rob05c commented on a change in pull request #5127:
URL: https://github.com/apache/trafficcontrol/pull/5127#discussion_r504840659
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File path: docs/source/overview/delivery_services.rst
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@@ -743,6 +743,20 @@ Routing Name
------------
A DNS label in the Delivery Service's domain that forms the :abbr:`FQDN (Fully
Qualified Domain Name)` that is used by clients to request content. All
together, the constructed :abbr:`FQDN (Fully Qualified Domain Name)` looks
like: :file:`{Delivery Service Routing Name}.{Delivery Service xml_id}.{CDN
Subdomain}.{CDN Domain}.{Top-Level Domain}`\ [#xmlValid]_.
+.. _ds-secondary-mode:
+
+Secondary Mode
+--------------
+By default, cache configuration will immediately try a Secondary Parent when
the first Primary Parent fails.
+
+.. note:: Recall for Delivery Services using pre-Topology
DeliveryService-Server Assignments, the Primary Parent is the Parent CacheGroup
of the Edge Cache's CacheGroup, and the Secondary Parent is the Secondary
CacheGroup of the Edge's Cachegroup. For Topology Delivery Services, the
Primary is the first assigned Parent in the Topology, and the Secondary is the
second assigned Parent in the Topology.
+
+This is typically ideal when the requested object is likely to be in the cache
of all CacheGroups, since the Secondary parent is the first Primary Parent in
the hash ring for its own CacheGroup. However, it is likely slightly higher
latency to go to the further parent.
+
+However, if the resource is unlikely to be in cache, for example with live
streaming video, it's probably ideal to try another parent in the same Primary
CacheGroup.
+
+To configure a Delivery Service to retry all Primary Parents first before
falling back to Secondary Parents, create a Parameter assigned to the Delivery
Service's Profile with the Config File ``parent.config`` and the Name
``try_all_primaries_before_secondary``. The Parameter Value is ignored.
+
Review comment:
Changed
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