Document restrictions for working on cells v1
Since the 2015/11/12 nova meeting we've said that cells v1 is in feature freeze and we won't focus on hard-to-fix latent bugs. We should include that information in the devref so it's treated as a policy. Change-Id: I31186463fd832c27accc05feb82713fe71f603a8
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@ -11,6 +11,55 @@
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License for the specific language governing permissions and limitations
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under the License.
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=======
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Cells
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=======
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Cells V1
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========
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Historically, Nova has depended on a single logical database and message queue
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that all nodes depend on for communication and data persistence. This becomes
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an issue for deployers as scaling and providing fault tolerance for these
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systems is difficult.
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We have an experimental feature in Nova called "cells", hereafter referred to
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as "cells v1", which is used by some large deployments to partition compute
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nodes into smaller groups, coupled with a database and queue. This seems to be
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a well-liked and easy-to-understand arrangement of resources, but the
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implementation of it has issues for maintenance and correctness.
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See `Comparison with Cells V1`_ for more detail.
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Status
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~~~~~~
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Cells v1 is considered experimental and receives much less testing than the
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rest of Nova. For example, there is no job for testing cells v1 with Neutron.
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The priority for the core team is implementation of and migration to cells v2.
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Because of this, there are a few restrictions placed on cells v1:
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#. Cells v1 is in feature freeze. This means no new feature proposals for cells
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v1 will be accepted by the core team, which includes but is not limited to
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API parity, e.g. supporting virtual interface attach/detach with Neutron.
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#. Latent bugs caused by the cells v1 design will not be fixed, e.g.
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`bug 1489581 <https://bugs.launchpad.net/nova/+bug/1489581>`_. So if new
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tests are added to Tempest which trigger a latent bug in cells v1 it may not
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be fixed. However, regressions in working function should be tracked with
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bugs and fixed.
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**Suffice it to say, new deployments of cells v1 are not encouraged.**
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The restrictions above are basically meant to prioritize effort and focus on
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getting cells v2 completed, and feature requests and hard to fix latent bugs
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detract from that effort. Further discussion on this can be found in the
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`2015/11/12 Nova meeting minutes
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<http://eavesdrop.openstack.org/meetings/nova/2015/nova.2015-11-12-14.00.log.html>`_.
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There are no plans to remove Cells V1 until V2 is usable by existing
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deployments and there is a migration path.
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Cells V2
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========
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@ -18,20 +67,6 @@ Cells V2
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Manifesto
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~~~~~~~~~
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Problem
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-------
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Nova currently depends on a single logical database and message queue that all
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nodes depend on for communication and data persistence. This becomes an issue
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for deployers as scaling and providing fault tolerance for these systems is
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difficult.
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We have an experimental feature in Nova called "cells" which is used by some
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large deployments to partition compute nodes into smaller groups, coupled with
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a database and queue. This seems to be a well-liked and easy-to-understand
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arrangement of resources, but the implementation of it has issues for
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maintenance and correctness.
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Proposal
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--------
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@ -106,8 +141,8 @@ The benefits of this new organization are:
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* Adding new sets of hosts as a new "cell" allows them to be plugged into a
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deployment and tested before allowing builds to be scheduled to them.
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Comparison with current cells
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-----------------------------
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Comparison with Cells V1
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------------------------
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In reality, the proposed organization is nearly the same as what we currently
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have in cells today. A cell mostly consists of a database, queue, and set of
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