2 clarifications needed for ObjectID fields:
1) Are ObjectID's globally unique across a subscription or across all of Rally's subscriptions?
2) Are The ObjectID's of built in Rally things constants and the exact same for all subscriptions? For example, in one of my workspaces, to get the allowed ScheduleState values for a UserStory, i have to hit this endpoint:
/AttributeDefinition/-41562/AllowedValues
where -41562 is the ObjectID. Can I assume that every other subscription uses -41562 for the ObjectID in this URL to get the valid Schedule States?
1) ObjectID's are unique per stack. So all of the ObjectID's on the SaaS stack (rally1.rallydev.com) are unique.
2) ObjectID's that are negative like the ScheduleState one mentioned above will be the same across workspaces. However things like custom fields and portfolioitem type and attributes will have unique ObjectID's across different workspaces.
Are you wanting to cache these values for perf reasons or what is it you're looking to do with them?