Git/Workflow/Topic

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Introduction

This workflow is based on the branchy development workflow documented by git help workflows.

Motivation

The primary goal of this workflow is to make release preparation and maintenance easier. We set the following requirements, of which some are themselves worthwhile goals:

  • Amortize the effort of release preparation throughout development
  • Support granular selection of features for release
  • Allow immature features to be published without delaying release
  • Keep unrelated development paths (topics) independent of one another

Design

The design of this workflow is based on the observation that meeting the highlighted goal makes the other goals easy. It is based on branchy development in which each branch has a well-defined purpose.

We define two branch types:

  • Topic Branch
    • Commits represent changes (real work)
    • Distinguished by feature (one topic per feature or fix)
    • Named locally by each developer (describe purpose of work)
    • Heads not published (no named branch on server)
  • Integration Branch
    • Commits represent merges (merge topics together)
    • Distinguished by stability (release maintenance, release preparation, development edge)
    • Named everywhere
    • Heads published on server

Notation

This document uses ascii-art to depict commit history:

Branch name 

master

Current branch 

*master

Commit with parent in same branch 

----o

Commit with parent in another branch 

\
 o

Commit with two parents (merge) 

 ----o
    /

Commit with arbitrary ancestry 

...o

Topic branches generally consist of a linear sequence of commits forked off an integration branch: 

...o  master
    \
     o----o----o----o  topic

Integration branches generally consist of a sequence of merge commits: 

        ...o      ...o
            \         \
...o----o----o----o----o  master
       /         /
   ...o      ...o

Integration Branches

We use an integration branch for each stage of development:

  • maint: Release maintenance (high stability). Only bug fixes should be published here. Only the release manager can push here.
  • master: Release preparation (medium stability). Only mature features and bug fixes should be published here.
  • next: Development (low stability). New features should be published here.

Each integration branch consists of merge commits whose first parent is the previous merge on the same branch, and whose second parent is a topic branch integrated by the merge. This is achieved by carefully following the steps described below. It is important that merges have their parents ordered correctly so that a mid-level overview of the development progress can be produced by 

$ git log --first-parent master

This traverses history starting at master but follows only the first parent of each merge. This produces a view of history like 

            \         \
...o----o----o----o----o  master
       /         /

in which topic merges are shown but the topic details are left out.

Development

We cover below the steps to take during each phase of development.

New Topic

Create a new topic branch for each separate feature or bug fix. Always start the topic from a stable integration branch, usually master. If the topic fixes a bug in the current release, use maint. Never start a topic from next! In the following table we review the steps and commands to create, develop, and publish a topic branch based on master.

Actions Results

Update master to base work on the most recently integrated features.

$ git checkout master
 
...o *master
$ git pull
 
...o----o *master

Create the local topic branch. Use a meaningful name for "topic". Name topics like you might name functions: concise but precise. A reader should have a general idea of the change to be made given just the branch name.

$ git checkout -b topic
 
...o----o  master
        ^ *topic

This is where the real work happens. Edit, stage, and commit files repeatedly as needed for your work. During this step, avoid the "urge to merge" from an integration branch. Your changes are safely committed; you can merge your work with others when you are finished.

$ edit files
$ git add -- files
$ git commit
 
...o----o  master
         \
          o *topic
$ edit files
$ git add -- files
$ git commit
 
...o----o  master
         \
          o----o *topic

When the topic is ready for publication it must be merged into next. Do not merge from next! It should be the current local branch when you merge.

Switch to next and update it. Use "git checkout -b next origin/next" to create a local next the first time.

$ git checkout next
$ git pull
 
...o----o  master
 .       \
  .       o----o  topic
   .
    ........o *next

Merge the topic and test it.

$ git merge topic
 
...o----o  master
 .       \
  .       o----o  topic
   .            \
    ........o----o *next

Finally, publish the change.

$ git push

Mature Topic

When a topic is ready for inclusion in the next release, we merge it into master.

Actions Results

Update master to get the latest work by others. We will merge the topic into it.

$ git checkout master
 
...o----o *master
 .       \
  .       o----o  topic
   .            \
    ........o----o  next
$ git pull
 
  ..........
 .          \
...o----o----o *master
 .       \
  .       o----o  topic
   .            \
    ........o----o  next

Merge the topic and test it.

$ git merge topic
 
  ..........
 .          \
...o----o----o---o *master
 .       \      /
  .       o----o  topic
   .            \
    ........o----o  next

Delete the local branch.

$ git branch -d topic
 
  ..........
 .          \
...o----o----o---o *master
 .       \      /
  .       o----o
   .            \
    ........o----o  next

Finally, publish the change.

$ git push

Note that master sees only the topics that have been merged into it. It cannot reach any of the merges into next: 

  ..........
 .          \
...o----o----o---o *master
         \      /
          o----o (topic)

Old Topic

Sometimes we need to continue work on an old topic that has already been merged to an integration branch and for which we no longer have a local topic branch. To revive an old topic, we create a local branch based on the last commit from the topic (this is not one of the merges into an integration branch).

First we need to identify the commit by its hash. It is an ancestor of the integration branch into which it was once merged, say next. Run git log with the --first-parent option to view the integration history: 

$ git log --first-parent next
commit 9057863...
Merge: 2948732 a348901
...
    Merge branch topicA

commit 2948732...
Merge: 1094687 b235725
...
    Merge branch topicB

commit 1094687...
Merge: 8267263 c715789
...
    Merge branch topicC

Locate the merge commit for the topic of interest, say topicB. It's second parent is the commit from which we will restart work (b235725 in this example).

Actions Results

Create a local topic branch starting from the commit identified above.

$ git checkout -b topic b235725
 
  ..........
 .          \
...o----o----o---o  master
 .       \      /
  .       o----o *topic (b235725)
   .            \
    ........o----o----o  next
           /         /
       ...o      ...o
     (c715789) (a348901)

Continue development on the topic.

$ edit files
$ git add -- files
$ git commit
 
  ..........
 .          \
...o----o----o---o  master
 .       \      /
  .       o----o----o *topic
   .            \
    ........o----o----o  next
           /         /
       ...o      ...o
$ edit files
$ git add -- files
$ git commit
 
  ..........
 .          \
...o----o----o---o  master
 .       \      /
  .       o----o----o----o *topic
   .            \
    ........o----o----o  next
           /         /
       ...o      ...o

When the new portion of the topic is ready, merge it into next and test.

$ git checkout next
$ git pull
$ git merge topic
 
  ..........
 .          \
...o----o----o---o  master
 .       \      /
  .       o----o----o----o  topic
   .            \         \
    ........o----o----o----o *next
           /         /
       ...o      ...o

Publish next.

$ git push

Optionally repeat the above, publishing to next, to continue development based on feedback from initial publication. Finally, when the new changes are mature, merge to master and publish.

$ git checkout master
$ git pull
$ git merge topic
 
  ..........
 .          \
...o----o----o---o---------o *master
 .       \      /         /
  .       o----o----o----o  topic
   .            \         \
    ........o----o----o----o  next
           /         /
       ...o      ...o
$ git branch -d topic
$ git push
 
  ..........
 .          \
...o----o----o---o---------o *master
 .       \      /         /
  .       o----o----o----o
   .            \         \
    ........o----o----o----o  next
           /         /
       ...o      ...o

Again, note that master sees only the topics that have been merged into it. It cannot reach any of the merges into next: 

  ..........
 .          \
...o----o----o---o---------o *master
         \      /         /
          o----o----o----o (topic)

Discussion

History Shape

The history graphs produced by this workflow may look complex compared to the fully linear history produced by a rebase workflow (used by CVS and Subversion): 

  ..........
 .          \
...o----o----o---o---------o  master
 .       \      /         /
  .       o----o----o----o  topic
   .            \         \
    ........o----o----o----o  next
           /         /
       ...o      ...o

However, consider the view of history along each branch:

topic 

      ...
         \
          o----o----o----o  topic

master 

            \
...o----o----o---o---------o  master
                /         /

next 

                \         \
         ...o----o----o----o  next
           /         /

Each branch by itself looks linear and has only commits with a specific purpose. The history behind each commit is unique to that purpose. Topics are independent.

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