What Does "git pull origin [branchname]" Mean?

On distributed software teams, using the git version control system enables developers to individually experiment and test changes even as everyone needs to integrate and sync those changes back into the shared codebase. At the heart of making this flexible collaboration model work is the essential "git pull" command for merging remote changes from teammate‘s branches into your local copy.

Specifically, the incantation:

git pull origin [branchname]

fetches the latest code from the named [branchname] on the GitHub remote repository and merges it into your matching local branch.

But what exactly does this ubiquitous git pull do under the covers? And what considerations and best practices should developers know when pulling branches?

Understanding How Git Pulls and Merges Branches

While it may seem simple on the surface, a git pull orchestrates a sequence of Git objects updates that enable branches to be distributedly edited and continually integrated with the latest code:

Step 1: Downloading the Remote Branch Commits

When pulling from GitHub‘s remote repository named "origin", Git first enumerates any new commits on the remote [branchname] that don‘t exist locally. This uses Git‘s underlying object model where branches are just pointers to the latest commit:

These new commits and their changes to files and directories are downloaded recursively using Git‘s packfile compression mechanism into the local .git directory.

Step 2: Fast-Forwarding the Local Branch

Next, Git advances the local branch pointer to point to the latest remote commit just fetched:

This ensures it tracks the upstream branch on GitHub and hasn‘t diverged.

Step 3: Merging Remote Changes into Local

Finally, Git analyses the files and changes between where the local and remote branches forked previously versus the latest remote commit, and automatically merges them into the local codebase:

The key thing is that git pull encapsulates both fetching the remote changes, fast-forwarding the local snapshot of that branch, AND integrating those changes via merge.

Now the local repository is synchronized, containing all the latest commits and file changes from teammates. Developers can add their own new commits locally, before pushing back up again to share with other branches.

Common Branch Collaboration Workflows Using Git Pull

This remote fetching and merging enables flexible workflows for developers collaborating across feature branches:

Centralized Workflow

In a basic centralized workflow, developers create local topic branches, make commits then push back up to a central "main" branch. Whenever they need the latest changes from teammates, they pull the main branch:

git checkout my-feature
# Edit files, git add and git commit
git push origin my-feature

git checkout main
git pull origin main

This keeps their local main branch and features up-to-date.

Gitflow Using Pull Requests

Popular branching models like Gitflow leverage pull requests to propose merging feature branches into main or release branches. Developers open PRs against the base branch which teammates review before pulling down and QAing locally:

# Open PR: my-feature -> dev branch
git push -u origin my-feature
# Other reviewer pulls branch 
git checkout dev
git pull origin my-feature
# Approve PR if changes check out!  

Continuous Integration and Delivery

For teams practicing CI/CD, shared pipelines automatically pull branches under test to validate builds before promoting releases:

integrationTests:
  stage: test 
  script:
    - git pull origin my-feature
    - ./runTests

Here pull requests kick off pipelines that fetch branches and feedback test results.

Behind Git‘s Branch Pull Model

Enabling these collaborative development patterns by pulling branches is powered by key aspects of Git‘s design:

Distributed VCS

Git‘s distributed architecture means each developer has a full copy of the branch commit history and files locally. This empowers local workflows while still able to sync across any remote repository like GitHub.

Cheap Local Branches

Creating, editing and deleting local branches is lightweight with Git‘s pointers and directed acyclic graph (DAG) structure. Developers can quickly work on features locally and isolate changes from mainline.

Robust Merge Tracking

Built-in merge commits explicitly track branching and reconvergence points in Git histories. Combined with Git‘s content tracking this handles conflicts and eases collaborating across parallel branches.

Non-Destructive Integrating

No centralized "locking" of files. Pulling branches merges remote changes while keeping local changes intact that haven‘t been committed yet. This avoids forced wipeouts or clashes.

Together these facets mean nearly any custom team workflow can clone, pull and push branches using local or centralized patterns. GitHub‘s pull requests augment lightweight branching with access controls, notifications and lifecycle tracking.

Common Challenges with git pull origin [branch]

While pulling branches enables flexible collaboration, at scale teams run into issues like:

Merge Conflict Hell

The most dreaded aspect of git pull – merge conflicts from overlapping edits across different branches:

Auto-merging file.txt
CONFLICT (content): Merge conflict in file.txt
Automatic merge failed; fix conflicts and then commit the result.

# Resolve both sets of edits then commit 

Careful branch isolation and testing minimizes conflicts appearing at pull time.

History Rewriting Headaches

If pulled commits modify local history shared branches can get Complex quickly. Force pushing overwritten shared branches tears down teammate‘s work.

Unstable Changes

Just because a branch passes CI tests doesn‘t guarantee it integrates cleanly or won‘t destabilize mainline. Peer code reviews before pulling branches reduces bad apples getting merged.

Developers mitigated issues by communicating changes early, limiting branch lifespan, making atomic commits and testing rigorously with CI.

Patterns for Successful Git Branching

When leveraged properly, pulling branches can amplify collaboration. Best practices include:

• Open short-lived local branches for each feature or bugfix
• Rebase and squash commits that make cohesive changes before sharing branches
• Push branches early for visibility and backup even if in progress
• Merge frequently from target integration branches to surface conflicts
• Delete local and remote branches once merged or closed

This keeps branches focused, integrated and history clean.

Combined with pull requests for code review and CI tests against proposed integration branches, teams reduce merge issues and safely build on each other‘s changes reported by GitHub branch analytics.

The Future of Distributed Git Collaboration?

Looking ahead, GitHub envisions turning collaborative coding into a real-time multiplayer experience through telemetry, commenting and merging workflows integrated into IDEs and developer tools.

"Imagine multi-caret editing in VS Code where you‘re watching your colleague type in real time, interacting via comments, reviewing in a minimized pane, pulling changes in instantly…" – Sana Kashif, GitHub Senior Software Engineer

As distributed version control becomes ubiquitous from open source to enterprise software, improving branch-based collaboration models unlocks the potential for safer, higher velocity team delivery.