For dse to use eventlets rather than threads

This spec proposes a change to Congress and the DSE to make
the DSE use greenthreads rather than regular threads.

Change-Id: If2ea9a54982c602e60dfda0cfc0852224846e876

specs/juno/convert-the-dse-to-eventlets.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
+
+=====================================================
+Convert the DSE from using threads to using eventlets
+=====================================================
+
+https://blueprints.launchpad.net/congress/+spec/dse
+
+Currently, the OpenStack standards are to use eventlets over threads.  The
+DSE framework was merged into Congress using threads and needs to be
+changed to use eventlets to be more in line with the OpenStack standards.
+
+Problem description
+===================
+
+The DSE currently uses threads for each of the services it loads, while
+the rest of OpenStack uses eventlets over threads.  Eventlets can scale
+better than threads and are able to be stopped, while threads cannot be
+stopped with a single method.  This raises a problem because if there
+is a limit to the amount of threads that can be run without causing
+performance issues.
+
+Proposed change
+===============
+
+The change that I propose is to make the deepSix class inherit from
+green thread.GreenThread (from the eventlet python module) rather than inheriting
+from threading.Thread.  This will make each service a greenthread rather than
+a thread.
+
+Also, when the DSE creates the services in the create service method, it
+will spawn them as greenthreads, rather than start threads.
+
+Alternatives
+------------
+
+None
+
+Policy
+------
+
+None
+
+Policy Actions
+--------------
+
+None
+
+Data Sources
+------------
+
+None
+
+
+Data model impact
+-----------------
+
+None
+
+REST API impact
+---------------
+
+None
+
+Security impact
+---------------
+
+None
+
+Notifications impact
+--------------------
+
+None
+
+Other end user impact
+---------------------
+
+The loading and starting of a d6service will change.
+
+Old way (taken from test_dse.py lines 31-35):
+    cage = congress.dse.d6cage.d6cage()
+    # so that we exit once test finishes; all other threads are forced
+    #    to be daemons
+    cage.setDaemon(True)
+    cage.start()
+
+New way:
+    cage = deepsix.spawnService(congress.dse.d6cage.d6Cage)
+    cage.d6stop() # Stops the cage and kills the greenthreads
+
+Performance Impact
+------------------
+
+This code will be called every time a d6service object is created. The
+implemented code should not have any effects on performance since it
+is executed as quickly as threading is.
+
+Other deployer impact
+---------------------
+
+None
+
+Developer impact
+----------------
+
+None
+
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  harrison.kelly (email: harrison.kelly@plexxi.com)
+
+Work Items
+----------
+
+Change d6cage.py to run each service as a greenthread instead of a thread.
+
+Change deepsix.py to inherit from greenthread.GreenThread instead of
+threading.Thread and spawn greenthreads instead of threads.
+
+
+Dependencies
+============
+
+None
+
+
+Testing
+=======
+
+Tests with DSE modules will be used to check if implementation does not effect
+the capabilities of the DSE.
+
+
+Documentation Impact
+====================
+
+None
+
+
+References
+==========
+
+None