[GitHub] [trafficserver] jrushford commented on pull request #7069: Move the direct self loop check later to HttpSM::do_http_server_open

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jrushford commented on pull request #7069:
URL: https://github.com/apache/trafficserver/pull/7069#issuecomment-667329221


   @SolidWallOfCode we have a use case where we want a peering edge cache group.  Say we have an edge cache group of cache1, cache2, and cache3.  Upstream, there is a mid-tier cache group in from of the origins, mid-tier1, and mid-tier2.  We want to send a client to an edge in the edge cache group where the parent's are an edge's peers and itself so as cache the video across the edge cache group and reduce lookups to the mid-tier using two remaps and two parent.config entries.
   
   remap.config on cache1:
     map http://cache1.foo.com http://cdn-back-remap.foo.com
     map http://cdn-back-remap.foo.com http://cdn-origin.foo.com
   
   remap.config on cache2:
     map http://cache2.foo.com http://cdn-back-remap.foo.com
     map http://cdn-back-remap.foo.com http://cdn-origin.foo.com  
   
   remap.config on cache3:
     map http://cache3.foo.com http://cdn-back-remap.foo.com
     map http://cdn-back-remap.foo.com http://cdn-origin.foo.com 
   
   parent.config on cache1, cache2, cache3
     dest_domain=cdn-origin.foo.com parent="mid-tier1:80|1.0;mid-tier2:80|1.0" round_robin=consistent_hash go_direct=false
     dest_domain=cdn-back-remap.foo.com parent="cache1:80|1.0;cache2:80|1.0;cache3:80|1.0" round_robin=consistent_hash go_direct=false ignore_self_detect=true
   
   With the above parent.configs, each edge, cache1, cache2, and cache3 will mark themselves down for SELF DETECTION but we say ignore the SELF DETECTION because ignore_self_detect for the parent.config entry is true.
   
   A request comes into cache1 on the first remap and is remapped to cdi-back-remap.foo.com.  it misses and hashes to one of the parents in the local edge cache group, possibly to itself.  when the request is forwarded to the back-remap, it gets remapped to the origin and if it misses, goes upstream to the mid-tiers and possibly the origin and gets cached locally on the edge.  In the end, a large asset is spread across the 3 edge caches drastically reducing the necessity to go to the higher tier or origin and makes better use of the cache in the edge group.  With the two remaps it does not cause any looping issues as long as you're careful.  In 8.1 a host will get marked down by parent selection if it detects itself in the parent list so, that's the reasoning for the ignore_self_detect flag.  Normally the self detection in parent selection prevents looping.
   
   This scenario also required setting proxy.config.http.insert_request_via_str INT 0 to prevent loop detection in HttpTransact.
   
   With these config settings, we're able to safely build a peering edge cache group to better utilize the edge's cache as a whole in ATS 8.1.  Now with that said, this is experimental and we have nothing in production yet that utilizes it.  We were taking advantage of the fact that we could loop safely one time in order to do this but with this PR, it will no longer be possible.
   
   


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