Git Merge

Git Merge
Name	Git Merge
Developer	Linus Torvalds, Junio Hamano, GitHub, GitLab
Released	2005
Latest release	Git 2.x
Programming language	C, Perl (programming language), Shell (computing)
Operating system	Linux, Windows, macOS
License	GNU General Public License

Git Merge Git Merge is a core operation in the Git (software) distributed version control system, used to integrate changes from different branches, reconcile divergent histories, and produce combined snapshots of a repository. It operates alongside commands such as git rebase and git checkout and underpins collaboration workflows used by organizations like Mozilla, Google, Facebook, Microsoft, and Netflix. The operation interacts with concepts and tools from projects and institutions such as Linux kernel, Open Source Initiative, Apache Software Foundation, and Free Software Foundation.

Overview

Git Merge takes two or more commit histories and produces a new commit that reconciles them, preserving ancestry metadata recorded by developers from teams at Red Hat, Canonical (company), Debian, SUSE, and IBM. The result is a merge commit (or a fast-forward update) that references parent commits similar to merge operations in systems used by Bitbucket, Atlassian, SourceForge, and GNU Savannah. Merge behavior is shaped by people and tools including Linus Torvalds, Junio Hamano, GitHub Actions, GitLab CI/CD, and integrations with Jenkins (software), Travis CI, and CircleCI.

How Git Merge Works

Git Merge reads commit objects, trees, and blobs from the object database introduced by Linus Torvalds for the Linux kernel workflow and computes three-way merges using a common ancestor (the merge base) via algorithms also relevant to projects like Mercurial and Bazaar (software). The operation uses the index to stage changes, writes tree objects, and creates commit objects signed optionally with GnuPG keys as practiced by contributors to GNU Project and OpenBSD. Merge can be invoked by users of environments like Visual Studio Code, IntelliJ IDEA, Eclipse, NetBeans, and Sublime Text, and is reflected in hosting services such as GitHub, GitLab, Bitbucket, and Azure DevOps.

Merge Strategies

Git offers strategies such as fast-forward, recursive, ours, theirs, and octopus, each used in workflows at companies like Spotify, Airbnb, Uber Technologies, Stripe (company), and Square (company). The recursive strategy, default for two-branch merges, performs a three-way merge and employs rename detection techniques used in Perforce and discussed in publications from ACM and IEEE. The octopus strategy handles multiple heads useful for large-scale merges in projects like KDE, GNOME, LibreOffice, and Blender (software). The ours and theirs semantics are used in conflict heuristics by teams at NASA and European Space Agency when integrating mission-critical repositories.

Conflicts and Resolution

Conflicts arise when concurrent edits touch the same lines or file modes, a scenario familiar to contributors to Linux kernel, Mozilla Firefox, Chromium (web browser), Apache HTTP Server, and OpenSSL. Resolution tools include git mergetool, external merge utilities like KDiff3, Meld (software), Beyond Compare, and IDE built-ins in PyCharm, Android Studio, and Xcode (IDE). Policies and code review practices from organizations such as Rackspace, Dropbox, Salesforce, Twitter, and LinkedIn guide conflict resolution and use conventions like commit message templates advocated by Conventional Commits and projects following standards by the IETF.

Best Practices and Workflow Integration

Best practices combine branching models like Git Flow and GitHub Flow with continuous integration patterns used by Travis CI, CircleCI, Jenkins (software), and GitHub Actions. Teams at Google, Facebook, Microsoft, Amazon (company), and Apple Inc. integrate merge strategies with code review systems such as Gerrit, Phabricator, and GitHub Pull Requests. Practices include feature toggles popularized by ThoughtWorks, trunk-based development advocated by Martin Fowler, and release engineering processes inspired by Google SRE and DevOps principles from DORA.

Examples and Use Cases

Common use cases include integrating feature branches created in repositories hosted on GitHub, GitLab, Bitbucket, and Azure DevOps; performing merges during release preparation for projects like Kubernetes, Docker (software), Terraform (software), and Ansible (software); and merging long-lived branches in large efforts such as the Linux kernel and Chromium (web browser). Educational resources from O'Reilly Media, Packt Publishing, Pluralsight, Coursera, and edX demonstrate merge scenarios, while community events like FOSDEM, OSCON, Git Merge (conference) and LinuxCon include talks and tutorials on merging techniques used by practitioners from Canonical (company), Red Hat, and SUSE.

Category:Version control systems