Backwards compatibility
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The ability of new software to work with older versions, data, or integrations without breaking them.
Backwards compatibility means new versions of your software continue to work with existing code, data, and integrations. If a developer built something on version 1 of your API, it still works when you release version 5. Their code does not break. Their data does not disappear. Their workflows keep running.
This is extraordinarily hard to maintain and extraordinarily valuable. Every new feature, every bug fix, every performance improvement must be evaluated against the question: will this break anything that exists today? The answer shapes everything from API design to database migrations to configuration file formats.
Companies that maintain backwards compatibility earn developer trust. Stripe has API versions dating back to 2011 that still work. Java maintained backwards compatibility so aggressively that code compiled in 1996 runs on modern JVMs. The cost of this discipline is real, but the cost of breaking developer trust is higher.
Examples
An API adds a new required field.
Instead of making the field required immediately (which would break existing clients), the team makes it optional with a sensible default value. Existing integrations continue working unchanged. New integrations can use the field for enhanced functionality. After 12 months, the team evaluates whether to make it required in the next major version.
A database migration preserves old data formats.
The system stores addresses as a single string field. The new version uses structured address components (street, city, state, zip). The migration script parses existing strings into components where possible and preserves the original string in a legacy_address field. No data is lost. Old API responses include both formats during the transition period.
A CLI tool maintains old flag names.
Version 2 of the CLI renames --output-format to --format for brevity. But the old flag still works as an alias. A deprecation warning appears when the old flag is used, pointing to the new name. Build scripts that use the old flag continue to work without modification.
In practice
Read more on the blog
Frequently asked questions
When is it acceptable to break backwards compatibility?
When the cost of maintaining it exceeds the cost of breaking it. Security vulnerabilities that cannot be patched without breaking changes are a clear case. Fundamentally flawed designs that limit the platform's future are another. But you need a long deprecation period, a clear migration path, and direct communication with affected users. Never break compatibility silently.
How do you test for backwards compatibility?
Keep a suite of tests that exercise old API versions and behaviors. Run these tests against every new release. Maintain a set of 'contract tests' that verify old clients still get expected responses. Some teams keep actual old client versions in their test pipeline. If the tests pass, compatibility is maintained. If they fail, you know exactly what broke.
Related terms
A code change that causes existing functionality or integrations to stop working.
Application programming interface: a defined way for software programs to communicate with each other.
A scripted, versioned change to a database schema that evolves the data structure over time.
Software development kit: a set of tools and libraries for building with a specific platform or API.
The process of releasing code to servers where users can access it.
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