What does 'build once, deploy many' mean, and why does it matter?

Summary

The same build artifact (a container image digest, a JAR, a package) is promoted unchanged from test to staging to production; only configuration differs per environment. You never rebuild for production.

Longer Answer

Rebuilding per environment means what you tested is not what you ship: dependencies may resolve differently, base images may have changed, and build steps may vary. Promoting a single artifact gives traceability (this digest came from this commit and passed these tests), simpler rollbacks (redeploy the previous digest) and supply-chain integrity (sign once, verify everywhere). It requires externalised configuration and environment-agnostic builds.

Key Points

• Same artifact through every environment.
• Config injected per environment.
• Traceability and easy rollback.
• Rebuilding breaks test validity.