Conflicts

Last updated on 2024-03-12 | Edit this page

Overview

Questions

  • What do I do when my changes conflict with someone else’s?

Objectives

  • Explain what conflicts are and when they can occur.
  • Resolve conflicts resulting from a merge.

A story of two collaborators

Two users can make independent sets of changes on the same document.

Different Versions Can be Saved

If multiple users make changes to different sections of the document you can incorporate two sets of changes into the same base document.

Multiple Versions Can be Merged

However, if multiple users make changes to the same sections of the document, this will generate a conflict.

The Conflicting Changes

As soon as people can work in parallel, they’ll likely step on each other’s toes. This will even happen with a single person: if we are working on a piece of software on both our laptop and a server in the lab, we could make different changes to each copy. Version control helps us manage these conflicts by giving us tools to resolve overlapping changes.

Before resolving conflicts, we’ll review two features that a good collaborator puts in practice: Create branches and make Atomic commits.

Group Challenge

A Typical Work Session

You sit down at your computer to work on a shared project that is tracked in a remote Git repository. During your work session, you take the following actions, but not in this order:

  • Make changes by appending the number 100 to a text file numbers.md
  • Update remote repository to match the local repository
  • Celebrate your success with some fancy beverage(s)
  • Update local repository to match the remote repository
  • Stage changes to be committed
  • Commit changes to the local repository

In what order should you perform these actions to minimize the chances of conflicts? Put the commands above in order in the action column of the table below. When you have the order right, see if you can write the corresponding commands in the command column. A few steps are populated to get you started.

order action . . . . . . command . . . . . . . . . .
1
2 echo 100 >> numbers.md
3
4
5
6 Celebrate! AFK
order action . . . . . . command . . . . . . . . . . . . . . . . . . .
1 Update local git pull origin main
2 Make changes echo 100 >> numbers.md
3 Stage changes git add numbers.md
4 Commit changes git commit -m "Add 100 to numbers.md"
5 Update remote git push origin main
6 Celebrate! AFK

Create a conflict

To see how we can resolve conflicts, we must first create one. The file sitrep.Rmd currently looks like this in both partners’ copies of our cases repository:

R 

usethis::edit_file("sitrep.Rmd")

OUTPUT 

Comparison of attack rates in different age groups
This can identify priority groups for interventions
Maps illustrate the spread and impact of outbreak

Let’s add a line to the collaborator’s copy only:

R 

usethis::edit_file("sitrep.Rmd")

OUTPUT 

Comparison of attack rates in different age groups
This can identify priority groups for interventions
Maps illustrate the spread and impact of outbreak
This line added to Wolfman's copy

and then push the change to GitHub:

BASH 

$ git add sitrep.Rmd
$ git commit -m "Add a line in our home copy"

OUTPUT 

[main 5ae9631] Add a line in our home copy
 1 file changed, 1 insertion(+)

BASH 

$ git push origin main

OUTPUT 

Enumerating objects: 5, done.
Counting objects: 100% (5/5), done.
Delta compression using up to 8 threads
Compressing objects: 100% (3/3), done.
Writing objects: 100% (3/3), 331 bytes | 331.00 KiB/s, done.
Total 3 (delta 2), reused 0 (delta 0)
remote: Resolving deltas: 100% (2/2), completed with 2 local objects.
To https://github.com/vlad/cases.git
   29aba7c..dabb4c8  main -> main

Now let’s have the owner make a different change to their copy without updating from GitHub:

R 

usethis::edit_file("sitrep.Rmd")

OUTPUT 

Comparison of attack rates in different age groups
This can identify priority groups for interventions
Maps illustrate the spread and impact of outbreak
We added a different line in the other copy

We can commit the change locally:

BASH 

$ git add sitrep.Rmd
$ git commit -m "Add a line in my copy"

OUTPUT 

[main 07ebc69] Add a line in my copy
 1 file changed, 1 insertion(+)

but Git won’t let us push it to GitHub:

BASH 

$ git push origin main

OUTPUT 

To https://github.com/vlad/cases.git
 ! [rejected]        main -> main (fetch first)
error: failed to push some refs to 'https://github.com/vlad/cases.git'
hint: Updates were rejected because the remote contains work that you do
hint: not have locally. This is usually caused by another repository pushing
hint: to the same ref. You may want to first integrate the remote changes
hint: (e.g., 'git pull ...') before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.
The Conflicting Changes

Git rejects the push because it detects that the remote repository has new updates that have not been incorporated into the local branch. What we have to do is pull the changes from GitHub, merge them into the copy we’re currently working in, and then push that.

Resolve conflict

The owner need to solve this conflict to be able to push to the remote repository.

Let’s start by pulling:

BASH 

$ git pull origin main

OUTPUT 

remote: Enumerating objects: 5, done.
remote: Counting objects: 100% (5/5), done.
remote: Compressing objects: 100% (1/1), done.
remote: Total 3 (delta 2), reused 3 (delta 2), pack-reused 0
Unpacking objects: 100% (3/3), done.
From https://github.com/vlad/cases
 * branch            main     -> FETCH_HEAD
    29aba7c..dabb4c8  main     -> origin/main
Auto-merging sitrep.Rmd
CONFLICT (content): Merge conflict in sitrep.Rmd
Automatic merge failed; fix conflicts and then commit the result.

The git pull command updates the local repository to include those changes already included in the remote repository. After the changes from remote branch have been fetched, Git detects that changes made to the local copy overlap with those made to the remote repository, and therefore refuses to merge the two versions to stop us from trampling on our previous work. The conflict is marked in in the affected file:

R 

usethis::edit_file("sitrep.Rmd")

OUTPUT 

Comparison of attack rates in different age groups
This can identify priority groups for interventions
Maps illustrate the spread and impact of outbreak
<<<<<<< HEAD
We added a different line in the other copy
=======
This line added to Wolfman's copy
>>>>>>> dabb4c8c450e8475aee9b14b4383acc99f42af1d

Our change is preceded by <<<<<<< HEAD. Git has then inserted ======= as a separator between the conflicting changes and marked the end of the content downloaded from GitHub with >>>>>>>. (The string of letters and digits after that marker identifies the commit we’ve just downloaded.)

It is now up to us to edit this file to remove these markers and reconcile the changes. We can do anything we want: keep the change made in the local repository, keep the change made in the remote repository, write something new to replace both, or get rid of the change entirely. Let’s replace both so that the file looks like this:

R 

usethis::edit_file("sitrep.Rmd")

OUTPUT 

Comparison of attack rates in different age groups
This can identify priority groups for interventions
Maps illustrate the spread and impact of outbreak
We removed the conflict on this line

To finish merging, we add sitrep.Rmd to the changes being made by the merge and then commit:

BASH 

$ git add sitrep.Rmd
$ git status

OUTPUT 

On branch main
All conflicts fixed but you are still merging.
  (use "git commit" to conclude merge)

Changes to be committed:

	modified:   sitrep.Rmd

BASH 

$ git commit -m "Merge changes from GitHub"

OUTPUT 

[main 2abf2b1] Merge changes from GitHub

Now we can push our changes to GitHub:

BASH 

$ git push origin main

OUTPUT 

Enumerating objects: 10, done.
Counting objects: 100% (10/10), done.
Delta compression using up to 8 threads
Compressing objects: 100% (6/6), done.
Writing objects: 100% (6/6), 645 bytes | 645.00 KiB/s, done.
Total 6 (delta 4), reused 0 (delta 0)
remote: Resolving deltas: 100% (4/4), completed with 2 local objects.
To https://github.com/vlad/cases.git
   dabb4c8..2abf2b1  main -> main

Pull before editing

Git keeps track of what we’ve merged with what, so we don’t have to fix things by hand again when the collaborator who made the first change pulls again:

BASH 

$ git pull origin main

OUTPUT 

remote: Enumerating objects: 10, done.
remote: Counting objects: 100% (10/10), done.
remote: Compressing objects: 100% (2/2), done.
remote: Total 6 (delta 4), reused 6 (delta 4), pack-reused 0
Unpacking objects: 100% (6/6), done.
From https://github.com/vlad/cases
 * branch            main     -> FETCH_HEAD
    dabb4c8..2abf2b1  main     -> origin/main
Updating dabb4c8..2abf2b1
Fast-forward
 sitrep.Rmd | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

We get the merged file:

R 

usethis::edit_file("sitrep.Rmd")

OUTPUT 

Comparison of attack rates in different age groups
This can identify priority groups for interventions
Maps illustrate the spread and impact of outbreak
We removed the conflict on this line

We don’t need to merge again because Git knows someone has already done that.

Git’s ability to resolve conflicts is very useful, but conflict resolution costs time and effort, and can introduce errors if conflicts are not resolved correctly. If you find yourself resolving a lot of conflicts in a project, consider these technical approaches to reducing them:

  • Pull from upstream more frequently, especially before starting new work
  • Use topic branches to segregate work, merging to main when complete
  • Make smaller more atomic commits
  • Push your work when it is done and encourage your team to do the same to reduce work in progress and, by extension, the chance of having conflicts
  • Where logically appropriate, break large files into smaller ones so that it is less likely that two authors will alter the same file simultaneously

Conflicts can also be minimized with project management strategies:

  • Clarify who is responsible for what areas with your collaborators
  • Discuss what order tasks should be carried out in with your collaborators so that tasks expected to change the same lines won’t be worked on simultaneously
  • If the conflicts are stylistic churn (e.g. tabs vs. spaces), establish a project convention that is governing and use code style tools (e.g. htmltidy, perltidy, rubocop, etc.) to enforce, if necessary

Solving Conflicts that You Create

Clone the repository created by your instructor. Add a new file to it, and modify an existing file (your instructor will tell you which one). When asked by your instructor, pull her changes from the repository to create a conflict, then resolve it.

Conflicts on Non-textual files

What does Git do when there is a conflict in an image or some other non-textual file that is stored in version control?

Let’s try it. Suppose Dracula takes a picture of the laboratory test and calls it lab.jpg.

If you do not have an image file of the lab available, you can create a dummy binary file like this:

BASH 

$ head -c 1024 /dev/urandom > lab.jpg
$ ls -lh lab.jpg

OUTPUT 

-rw-r--r-- 1 vlad 57095 1.0K Mar  8 20:24 lab.jpg

ls shows us that this created a 1-kilobyte file. It is full of random bytes read from the special file, /dev/urandom.

Now, suppose Dracula adds lab.jpg to his repository:

BASH 

$ git add lab.jpg
$ git commit -m "Add picture of laboratory test"

OUTPUT 

[main 8e4115c] Add picture of laboratory test
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 lab.jpg

Suppose that Wolfman has added a similar picture in the meantime. His is a picture of the laboratory site, but it is also called lab.jpg. When Dracula tries to push, he gets a familiar message:

BASH 

$ git push origin main

OUTPUT 

To https://github.com/vlad/cases.git
 ! [rejected]        main -> main (fetch first)
error: failed to push some refs to 'https://github.com/vlad/cases.git'
hint: Updates were rejected because the remote contains work that you do
hint: not have locally. This is usually caused by another repository pushing
hint: to the same ref. You may want to first integrate the remote changes
hint: (e.g., 'git pull ...') before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.

We’ve learned that we must pull first and resolve any conflicts:

BASH 

$ git pull origin main

When there is a conflict on an image or other binary file, git prints a message like this:

OUTPUT 

$ git pull origin main
remote: Counting objects: 3, done.
remote: Compressing objects: 100% (3/3), done.
remote: Total 3 (delta 0), reused 0 (delta 0)
Unpacking objects: 100% (3/3), done.
From https://github.com/vlad/cases.git
 * branch            main     -> FETCH_HEAD
   6a67967..439dc8c  main     -> origin/main
warning: Cannot merge binary files: lab.jpg (HEAD vs. 439dc8c08869c342438f6dc4a2b615b05b93c76e)
Auto-merging lab.jpg
CONFLICT (add/add): Merge conflict in lab.jpg
Automatic merge failed; fix conflicts and then commit the result.

The conflict message here is mostly the same as it was for sitrep.Rmd, but there is one key additional line:

OUTPUT 

warning: Cannot merge binary files: lab.jpg (HEAD vs. 439dc8c08869c342438f6dc4a2b615b05b93c76e)

Git cannot automatically insert conflict markers into an image as it does for text files. So, instead of editing the image file, we must check out the version we want to keep. Then we can add and commit this version.

On the key line above, Git has conveniently given us commit identifiers for the two versions of lab.jpg. Our version is HEAD, and Wolfman’s version is 439dc8c0.... If we want to use our version, we can use git checkout:

BASH 

$ git checkout HEAD lab.jpg
$ git add lab.jpg
$ git commit -m "Use image of test instead of site"

OUTPUT 

[main 21032c3] Use image of test instead of site

If instead we want to use Wolfman’s version, we can use git checkout with Wolfman’s commit identifier, 439dc8c0:

BASH 

$ git checkout 439dc8c0 lab.jpg
$ git add lab.jpg
$ git commit -m "Use image of site instead of test"

OUTPUT 

[main da21b34] Use image of site instead of test

We can also keep both images. The catch is that we cannot keep them under the same name. But, we can check out each version in succession and rename it, then add the renamed versions. First, check out each image and rename it:

BASH 

$ git checkout HEAD lab.jpg
$ git mv lab.jpg lab-test.jpg
$ git checkout 439dc8c0 lab.jpg
$ mv lab.jpg lab-site.jpg

Then, remove the old lab.jpg and add the two new files:

BASH 

$ git rm lab.jpg
$ git add lab-test.jpg
$ git add lab-site.jpg
$ git commit -m "Use two images: test and site"

OUTPUT 

[main 94ae08c] Use two images: test and site
 2 files changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 lab-site.jpg
 rename lab.jpg => lab-test.jpg (100%)

Now both images of the lab are checked into the repository, and lab.jpg no longer exists.

Good practices

A good collaborator:

  • Pull from upstream frequently before new edits.
  • Use topic or feature branches to segregate work.
  • Make atomic commits.
  • Push your work when it is done to reduce work in progress.
  • Break large files when appropriate.
  • Read contributing guidelines from the project owner.

Create branches

Sometimes you want to experiment with your project without affecting the main version. You can do this by using branches. A branch is a local copy of the main project (also called the main branch) where you can make changes and test new ideas. The main branch stays safe and unchanged while you work on your branch (a.k.a., feature branch). When you are satisfied with the changes, you can merge them into the main branch. This means that the separate lines of development in your branch are combined with the main branch.

One feature branch and one main branch in Git.
One feature branch and one main branch in Git.

You can have more than one branch off of your main copy. If one of your branches ends up not working, you can either abandon it or delete it without impacting the main branch of your project.

Two feature branches and one main branch in Git.
Two feature branches and one main branch in Git.

To create a branch, first, verify that you are working on the main branch:

BASH 

$ git status

OUTPUT 

On branch main

The git branch command creates a branch of the name newbranch:

BASH 

$ git branch newbranch

The git checkout command switch from the current branch (in this case, main) to the newbranch:

BASH 

$ git checkout newbranch

OUTPUT 

Switched to branch 'newbranch'

Now, you can confirm that we are in newbranch using git status:

BASH 

$ git status

OUTPUT 

On branch newbranch
nothing to commit, working tree clean

Now, let’s make an edit to the sitrep.Rmd file:

R 

usethis::edit_file("sitrep.Rmd")

OUTPUT 

Comparison of attack rates in different age groups
This can identify priority groups for interventions
Maps illustrate the spread and impact of outbreak
Use packages listed in the CRAN Task View: Epidemiology

Add and commit this change to the local repository:

BASH 

$ git add sitrep.Rmd
$ git commit -m "Add edit in new branch"

OUTPUT 

[newbranch 60d5ff9] Add edit in new branch
 1 file changed, 1 insertion(+)

Push commit to the remote repository. NOTE: instead of git push origin main, we replace main with the name of the new branch:

BASH 

$ git push origin newbranch

OUTPUT 

Enumerating objects: 5, done.
Counting objects: 100% (5/5), done.
Delta compression using up to 12 threads
Compressing objects: 100% (3/3), done.
Writing objects: 100% (3/3), 339 bytes | 169.00 KiB/s, done.
Total 3 (delta 2), reused 0 (delta 0), pack-reused 0
remote: Resolving deltas: 100% (2/2), completed with 2 local objects.
remote:
remote: Create a pull request for 'newbranch' on GitHub by visiting:
remote:      https://github.com/vlad/cases/pull/new/newbranch
remote:
To https://github.com/vlad/cases.git
 * [new branch]      newbranch -> newbranch

On GitHub, we get a notification to Create a Pull Request:

In your local repository, you can leave your work parked for a while on that branch. To return to main (or to any other branch), you can use the same command to switch between branches:

BASH 

$ git checkout main

OUTPUT 

Switched to branch 'main'
Your branch is up to date with 'origin/main'.

Atomic commits

A good commit is also an atomic commit, like in the commit of the last challenge:

BASH 

$ git add lab-test.jpg
$ git add lab-site.jpg
$ git commit -m "Use two images: test and site"

Commits should be ‘atomic’, meaning that they should do one simple thing and they should do it completely. (The Turing Way Community). Atomic commits prioritize one unit of change instead of the quantity of changes.

Two related modified lines must be part of the same commit.
Two related modified lines must be part of the same commit.

In Rstudio, the Review changes window has a button called “Stage chunk”. This helps to make atomic commits, even if you change a lot of lines in a single edit. You can either select them to make isolated commits or to unite them to be part of the same commit.

Two unrelated edits must be part of two isolated commits.
Two unrelated edits must be part of two isolated commits.

Good practice

A good atomic commit:

  • Includes more than one file that involves one unit of change.

  • Isolate or includes multiple edited lines using the “Stage chunk” button in Rstudio.

  • Does not include all the files in one commit.

Key Points

  • Version control also allows many people to work in parallel.
  • Conflicts occur when two or more people change the same lines of the same file.
  • The version control system does not allow people to overwrite each other’s changes blindly, but highlights conflicts so that they can be resolved.
  • Feature branches help to segregate work and minimize conflicts.
  • Atomic commits prioritize one unit of change instead of the quantity of changes.