Availability: Adds account-level read regions as effective preferred regions when preferred regions is not set on client. #4709
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Problem Statement
As of today, customers who do not configure
ApplicationPreferredRegionsorApplicationRegionare pinned to either the hub region (in multi-write accounts) or the primary region (in single-write accounts). In outage scenarios, availability is scoped to just a single region and is not ideal (both reads/writes in multi-write accounts and reads in single-write multi-region accounts have their availability curbed to just the one region). SettingApplicationPreferredRegionsorApplicationRegionas empty is not an opt-out of availability decision from the customer's perspective unless of course on the client a regional endpoint has been set. This PR aims to fix this issue when preferred regions is not set, and a global endpoint is set on the client.Approach taken in this PR
The idea is to construct an effective preferred region list and to rely on account-level read and account-level write regions for it. If there is client-perceived unavailability or account-level topology changes - then this effective preferred regions list is reordered accordingly or reflects the account-level regions post a cached
DatabaseAccountrefresh in the SDK.There are also changes made to the flow which decides when
DatabaseAccountrefresh is triggered. The decision depends on a check whether the SDK has a different effective first preferred read / write region from the first account-level read / write region.Closing issues
closes #4665