Integrations Points for Purging on D8

[aweingarten]

There are a bunch of integration points that need to happen to allow end-to-end purging.

• Work with Acquia hosting to support cache-tag clearing on varnish.
• Work with CloudFlare to support 16k http cache headers currently only 255 bytes are supported.
• Work with CloudFlare to support high-traffic, high velocity cache clearing. Currently according to them "You may purge up to 30 tags in one API call, and make up to 200 purge API calls in every 24 hour period."

see
https://support.cloudflare.com/hc/en-us/articles/206596608-How-to-Purge-Cache-Using-Cache-Tags