One Customer's Config Took Down the Edge — JavaScript Bug Hunt

Briefing

Modelled on the Fastly global outage (8 June 2021): a latent bug shipped weeks earlier was triggered when one customer pushed a valid configuration change. 85% of the CDN's network started returning errors within a minute.

router.js resolves a hostname to a backend and assumes the backend map is always populated. A customer whose config legitimately has no explicit backends takes the whole request path down.

Fix resolveBackend so an empty or missing map degrades to the default backend instead of failing.

Bug report

BUG-FASTLY · Priority: Critical · Reported by: edge SRE

resolveBackend(config, host) must return:

• config.backends[host] when that entry exists
• config.defaultBackend when it does not, or when backends is missing/empty
• the string "origin" when there is no default either

Observed: a config whose backends object is absent throws, and a config whose backends is empty returns undefined — both surface as a 503 for every request served by that edge node.

Logs

[edge] TypeError: Cannot read property 'www.example.com' of undefined
[edge] 85% of POPs returning 503

The code as shipped

src/edge/router.js (editable)

// Maps a hostname to the backend that should serve it.
exports.resolveBackend = function (config, host) {
  return config.backends[host];
};

Read-only context: src/edge/CONTRACT.js.