[WIP] Introduce generation id in openstack delimiter

This document describes the use of generation id in openstack
delimiter. I have neither fixed the format of the document nor
it may be the right place to host it. But this is something to
start the discussion within the team.

Change-Id: I55451c44c3b95a82f629fed44e19155a377c64d7

doc/source/gen_id.rst

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+..
+ This work is licensed under a Creative Commons Attribution 3.0 Unported
+ License.
+
+ http://creativecommons.org/licenses/by/3.0/legalcode
+
+=========================================
+Openstack Delimeter - Generation ID usage
+=========================================
+
+No blueprint, this is intended as a reference document.
+
+Statement of intent
+===================
+
+How to resolve a problem of cross project quota management seemlessly
+by ensuring that atomicity of each transaction is kept intact in a
+multi-user environment.
+
+
+Sequencing transactions using generation id
+===========================================
+
+- There are various ways to do locking of resources to ensure data
+consistency.
+
+- The generation id is a concept of sequencing parallel writes to a
+database via multiple threads in a highly concurrent environment in such
+a way that data consistency is retained.
+
+- To a layman, generation id is just a sequencing number that ensures
+that the state of the database is correctly updated in a multi-threaded
+world by making sure that all clients writing to the database have a
+consistent view/single view.
+
+- The generation id concept goes in parallel with a retry mechanism to
+ensure that if the generation ids don't match for a client during
+updates to the DB - a retry is performed to ensure that the data is 
+updated on the right state of the DB.
+
+
+Delimeter's use of generation id
+=================================
+
+- Two aspects - One on the producer/resource provider side and the other
+on the consumer side. However, the resource provider side of the story is
+NOT the responsibility of delimeter and hence it's purely left out to the
+individual services.
+
+- generation id is a generic concept that could be applied in case of
+delimeter to ensure that the consumers of the quotas have a consistent
+view in a highly concurrent environment. 
+
+
+Example usage of generation id in OpenStack Delimiter
+=====================================================
+
+<TBD>