert/bioformats.git
$ git remote
origin
melissalinkert
sbesson
$ git checkout -b new_branch origin/develop
$ vim Readme.txt
$ git merge melissalinkert/branch
$ git commit -m "Merge branch"
$ git push sbesson new_branch

Additionally, you can perform an update of the submodule from the parent project, i.e. checkout a specific commit. After updating, the submodule ends up in a detached HEAD state:

$ cd code/openmicroscopy
$ git submodule update
Submodule path 'components/bioformats': checked out '9328b869b9ba61851adaa3db428ce25f0ca56845'
$ cd components/bioformats
$ git branch
* (no branch)
  develop

If you move between branches in the project, you may end up in a different state of the submodule:

$ cd ../..
$ git checkout my-branch
M   components/bioformats
Switched to branch 'my-branch'

$ git status
# On branch my-branch
# Changes not staged for commit:
#   (use "git add <file>..." to update what will be committed)
#   (use "git checkout -- <file>..." to discard changes in working directory)
#
#   modified:   components/bioformats (new commits)
#
# no changes added to commit (use "git add" and/or "git commit -a")

If you do not want to modify the submodule state, run git submodule update. Be careful though, the git subdmodule update command will silently delete all local changes under the submodule. If you want to keep your changes, make sure you have pushed them to GitHub.

To advance the submodule to another commit, you can run the git add command:

cd components/bioformats
git merge gh/branch
git commit -m "Merged branch"
git push

cd ..
git add bioformats
git commit -m "Move to latest bioformats"

Warning

Be careful NOT to add a trailing slash when adding the submodule, the following command would want to delete the submodule and add all the files in the submodule directory:

git add components/bioformats/

There are Git hooks available to make working with submodules safer. See post-merge-checkout for an example.

Commit messages

All commit messages in git should start with a single line of 72 characters or less, following by a blank line, followed by any other text.

Add feature X (See #123, Fix #321)
<this line left blank>
More description about X. It’s really great …

Many git tools expect exactly this format, not the least of which is GitHub. If you would like to see how these commit messages are rendered on GitHub, take a look at the repository https://github.com/kneath/commits

You can read more about commit messages at A Note About Git Commit Messages.

Rebasing to keep code clean

Rebasing allows you to update the ‘base’ point at which you branched from another branch (as described above). You can also use ‘rebase’ to organize your commits before merging.

  • Strip whitespace: It is good practice not to commit extra whitespace at the end of lines or files. Git allows you to remove all extra whitespace during rebase - E.g. to origin/develop branch

    git rebase --whitespace=strip origin/develop
    
  • If you used the set-up script above, the alias ‘ws’ was added to allow you to achieve the same action with:

    git ws origin/develop
    
  • Rebase “interactive”: To remove, edit, combine etc. your commits, use the -i flag. Git will open an editor to allow you to edit the commit summary (gives instructions too). For example, to rebase onto origin/develop branch:

    git rebase -i origin/develop
    

Branch naming

We roughly follow the git-flow style of naming and managing branch. Info about the idea can be found under A successful Git branching model. There is also a screencast available on Vimeo.

Branching Model

The master branch is always “releasable”, almost always by having a tagged version merged into it. The develop branch is where unstable work takes place. At times, another stable branch with the version name appended (“dev_x_y”) is also active. PRs merged into this stable branch are also rebased onto develop.

For more information about how multiple branches are being maintained currently, see Continuous integration.

Advanced: Branch management

One large goal of the work with the forked repository model is to have both team members as well as external collaborators be aware of upcoming features as they happen, and have them be able to comment on the work as quickly and easily.

There is a danger of some members of the community not being aware of which branches are active and applicable, but if our weekly meetings contain a summary of what work is happening in which branch as opposed to just which tickets are in progress on the whiteboard, then it should be fairly easy for someone from the outside to get involved.

What follows is an explanation of the overarching way we categorize and review our branches. This is not required reading for everyone.

Branch types

To make working with a larger number of branches easily, we will initially introduce some terminology. Branches should typically be in one of three states: investigations, works-in-progress (WIP), or deliverables.

Description of branch types

Investigations

At the bottom of the figure above are the investigation branches. These are efforts which are being driven possibly by a single individual and which are possibly not a part of the current milestone. They may not lead to released code, or they may be put on hold for some period of time while other avenues are also investigated.

WIPs

For an investigation to move up to being a work-in-progress, it should have more involvement from the rest of team and have been discussed and documented via stories, mini-group meetings, etc. Where necessary – which will usually be the case – the major components (Bio-Formats, the model, the database, the server, at least one client) should be under way.

Deliverables

Finally, deliverable branches are intended for inclusion in the upcoming milestone. They have all the necessary “paper work” – requirements, stories, tasks, scenarios, tests, screencasts, etc. Where support is needed to get all of the pieces in place, the rest of the team can be involved. And when ready a small number (mostly likely just one) will be finalized and merged into “develop” at a time. This represents the post-sprint “demo” concept that has been discussed elsewhere.

The backlog

One non-branch type that should also be kept in mind is the backlog. Between major deliverables and while a WIP is being ramped up to a deliverable, the backlog should be continually worked on and the fix branches also merged in once tested and verified.

Branch workflow

With the definitions, we can walk through the progression of a branch from inception to delivered code.

First, someone, perhaps even an external collaborator, creates a branch, typically starting from master or develop (having them branch from the mainline should hopefully makes things easier later on). Work is first done locally, and then eventually pushed to github.com/YOURUSER/openmicroscopy. If you have given access to particular members of the team, then they may want to work directly on that branch. Alternatively, they may create branches from your branch, and send you commits – either via Pull Request or as patches – for you to include in your work.

You will want to start making others aware of your work so you can get feedback, but at this point there may not even be a well-defined requirement / story for your work. There should, however, at least be tasks for what you are working on in the whiteboard, so others know what is happening. It may be necessary, regardless, to also email ome-nitpick, ome-devel, or some subset of OME developers, with the branch name you want them to take a look at. A Pull Request could also be used.

After it is clear that there is some interest in your investigation branch, then the related stories and possibly requirement should be flushed out. The design of the work should be checked against the other parts of OME. For example, a GUI addition should fit into other existing workflows, and the implications on the other client (i.e. OMERO.web’s impact on OMERO.insight, or the other way around) should be evaluated.

At this point, the branch will most likely be considered a work-in-progress and will need to start getting ready for release. The various related branches will need to be kept in sync. Whether through a rebase or a merge workflow, all involved parties will need to make sure they regularly have an up-to-date view of the work going on.

For example, the “remotes.default” has been configured as above, a sensible thing to do every morning on coming to work is to run:

git remote update

and see all changes that the team have made:

~/git $ git remote update
Fetching team
Fetching origin
remote: Counting objects: 22, done.
remote: Compressing objects: 100% (8/8), done.
remote: Total 8 (delta 7), reused 0 (delta 0)
Unpacking objects: 100% (8/8), done.
From ssh://lust/home/git/omero
3f2ab6f..f80cbc4  dev_4_1_jcb -> origin/dev_4_1_jcb
Fetching gh
...
Fetching jm
remote: Counting objects: 46, done.
remote: Compressing objects: 100% (2/2), done.
remote: Total 24 (delta 19), reused 24 (delta 19)
Unpacking objects: 100% (24/24), done.
From git://github.com/jburel/openmicroscopy
* [new branch]      feature/plateAcquisitionAnnotation -> jm/feature/plateAcquisitionAnnotation
Fetching colin
From git://github.com/ximenesuk/openmicroscopy
* [new branch]      909-Proposal2 -> colin/909-Proposal2

If you want to get the changes for all submodules, you can use:

git submodule foreach --recursive git remote update

At this point, you may need to “merge –ff-only” or just “rebase” your work to incorporate the new commits:

git checkout 909-Proposal2
git show-branch 909-Proposal2 colin/909-Proposal2
git rebase colin/909-Proposal2

Finally, the WIP branch will have advanced far enough that it should be made release-ready, which will need to be discussed at a weekly meeting. Often at this point, the involved developers will need help from others getting the documentation, the testing, the screencasts, the scenarios, and all the other bits and bobs (the “paper work”) ready for release.

One at a time (at least initially), WIP branches will be picked and made into a deliverable. At this point, several people will have looked over the code and all the paper work, and the whole team should feel comfortable with the release-state of the branch. At this point, a Pull Request should be issued to the official openmicroscopy/openmicroscopy repository for the final merge. All the related branches in each individual’s repository can now be deleted.

A major benefit of having the paper work per deliverable done immediately is that if it becomes necessary the mainline, i.e. the “develop” branch of openmicroscopy/openmicroscopy, could be released far more quickly than if we have several deliverables simultaneously in the air.

Merge branches

A significant disadvantage to having separate lines of inquiry in separate branches is the possibility that there will be negative interactions between 2 or more branches when merged, and that these problems won’t be found until late in development. To offset this risk, it is possible and advisable to begin creating “temporary merge branches” earlier in development.

For example, if we assume that two of the branches from the git remote update command from above are intended for release fairly soon:

  • jm/feature/plateAcquisitionAnnotation
  • colin/909-Proposal2

Then we can create a temporary test branch:

git checkout -b test-909-and-plate origin/develop
git merge --no-ff jm/feature/plateAcquisitionAnnotation
git merge --no-ff colin/909-Proposal2

and build and test this composite. This need not be done manually, but assuming there’s a convention like “all branches for immediate release are prefixed with ‘deliverable/’ ”, then a jenkins job can attempt the merge, failing if it is not possible, and run all tests if it succeeds. Any weekly testing we do can use the artifacts generated by this build to be as sure as possible that nothing unexpected has leaked in.

Code reviews and comments

On the flip-side, a major advantage to having the above branching workflow is that is far easier to review the entire impact and style of a deliverable before it is integrated into the mainline. Any commit or even line which is being proposed for release can be commented as shown on https://github.com/features/#code-review

If you would like to include other users beyond just the branch owner in the discussion, you can use a twitter-style name to invite them (“@SOMEUSER”): https://github.com/blog/821-mention-somebody-they-re-notified

Pull Requests

Several times above “Pull Requests” (PR) have been mentioned. A Pull Request is a way to invite someone to merge from one repository to another. If the commits included in the PR can be seamlessly merged, then the target user need only click on a button. If not, then there may be some back-and-forth on the work done, similar to the code reviews of a deliverable branch. For background, see

If you have discovered that something in the proposed branch needs changing (and you do not have write access to the branch itself), then you can checkout the branch, make the fixes, push the branch, and open a Pull Request.

git checkout -b new_stuff SOMEUSER/new_stuff
# Modifications
git commit -a -m "My fix of the new_stuff"
git push gh new_stuff
# Now go to the new_stuff branch on github.com and open the PR

GitHub’s “Open a pull request” page invites you to leave a comment under the PR title: we use this comment to describe the PR. A good choice of PR title and description are both helpful to reviewers of your work. For the PR description there may be template text already provided for you to edit. If so then do consider what it says but also feel free to change that template as much as makes sense for describing your PR.

Pull Request conflicts

When issuing a pull request, usually you will the following message “This pull request can be automatically merged”. If this is not the case, follow a possible workflow to fix the problem. For the sake of this example, bugs is the branch we are working on:

# push the branch to GitHub
git push gh bugs:refs/heads/bugs
# issue a pull request, not possible to merge due to a conflict.

Now we need to fix the conflict:

# checkout your local branch
git checkout bugs
# fetch and merge origin/develop
git fetch origin
git merge origin/develop            # Any conflicts will be listed
# Edit the conflicting files to fix conflicts, then
git add path/to/file
git commit                          # Use the suggested 'merging...' message
git push gh bugs

Your branch should now be able to merge back into develop. This should only be done at the very end of a pull request just before it is merged into origin/develop. Multiple “pull origin/develop” messages in a branch would be very bad style.