Merge "Libvirt driver class refactoring"
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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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Libvirt driver class refactoring
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================================
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https://blueprints.launchpad.net/nova/+spec/libvirt-driver-class-refactor
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The libvirt driver.py class is growing ever larger and more complicated.
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There are circular dependencies between this class and other libvirt
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classes. This work aims to split some of the functionality out into
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new classes
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Problem description
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===================
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The libvirt driver.py class is growing ever larger over time. This is
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increasing the complexity of the code and also resulting in larger
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test suites.
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The driver.py class is serving what are really a number of distinct
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use cases. Primarily it is the interface for the compute manager
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class to consume. It also, however, has alot of helper APIs for
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dealing with the libvirt connection and the host operating system,
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as well as helpers for dealing with guest instance configuration.
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A number of these helpers are required by other libvirt modules
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such as the vif, volume and image backend drivers. This has resulted
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in circular dependancies between the driver.py and the other libvirt
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modules. For example, LibvirtDriver uses NWFilterFirewall, but also
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has to pass a 'get_connection' callback so that NWFilterFirewall can
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obtain the libvirt connection from the LibvirtDriver class. There are
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a number of other similar deps.
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Proposed change
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===============
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The intention is to introduce two new modules to the codebase
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* host.py - this will encapsulate access to libvirt and the host
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operating system state. It will contain a 'Host' class, which
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manages a single libvirt connection. It will contain the methods
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for connecting to libvirt, getting lists of domains, querying
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host performance metrics and so on (see the work-items section
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for specifics). This is not to be confused with the existing
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HostState class which is just a trivial helper for the driver
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'host_state' method.
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* guest.py - this will encapsulate interaction with libvirt guest
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domain instances. It will contain a 'Guest' class, which manages
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a single libvirt guest domain. It will contain all methods used
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to construct the guest XML configuration during instance startup
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that currently live in driver.py
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The code for host.py and guest.py will be pulled out of the
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existing driver.py class. Other libvirt modules will be updated
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as needed to access the new APIs. To minimize the risk of creating
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regressions changes to the methods being moved will be minimized,
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to just minor renames & fixups where appropriate.
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The intended end result is that none of the modules in the libvirt
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driver directory should need to access the driver.py file. They
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should be able to consume the host.py and guest.py APIs instead,
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thus breaking the circular dependancies. For example the
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NWFilterFirewall class can be given an instance of the Host class
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instead of a callback to LibvirtDriver.
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The new structure should also reduce the size of the test_driver.py
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file and make it possible to create simpler, self contained tests
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for the functionality that's in host.py and guest.py, since it will
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be isolated from the overall virt driver API.
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It is not anticipated that any configuration parameters will move.
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The high level desire is that the new APIs will not directly use
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any Nova configuration parameters. Instead the LibvirtDriver would
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be responsible for reading the config parameters and then setting
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attributes on the new class or passing method parameters where
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appropriate.
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At the end of the work there should be absolutely no functional
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change on the libvirt driver. This is intended to be purely
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refactoring work that is invisible to anyone except the people
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writing libvirt driver code.
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Alternatives
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------------
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Doing nothing is always an option, but it isn't very appealing
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because it leaves us with an ever growing monster ready to
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devour us at any moment.
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Data model impact
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-----------------
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None
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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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None
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Developer impact
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----------------
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There are liable to be conflicts with any developers who have patches
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touching driver.py or test_driver.py
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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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berrange
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Work Items
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----------
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* Create a host.py module and move the basic connection handling code
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out of driver.py into the new Host class. This will cover the following
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methods:
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* _conn_has_min_version
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* _has_min_version
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* _native_thread
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* _dispatch_thread
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* _event_lifecycle_callback
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* _queue_Event
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* _dispatch_events
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* _init_events_pipe
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* _init_events
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* _get_new_connection
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* _close_callback
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* _test_connection
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* _connect
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* Move helpers used by HostState out into the Host class. This will
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cover the following methods
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* _get_vcpu_total
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* _get_memory_mb_total
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* _get_vcpu_used
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* _get_memory_mb_used
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* _get_hypervisor_type
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* _get_hypervisor_version
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* _get_hypervisor_hostname
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* _get_cpu_info
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* _get_disk_available_least
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* Create a guest.py module and move the code for creating the guest XML
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configuration out of driver.py into the new Guest class. This will cover
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the following methods
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* _get_guest_cpu_model_config
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* _get_guest_cpu_config
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* _get_guest_disk_config
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* _get_guest_storage_config
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* _get_guest_config_sysinfo
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* _get_guest_pci_device
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* _get_guest_config
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* _get_guest_xml
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* Move the code for listing domains into the new Host class. This
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will cover the '_list_instance_domains' method.
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* Change NWFilterFirewall and LibvirtBaseVIFDriver so that they
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accept a 'Host' object instance, instead of requiring a callback
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to the LibvirtDriver class.
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* Anything else that appears relevant to move :-)
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Dependencies
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============
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* None
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Testing
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=======
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Since it is intended that there is no functional change in this work,
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the existing test coverage should be sufficient. The existing unit
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tests will need some refactoring as code is moved, and some more unit
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tests will be written where appropriate.
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Documentation Impact
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====================
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None
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References
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==========
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None
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