503 Service Unavailable on Fastly Magento2 admin endpoints (Edge Dictionary ListAll)

The problem

After enabling Fastly on a Magento Cloud staging environment, admin pages under Stores > Configuration intermittently showed a red error box ("A technical problem with the server created an error"), with XHR requests like admin/fastlyCdnEdgeDictionary/ListAll and admin/fastlyCdn_Vcl/checkAuthDictionary returning 503 Service Unavailable from the Varnish cache server. The failures were random: sometimes one endpoint failed, sometimes another, sometimes none. Magento 2.2.2, fastly/magento2 1.2.48. Another user confirmed the same on Magento Cloud Pro on every admin page load.

What fixed it

The module's admin XHR calls tripped over Magento Cloud session handling, producing intermittent 503s from Varnish on the Fastly admin endpoints. The Fastly team fixed it in PR #177, released in fastly/magento2 1.2.54. The reporter upgraded to 1.2.54 and confirmed the 503s were gone. Upgrade the fastly/magento2 extension to 1.2.54 or later.

A user asked: After enabling Fastly on a Magento Cloud staging environment, admin pages under Stores > Configuration intermittently showed a red error box ("A technical problem with the server created an error"), with XHR requests like admin/fastlyCdnEdgeDictionary/ListAll and admin/fastlyCdnVcl/checkAuthDictionary returning 503 Service Unavailable from the Varnish cache server. The failures were random: sometimes one endpoint failed, sometimes another, sometimes none. Magento 2.2.2, fastly/magento2 1.2.48. Another user confirmed the same on Magento Cloud Pro on every admin page load. The verified answer: The module's admin XHR calls tripped over Magento Cloud session handling, producing intermittent 503s from Varnish on the Fastly admin endpoints. The Fastly team fixed it in PR #177, released in fastly/magento2 1.2.54. The reporter upgraded to 1.2.54 and confirmed the 503s were gone. Upgrade the fastly/magento2 extension to 1.2.54 or later. Thread: https://vectle.com/threads/thrh65vZPmFMmowuQNVEGtFWw