mitchell852 commented on a change in pull request #4537: Add blueprint for 
Flexible Topologies

 File path: blueprints/
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+# Flexible Topologies
+## Problem Description
+Today, a Traffic Control CDN is limited to 2 tiers -- *EDGE* and *MID* -- with 
the option to skip the *MID* tier for certain Delivery Service types (e.g. 
`HTTP_LIVE` and `HTTP_NO_CACHE`). In addition, a CDN is limited to one global 
parent hierarchy, which is defined via the `parent_cachegroup` and 
`secondary_parent_cachegroup` fields of cachegroups. Both of these problems 
limit a CDN's ability to scale with increased demand and changing usage 
patterns, and providing the ability to add more tiers to a CDN helps it keep up 
with that growth. A Topology that works well for one set of Delivery Services 
might not be ideal for a different set of Delivery Services, and a CDN needs 
the flexibility to provide the best Topology for any given Delivery Service -- 
with any number of tiers and custom caching hierarchies.
+## Proposed Change
+Traffic Control will provide the ability to define one or more Topologies, and 
a Topology can have any number of Delivery Services assigned to it. A Topology 
will be composed of Cachegroups along with their primary/secondary parent 
relationships to other Cachegroups as defined by the Topology.
+If a Delivery Service is assigned to a Topology, any `deliveryservice_server` 
assignments it has to `EDGE` caches will be ignored, because it will be 
assigned to all caches in the Delivery Service's CDN (filtered by server 
capabilities) that belong to the Topology's cachegroups. Ideally, this feature 
will obsolete legacy `deliveryservice_server` assignments, since Topologies 
negate the need to assign Delivery Services to individual `EDGE` caches. 
Nonetheless, legacy `deliveryservice_server` assignments will be supported 
alongside Topology-based Delivery Services for some time until all Delivery 
Services have been migrated to Topologies.
+### Traffic Portal Impact
+Traffic Portal will need new pages for creating and viewing Topologies, and 
the Delivery Service form will need to be updated to add a new Topology field 
for assigning a Delivery Service to a Topology. If a Delivery Service is 
assigned to a Topology, Traffic Portal should prohibit assigning `EDGE` servers 
to the Delivery Service (`ORIGIN` servers may still need to be assignable for 
 Review comment:
   > If a Delivery Service is assigned to a Topology, Traffic Portal should 
prohibit assigning `EDGE` servers to the Delivery Service
   As should the API

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