I am going through the first free chapter - Does it mean that all the content repositories are hidden behind a facade?
So I can fire a query and no matter where it lies - the query will pull out the answer for me?
Taking the example as a reference point, I could equal the three ECM systems ( Share Point, FileNet, Alfresco) as three tables in an RDBMS -
and some kind of joining capability is provided by CMIS... The query can be fulfilled by consulting some content from Share Point - and some content
You are close, but you are thinking about it as a "federated" model, which the specification really does not provide for. The CMIS specification allows you to use one common API and query language, though. So if you have those three systems you can use the exact same code to query each one in turn and then aggregate those results using your own logic. But CMIS does not federate multiple CMIS servers into a common logical server--you cannot issue one query and have CMIS execute it against all of the CMIS-compliant servers in your organization.