Mike,

Metron chooses a randomly-available Doppler for each message
<https://www.pivotaltracker.com/story/show/96801752>. Availability prefers
same-zone Doppler servers:

- If a Metron instance knows about any same-zone Dopplers, it chooses
one at random for each message.
- If no same-zone Dopplers are present, the random choice is made from
the list of all known servers.


In fact, the behavior you describe is the behavior of DEA Logging Agent
before Metron existed. What we discovered with that approach is that it
balances load very unfairly, as a single high-volume app is stuck on one
server. While the "new" mechanism does not guarantee consistency, it does
enable the Doppler pool to more-evenly share load.

If you're seeing that a single app instance is routed to the same Doppler
server every time, then (without further information) I would guess that
you're either running a single Doppler instance in each availability zone,
or your deck is stacked. :-) If neither of those is true and you're still
observing that Metron routes messages from an app instance to a single
Doppler, I'd love to investigate how that is happening.

– John Tuley