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The key string contains key-value pairs allowing us to uniquely identify the instance of type defined by resource.
When looking at the XSD document, please notice that the first data member definition represents the identifier, in the example above, the identifier for esr-ems type is ems-id. That id is then used in creating path.
AAIService uses a construct that I would like to call Fully Qualified Object Identifier, or FQOID that is made of resource type name and resource identifier.
In this exampe, that identifier will be esr-ems.ems-id.
To write the request key string user needs to use format: "{FQOID} = 'value'" or "{FQOID} = $some.variable.pointing.to.data.in.SvcLogicContext.memory.id".