What is Blast Radius?

Last updated: 2025-04-11

Blast radius is a measure of how far the impact of a code change propagates through a system — how many modules, services, endpoints, and data flows are affected if the change introduces a defect. A small, localized change has a small blast radius; a change to a shared utility has a large one.

Why does blast radius matter for engineering teams?

Not all PRs carry equal risk. A typo fix in a README has a blast radius of zero; a parameter change in a shared authentication middleware has a blast radius spanning every authenticated endpoint. Quantifying blast radius lets reviewers allocate attention proportionally — more scrutiny where the impact is wider.

How does Argus handle blast radius?

Argus calculates blast radius using the dependency graph. Each PR's changed files are traced through the graph to count affected downstream modules, cross-boundary calls, and critical path intersections. The result is a risk score that determines review depth and surfaces which downstream consumers reviewers should verify.

PRs with blast radius >10 modules have 4.7× higher post-merge defect rate than isolated changes — Facebook engineering review data, 2023

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