How to Change the Base Branch of a Pull Request in Git

As a full-stack developer working on large projects with multiple branches, you may find yourself needing to change the base branch of an existing pull request. Whether you accidentally targeted the wrong branch initially or requirements changed mid-stream, Git allows you to alter the base branch with just a few clicks.

But as codebases and team sizes scale, managing shifting pull requests and keeping context in sync becomes critical to avoiding downstream issues that cripple productivity.

In this comprehensive guide, we‘ll cover:

• The hidden costs of inflexible branches
• Step-by-step instructions for changing pull request bases
• Reference git architecture for managing fluid branches
• Special considerations for monorepos vs. multi-repos
• Integrating dynamic base branches into CD pipelines
• Additional tips from an enterprise full-stack perspective

By the Numbers: The Hidden Costs of Inflexible Branches

To frame why engineering teams should care about easily changing pull request bases, it helps to quantify the overhead imposed by inflexible workflows:

• 17+ hours lost per developer each month to merge conflict resolution (State of Octoverse)
• 47% of developers need to rework the same feature multiple times due to upstream code shifts (Atlassian)
• 62% of developers spend more than 10 hours per week context switching between branches (SourceLevel)

These statistics indicate just how costly merge issues, rework, and cross-branch context switching can compound over time.

By adopting fluid branch management practices (like changing PR bases freely), teams can dramatically simplify coordination and reduce waste.

Step-by-Step Guide

Now that we‘ve established the critical role adaptable branches play at scale, let‘s walk through the specific steps to change the base branch of an existing pull request:

Initial Pull Request

We‘ll assume you‘ve already created a pull request targeting the incorrect base branch:

The key things to note are:

• The PR diff shows changes between the feature branch (new-feature) and initial base branch (release/1.1.0)
• The contingency GitHub selects reflects the original base branch (release/1.1.0)

Edit the Base Branch

To change the base branch, click on the Edit button in the upper right:

This will display a dropdown that lists all potential base branches:

From here, simply select the desired new base branch.

In our example, we‘ll change it to main since the original target release already shipped:

Updated Pull Request

Once selected, GitHub will automatically recalculate the PR diff against the new base branch:

Now the contingency shows what changes would occur if merging new-feature into the main branch instead of the original release/1.1.0 target.

Behind the scenes, Git handles the branch change by:

1. Closing out the existing pull request
2. Creating a new pull request with the specified base branch

From your perspective as a developer, the pull request appears to update seamlessly.

Merge the Pull Request

With the base branch updated, you can move forward with the normal pull request workflow:

• Build and validate in the updated context
• Update documentation
• Seek peer reviews and approvals

Once satisfied, merge into the new base just as you would any other pull request.

For teams with branch protection rules enabled, this may require another round of reviews/approvals after changing the base branch.

Proposed Git Architecture for Fluid Branch Management

The simplicity of changing pull request bases makes continuously realigning branches possible. But to scale this flexibility, having an intentional branching model and workflow is key.

Here is one proposed git architecture that balances agility with governance:

A few key principles relevant to fluid base branches:

• Maintain a single, evergreen main branch that serves as the integration anchor
• Create descriptive feature branches off main for specific issues/stories
• Open draft pull requests early against main to establish context
• Change PR bases liberally as requirements evolve or new target contexts emerge
• Enforce approvals on PR merges to main depending on risk levels
• Continually groom stale/merged branches to limit noise

Clarifying team workflows through architecture documents helps reduce confusion when leveraging advanced functionality like swap-able pull request bases.

Special Scenarios: Monorepo vs. Multi-Repo Setups

When working with complex git architectures involving multiple interconnected repositories, changing pull request bases has additional considerations:

Monorepos

In monolithic repository models, all code exists within a single git repository.

Benefits relative to base branch changes:

• Simpler branch interdependencies since everything branches from same origin
• Easier test cascading impacts of base branch shifts
• Single source of truth for all in-flight branches/pull requests

Considerations:

• More tedious to propagate base branch edits across many interrelated pull requests
• Increased chance of breaking unseen dependencies across internal packages

Multi-Repos

In multi-repo setups, code gets partitioned across many coupled git repositories.

Benefits relative to base branch changes:

• Compartmentalizes risk of major framework upgrades tied to base branch shifts
• Generally easier to rebase/resync a single repository at a time

Considerations:

• Difficult to reason about cross-repository dependencies when changing bases
• No central hub to track branch/PR changes across repos
• Fan-out updates to linked repositories with long daisy chain effects

Understanding these architectural trade-offs helps inform how to best leverage dynamic base branches in your specific environment.

Incorporating Fluid Bases Into Continuous Delivery Pipelines

Beyond day-to-day git workflows, allowing pull request base branches to change fluidly has major implications on continuous integration and delivery (CI/CD) pipelines.

A few ways this can be addressed:

Dynamic Job Definitions

Hardcoding base branch names when defining build/deploy jobs leads to broken pipelines any time bases shift. For example:

job: Build FeatureX against release/1.5

Instead, leverage symbolic placeholders that you substitutions with parameters at runtime.

job: Build $PR_BASE_BRANCH against $PR_TARGET_BRANCH 

Generate Branches Programmatically

Rather than manually specifying base branches which immediately go out of date, auto-generate symbolic branch names aligned to conventions like releases or semver:

release_branch = f”release/x.{minor_version}” 
pr_base = generateBaseBranch(app, release_branch)

Trigger Automated Rebuilds

Wrap key jobs with handlers that detect base branch updates on linked pull requests and automatically re-execute builds.

This shifts burden away from developers to manually restart pipelines after changing PR bases.

Conditional Pipeline Stages

Use policy-driven pipelines that analyze pull requests context before executing later stages. For example:

if PR base branch changed:
   rerun all validation tests
else: 
   provision infrastructure 

Building CI/CD systems robust to fluid git changes unblocks teams from greater agility.

Additional Tips for Full-Stack Developers

Here are some extra pointers when changing pull request bases specifically for full-stack developers managing complex cloud-based apps:

Sandbox Base Branch Upgrades

Proactively create isolated environments to test rebasing and merging large framework upgrades introduced directly in base branches. This mitigates risk of locking production due to untested migrations.

Automate Schemas/Infrastructure

Rely on infrastructure-as-code templates coupled with automated schema migrations rather than manually tracking database changes across long-lived feature branches. This simplifies validating infrastructure changes against fluid base branches.

Containerize Dependencies

Utilize containerized build environments to encapsulate and port all required dependencies instead of assuming presence on base branches. Containers help insulation from base branch changes.

Implement Feature Flags

Wrap new functionality with configurable flags to coordinate staged rollouts across environments, regardless of shifting base branches changes get merged into. This avoids incomplete transitions.

Model Baseless Services

Structure services to be as stateless and base branch agnostic as possible. For example, leverage message queues vs. direct service-to-service requests. This reduces coupling to base branches.

Building in these safeguards around adaptability enables full-stack teams to scale git fluidity and velocity in complex environments.

Key Takeaways

Mature engineering teams aim to remove as much waste and friction from their development lifecycle as possible. Providing low overhead mechanisms for developers to quickly change pull request base branches without losing context allows just that.

In this extended guide, we walked step-by-step how to alter pull request bases within GitHub, while framing key considerations for larger teams around architecture, tooling, and processes.

The goal of a holistic git branching strategy should be maximizing outcome (code deployed, issues fixed, user value) while minimizing output (PRs raised, branches managed, keystrokes spent).

Fluid base branches streamline these signals – enabling developers to spend more time building features rather than rebuilding context.

Hopefully by illuminating these dynamics from an enterprise full-stack angle, other scaling organizations can further optimize throughput by embracing adaptable git workflows.

Let me know if any other base branch questions come up!