Conflicts
Last updated on 2024-12-04 | 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.
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.
To see how we can resolve conflicts, we must first create one. The
file mars.txt
currently looks like this in both partners’
copies of our planets
repository:
OUTPUT
Cold and dry, but everything is my favorite color
The two moons may be a problem for Wolfman
But the Mummy will appreciate the lack of humidity
Let’s add a line to the collaborator’s copy only:
OUTPUT
Cold and dry, but everything is my favorite color
The two moons may be a problem for Wolfman
But the Mummy will appreciate the lack of humidity
This line added to Wolfman's copy
and then push the change to GitHub:
OUTPUT
[main 5ae9631] Add a line in our home copy
1 file changed, 1 insertion(+)
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/planets.git
29aba7c..dabb4c8 main -> main
Now let’s have the owner make a different change to their copy without updating from GitHub:
OUTPUT
Cold and dry, but everything is my favorite color
The two moons may be a problem for Wolfman
But the Mummy will appreciate the lack of humidity
We added a different line in the other copy
We can commit the change locally:
OUTPUT
[main 07ebc69] Add a line in my copy
1 file changed, 1 insertion(+)
but Git won’t let us push it to GitHub:
OUTPUT
To https://github.com/vlad/planets.git
! [rejected] main -> main (fetch first)
error: failed to push some refs to 'https://github.com/vlad/planets.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.
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. Let’s start by pulling:
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/planets
* branch main -> FETCH_HEAD
29aba7c..dabb4c8 main -> origin/main
Auto-merging mars.txt
CONFLICT (content): Merge conflict in mars.txt
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:
OUTPUT
Cold and dry, but everything is my favorite color
The two moons may be a problem for Wolfman
But the Mummy will appreciate the lack of humidity
<<<<<<< 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:
OUTPUT
Cold and dry, but everything is my favorite color
The two moons may be a problem for Wolfman
But the Mummy will appreciate the lack of humidity
We removed the conflict on this line
To finish merging, we add mars.txt
to the changes being
made by the merge and then commit:
OUTPUT
On branch main
All conflicts fixed but you are still merging.
(use "git commit" to conclude merge)
Changes to be committed:
modified: mars.txt
OUTPUT
[main 2abf2b1] Merge changes from GitHub
Now we can push our changes to GitHub:
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/planets.git
dabb4c8..2abf2b1 main -> main
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:
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/planets
* branch main -> FETCH_HEAD
dabb4c8..2abf2b1 main -> origin/main
Updating dabb4c8..2abf2b1
Fast-forward
mars.txt | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
We get the merged file:
OUTPUT
Cold and dry, but everything is my favorite color
The two moons may be a problem for Wolfman
But the Mummy will appreciate the lack of humidity
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 Martian surface and
calls it mars.jpg
.
If you do not have an image file of Mars available, you can create a dummy binary file like this:
OUTPUT
-rw-r--r-- 1 vlad 57095 1.0K Mar 8 20:24 mars.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 mars.jpg
to his
repository:
OUTPUT
[main 8e4115c] Add picture of Martian surface
1 file changed, 0 insertions(+), 0 deletions(-)
create mode 100644 mars.jpg
Suppose that Wolfman has added a similar picture in the meantime. His
is a picture of the Martian sky, but it is also called
mars.jpg
. When Dracula tries to push, he gets a familiar
message:
OUTPUT
To https://github.com/vlad/planets.git
! [rejected] main -> main (fetch first)
error: failed to push some refs to 'https://github.com/vlad/planets.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:
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/planets.git
* branch main -> FETCH_HEAD
6a67967..439dc8c main -> origin/main
warning: Cannot merge binary files: mars.jpg (HEAD vs. 439dc8c08869c342438f6dc4a2b615b05b93c76e)
Auto-merging mars.jpg
CONFLICT (add/add): Merge conflict in mars.jpg
Automatic merge failed; fix conflicts and then commit the result.
The conflict message here is mostly the same as it was for
mars.txt
, but there is one key additional line:
OUTPUT
warning: Cannot merge binary files: mars.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 mars.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 mars.jpg
$ git add mars.jpg
$ git commit -m "Use image of surface instead of sky"
OUTPUT
[main 21032c3] Use image of surface instead of sky
If instead we want to use Wolfman’s version, we can use
git checkout
with Wolfman’s commit identifier,
439dc8c0
:
BASH
$ git checkout 439dc8c0 mars.jpg
$ git add mars.jpg
$ git commit -m "Use image of sky instead of surface"
OUTPUT
[main da21b34] Use image of sky instead of surface
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 mars.jpg
$ git mv mars.jpg mars-surface.jpg
$ git checkout 439dc8c0 mars.jpg
$ mv mars.jpg mars-sky.jpg
Then, remove the old mars.jpg
and add the two new
files:
BASH
$ git rm mars.jpg
$ git add mars-surface.jpg
$ git add mars-sky.jpg
$ git commit -m "Use two images: surface and sky"
OUTPUT
[main 94ae08c] Use two images: surface and sky
2 files changed, 0 insertions(+), 0 deletions(-)
create mode 100644 mars-sky.jpg
rename mars.jpg => mars-surface.jpg (100%)
Now both images of Mars are checked into the repository, and
mars.jpg
no longer exists.
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 filenumbers.txt
- 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.txt |
|
3 | ||
4 | ||
5 | ||
6 | Celebrate! |
order | action . . . . . . | command . . . . . . . . . . . . . . . . . . . |
---|---|---|
1 | Update local | git pull origin main |
2 | Make changes | echo 100 >> numbers.txt |
3 | Stage changes | git add numbers.txt |
4 | Commit changes | git commit -m "Add 100 to numbers.txt" |
5 | Update remote | git push origin main |
6 | Celebrate! |
Key Points
- 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.