Just resolved a major outage at my organization - the error involved changing the map service in question for using dedicated instances to using the shared instance pool.
(Published to ArcGIS Server 10.9.1 using Pro 3.0)
Details below, but my understanding is that dedicated vs shared instance pool is purely about resource usage on the server, and should never impact the capabilities or behavior of a map service outside of the resources used.
Brief summary - we have a custom ArcMap add-in to use for some complex editing to enforce data quality and simplify editing for non-ArcMap experts. The tool consumes a Map Service and Geodata service (named the same thing) to allow users to check out data for editing, then check it back in. The tool was broken after a migration to newer OS and Arc Enterprise versions and infrastructure (Win 2019 and Enterprise 10.9.1, from 2012 and 10.7.1 respectively). We figured there were just some inherent compatibility issues as the tool is older, custom, and the services are published through Pro.
The error the tool returns when we use dedicated instances on the map service is:
"Error getting unique IDs from map server: value does not fall in expected range"
However, the tool will allow users to check out data and edit/check in when we set the map service to use the Shared Instance Pool vs. the dedicated pool.
the related Geodata service is using dedicated instances and that does not seem to be causing any issues.
This feels very much like a bug, or maybe just a sign of creeping compatibility issues in our infrastructure.
But I am looking to confirm that Shared vs. Dedicated only comes in to play for service performance tuning and resource usage, and should not impact functionality.
I would have never thought that would be the cause of our problem.
Thanks -
Allen