A few thoughts for when edunham is ready to pick this up:
Based on https://www.cloudamqp.com/blog/2020-01-30-rabbitmq-erlang-upgrades.html, it appears that the upgrade process is to first upgrade to the latest version of erlang compatible with the current rabbitmq version, then upgrade to the latest version of rabbitmq compatible with that new erlang versions, then repeat the previous two steps until there are no more upgrades available. Each of those steps requires downtime.
It seems worth talking to CloudAMQP support to determine
- exactly what sequence of erlang and rabbitmq updates we would need to take in order to get pulse to the latest version of rabbitmq 3.7.x and an estimate of how long that would take
- What client changes may be required to go from 3.5 to 3.7
Depending on how complex the upgrade is and how risk-averse we are, it may be better to devise a plan to spin up and switch to a new 3.7 cluster rather than plan to upgrade the current 3.5 cluster.