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This proposes a place to store instances that could not be scheduled to a cell. blueprint cells-cell0 Change-Id: I04c260aa6d283899808f59686efd71b79fdd50a8
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157 lines
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This work is licensed under a Creative Commons Attribution 3.0 Unported
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License.
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http://creativecommons.org/licenses/by/3.0/legalcode
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==============
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Add a CellZero
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==============
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https://blueprints.launchpad.net/nova/+spec/cells-cell0
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In order to maintain the API contract when using cells we need to store enough
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information to fulfill an instance show request even when the instance could
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not be scheduled to a cell.
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Problem description
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===================
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When an API request is made to build an instance there is a certain response
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contract that we need to honor. This means that we need to have stored certain
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information from the request such as image, flavor, name, uuid, etc... Under
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ideal circumstances the instance will have been scheduled to a host in a cell
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and the data will be in that cells database. In the event that no cell/host
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can hold the instance that data needs to be stored outside of any live cells.
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Use Cases
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----------
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* Operators want to partition their deployments into cells for scaling, failure
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domain, and buildout reasons. When partitioned, we need to maintain the
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current API contract.
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Proposed change
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===============
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A cell0 will be added which consists of the database tables needed by an
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instance within a live cell. Instances, and necessary relations, can be stored
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here and included in API responses. The only available action for instances
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within this cell will be to delete them as they are in an error state.
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Instances that are stored here will have a regular instance_mapping set so that
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requests for those instances can follow the normal code path.
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The boot process for instances will be updated to create instances in cell0
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when the scheduler fails to pick a location for the instance.
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Alternatives
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------------
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An alternative would be to store the instance records, and relations, within
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the nova api database in a serialized format for easy retrieval for API
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responses. This was dismissed because it would require multiple upgrade paths
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when making changes to the instance table schema or related schemas.
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Another alternative would be to maintain an instance table and related tables
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in the nova api database. This is very similar to what's being proposed
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however having a cell0 construct is beneficial to avoid special casing
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lookups/deletes for instances here. They will have a normal instance_mapping
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set so operations will find them normally.
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Data model impact
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-----------------
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All database tables necessary for storing an instance and related objects, like
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security_groups or instance_extra, will be created and managed for this new
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cell. For now it would be best to just deploy a normal cell database for cell0
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though it will contain unnecessary tables such as compute_nodes. Later work
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can trim this down if it's deemed worthwhile.
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REST API impact
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---------------
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None
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Security impact
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---------------
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None
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Notifications impact
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--------------------
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None
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Other end user impact
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---------------------
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None
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Performance Impact
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------------------
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None
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Other deployer impact
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---------------------
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Instances that were not schedulable will exist in this special cell.
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Deployers will need to be aware of this to aid proper debugging.
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Developer impact
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----------------
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None
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Implementation
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==============
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Assignee(s)
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-----------
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Primary assignee:
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alaski
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Other contributors:
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None
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Work Items
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----------
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* Devstack changes to setup a cell0.
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* Add logic to create instances in cell0 when they can not be scheduled.
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* Document deployment instructions for cell0.
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Dependencies
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============
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* Spec to call the scheduler earlier in the boot process
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https://review.openstack.org/#/c/239995/
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Testing
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=======
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Since this is designed to be an internal re-architecting of Nova with no user
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visible changes the current suite of Tempest or functional tests should
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suffice. At some point we will want to look at how to test multiple cells or
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potentially exposing the concept of a cell in the API and we will tackle
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testing requirements then.
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Documentation Impact
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====================
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Documentation will be written describing the flow of an instance build and how
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this affects it.
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References
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==========
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