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

 File path: blueprints/
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+"License"); you may not use this file except in compliance
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+Unless required by applicable law or agreed to in writing,
+software distributed under the License is distributed on an
+KIND, either express or implied.  See the License for the
+specific language governing permissions and limitations
+under the License.
+# 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 
+Since Delivery Services will no longer be constrained to one global Topology 
as they are today, it would be extremely useful to be able to visualize a 
Delivery Service's Topology like a tree, where each node in the tree is a 
cachegroup, and the edges between nodes are the primary/secondary parent 
relationships between them. Clicking on a particular node would show all the 
servers in that cachegroup that could serve a request for the Delivery Service. 
This visualization will most likely be different from the Topology form for 
creating a Topology and does not necessarily need to be provided by Traffic 
+### Traffic Ops Impact
+Traffic Ops will provide the ability to create Topologies, composed of 
cachegroups and parent relationships, which will be assignable to one or more 
Delivery Services.
+#### REST API Impact
+The following is the JSON representation of a `Topology` object:
+    "name": "foo",
+    "description": "a foo topology",
+    "nodes": [
+        {
+            "cachegroup": "child-cachegroup",
+            "parents": [1, 2]
+        },
+        {
+            "cachegroup": "parent-cachegroup",
+            "parents": []
+        },
+        {
+            "cachegroup": "secondary-parent-cachegroup",
+            "parents": []
+        }
+    ]
+The following table describes the top-level `Topology` object:
+| field       | type                        | optionality | description        
+| ----------- | --------------------------- | ----------- | 
------------------------------------------------------------------- |
+| name        | string                      | required    | a unique name for 
identifying this Topology                         |
+| description | string                      | required    | the description of 
this Topology                                    |
+| nodes       | array of `node` sub-objects | required    | the set of `nodes` 
in this topology, similar to an *adjacency list* |
+The following table describes the `node` sub-object:
+| field      | type              | optionality | description                   
+| ---------- | ----------------- | ----------- | 
+| cachegroup | string            | required    | the `short_name` of a 
cachegroup this node maps to in the Topology                                    
+| parents    | array of integers | required    | zero-based indexes to other 
nodes in the Topology's `nodes` array, where the 1st element is for the 
*primary* parent relationship and the 2nd element is for the *secondary* parent 
relationship, and so on |
+API constraints:
+- a Topology must have at least 1 `node`; otherwise, it is useless
+- there cannot be multiple `nodes` for the same cachegroup in a Topology
+- `parents` must have 0, 1 or 2 elements, cannot contain duplicates, cannot 
contain the index of its own `node`, and cannot contain the index of `nodes` 
whose cachegroup is of type `EDGE_LOC`
+- leaf `nodes` must be cachegroups of type `EDGE_LOC`
+- all `nodes` in the Topology must be reachable -- i.e. a `node` is either a 
leaf (which would be an `EDGE_LOC`) or is a parent of at least one other node
+- a Topology cannot contain a cycle (through any combination of 
primary/secondary parent relationships)
+The following new endpoints will be required:
+##### `GET /topologies`
+response JSON:
+{ "response": [
+    {
+        "name": "foo",
+        "description": "a foo topology",
+        "nodes": [
+            {
+                "cachegroup": "child-cachegroup",
+                "parents": [1, 2]
+            },
+            {
+                "cachegroup": "parent-cachegroup",
+                "parents": []
+            },
+            {
+                "cachegroup": "secondary-parent-cachegroup",
+                "parents": []
+            }
+        ]
+    }
+##### `POST /topologies`
 Review comment:
   scratch that. you mention it later under security section.

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