While upgrading cache_upload (T131502) we've noticed issues when varnish 4 fetches from varnish 3. To avoid those problems with the cache_text transition to v4 we're planning to:
- Make ulsfo fetch from eqiad instead of codfw
- Make codfw fetch straight from the applayer instead of eqiad
- Upgrade cache_text codfw to varnish 4
At that point we'll be able to observe closely codfw's behavior (v4-only).
Assuming everything goes fine up to this point, we can start testing the behavior of v4<->v4 by routing ulsfo to codfw and upgrading ulsfo to v4.