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Meta Belljar

Belljar is Meta's CI/CD testing framework for continuously detecting critical startup dependencies among control-plane services. It runs in Meta's CI/CD pipelines to uncover and eliminate circular dependency risks before they are deployed to production.

Originally disclosed in Meta's May 2022 blog post on developer tools.

Role in PowerLoss Recovery

Belljar forms the prevention half of Meta's belt-and-braces approach to circular dependencies during region bootstrap:

• Belljar (prevention) — continuously detects dependency risks early and often in CI/CD, eliminating most, if not all risks before production deployment
• Twrko (recovery) — purpose-built jumpstart capability to break any unexpected circular dependencies that slip past Belljar

Given the rapid evolution of Meta's infrastructure, both are required — prevention alone cannot guarantee zero new circular dependencies, and recovery alone accepts too much production risk.

Seen in

• sources/2026-06-03-meta-lights-out-systems-on-validating-instant-power-loss-readiness — named as the CI/CD detection layer for startup dependencies