Hey folks,

At Comcast we have a need to augment CLIENT_STEERING (also regular
STEERING while we're at it) to allow targets to be ordered/sorted
based upon the client's proximity to the origin of the target delivery
services. I'd like to get your feedback on my proposed design and
address any of your concerns.

For HTTP_LIVE targets for instance, we'd like edge caches to ideally
retrieve/serve data from an Origin that is close to the client and
fall back to Origins that are farther and farther away. This allows
for increased redundancy while ordering optimal Origins (Delivery
Services) for a client to choose from.

For example, I have 3 Origins in different locations: Seattle, Denver,
and Boston. I would create an HTTP_LIVE delivery service for each
origin and a CLIENT_STEERING delivery service with those delivery
services as targets. I would then like to have those targets ordered
based upon proximity to the client. So a client in Seattle would get
the list [Seattle, Denver, Boston], while a client in Boston would get
the list [Boston, Denver, Seattle].

To make things more complicated, I want to add a redundant origin in
each location and split traffic between them (like STEERING_WEIGHT
today) while taking into account the geo-ordering. I also want to be
able to force an ordering (like STEERING_ORDER today) among co-located
targets.

In order to accomplish this I propose to:
1. add two new steering types: GEO_ORDER and GEO_WEIGHT (by adding a
target of type GEO_*, a steering DS would then enable geo-ordering)
2. associate a Delivery Service Origin with a latitude/longitude,
thereby associating a lat/long to a GEO_* target

Item 1 is pretty straightforward and will also play nicely with the
current steering types (STEERING_ORDER and STEERING_WEIGHT). I've
completed a POC within Traffic Router that basically provides the
following ordering when mixing all 4 types in a single steering
delivery service:
- Negative STEERING_ORDER targets
- GEO_WEIGHT and GEO_ORDER targets, grouped by proximity to the
client, ordered by geo-order and the consistent-hashing from the
weightings
- STEERING_WEIGHT targets (consistent-hashed)
- Positive STEERING_ORDER targets

Item 2 is not as straightforward because the simple thing would be to
just add an Origin Lat/Long field to the Delivery Service and call it
a day. However I don't think we should do that, and I'll dive more
into that in a separate thread (coming soon).

Does anyone have questions/concerns about adding these new GEO_ORDER
and GEO_STEERING target types and geo-sorting them based upon
proximity to the client? Are you okay with the proposed ordering when
all the steering types are mixed together?

- Rawlin

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