Cosmic Module

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Qubits of DPK

March 30, 2026

Core SWE @ Google
Large software organizations maintain enormous codebases with thousands of engineers contributing code.
Sometimes engineers must make changes that affect many parts of the codebase at once. These are known as large-scale changes (LSCs).
Examples include:
  • updating a commonly used API
  • renaming a shared function
  • changing a security rule across the entire system
  • migrating to a new library
This chapter explains how large organizations safely perform such changes without breaking systems.
The central idea of this chapter is:
Large-scale changes allow organizations to evolve their codebases consistently across thousands of files and teams.
Without a structured approach, updating large systems would be extremely difficult.

What Are Large-Scale Changes?

A large-scale change is a modification that affects many parts of a codebase simultaneously.
These changes may involve:
  • hundreds or thousands of files
  • multiple services or libraries
  • many engineering teams
Large-scale changes are common in long-lived systems where shared components are widely used.

Why Large-Scale Changes Are Necessary

Over time, software systems evolve.
Organizations may need to perform large-scale changes for several reasons.
Large-scale changes help keep the entire codebase modern and maintainable.

Challenges of Large-Scale Changes

Large-scale changes introduce several challenges.
Examples include:
Organizations must develop tools and processes to handle these changes safely.

Automated Refactoring

To perform large-scale changes efficiently, engineers often use automated tools.
These tools can:
  • search for patterns in the codebase
  • automatically update code
  • apply consistent modifications across files
Automation reduces human error and significantly speeds up the process.

Example of a Large-Scale Change

Suppose a company decides to rename a function used across many services.
Old function:
plain text
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1getUserData()
New function:
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1fetchUserProfile()
If hundreds of services use the old function, engineers must update every reference.
Automated tools can search the entire codebase and update these references consistently.

Incremental Migration

Sometimes large-scale changes are performed gradually rather than all at once.
Steps may include:
  1. #
    Introduce the new system.
  2. #
    Encourage teams to adopt the new approach.
  3. #
    Monitor usage of the old system.
  4. #
    Eventually remove the outdated implementation.
This gradual approach reduces disruption.

Ownership and Coordination

Large-scale changes often require coordination across teams.
Teams responsible for shared components must:
  • communicate upcoming changes
  • provide migration guidelines
  • support teams during transitions
Clear ownership ensures that these changes proceed smoothly.

Testing During Large Changes

Testing becomes especially important during large-scale changes.
Organizations rely on:
  • automated unit tests
  • integration tests
  • continuous integration pipelines
These systems verify that changes do not break existing functionality.

Monitoring Impact

After applying large-scale changes, engineers monitor systems to detect potential issues.
Monitoring may include:
  • system logs
  • performance metrics
  • error reports
If problems occur, engineers can quickly revert or fix the change.

Benefits of Large-Scale Changes

Large-scale changes provide important advantages.
These changes allow large codebases to evolve over time.

Key Lessons from Chapter 21

This chapter emphasizes several important ideas.
  1. #
    Large-scale changes affect many parts of a codebase simultaneously.
  2. #
    Automation helps apply consistent updates across thousands of files.
  3. #
    Coordination between teams is essential during large changes.
  4. #
    Testing ensures that updates do not break existing systems.
  5. #
    Large-scale changes help maintain long-term system health.

Simple Explanation (For Non-Technical Readers)

Imagine a company deciding to update the address format used in all its documents.
If thousands of documents exist, updating them manually would take a long time.
Instead, automated tools can update all documents at once.
Large-scale software changes work the same way.
They allow organizations to update many parts of a system efficiently and consistently.