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Bob, this is fantastic, I really appreciate all the detail. A couple of
questions ...

On Wed, Feb 05, at 2:16 am, Robert Kukura <rkukura at redhat.com> wrote:

> A couple of interrelated issues with the ML2 plugin's port binding have
> been discussed over the past several months in the weekly ML2 meetings.
> These effect drivers being implemented for icehouse, and therefore need
> to be addressed in icehouse:
> 
> * MechanismDrivers need detailed information about all binding changes,
> including unbinding on port deletion
> (https://bugs.launchpad.net/neutron/+bug/1276395)
> * MechanismDrivers' bind_port() methods are currently called inside
> transactions, but in some cases need to make remote calls to controllers
> or devices (https://bugs.launchpad.net/neutron/+bug/1276391)
> * Semantics of concurrent port binding need to be defined if binding is
> moved outside the triggering transaction.
> 
> I've taken the action of writing up a unified proposal for resolving
> these issues, which follows...
> 
> 1) An original_bound_segment property will be added to PortContext. When
> the MechanismDriver update_port_precommit() and update_port_postcommit()
> methods are called and a binding previously existed (whether its being
> torn down or not), this property will provide access to the network
> segment used by the old binding. In these same cases, the portbinding
> extension attributes (such as binding:vif_type) for the old binding will
> be available via the PortContext.original property. It may be helpful to
> also add bound_driver and original_bound_driver properties to
> PortContext that behave similarly to bound_segment and
> original_bound_segment.
> 
> 2) The MechanismDriver.bind_port() method will no longer be called from
> within a transaction. This will allow drivers to make remote calls on
> controllers or devices from within this method without holding a DB
> transaction open during those calls. Drivers can manage their own
> transactions within bind_port() if needed, but need to be aware that
> these are independent from the transaction that triggered binding, and
> concurrent changes to the port could be occurring.
> 
> 3) Binding will only occur after the transaction that triggers it has
> been completely processed and committed. That initial transaction will
> unbind the port if necessary. Four cases for the initial transaction are
> possible:
> 
> 3a) In a port create operation, whether the binding:host_id is supplied
> or not, all drivers' port_create_precommit() methods will be called, the
> initial transaction will be committed, and all drivers'
> port_create_postcommit() methods will be called. The drivers will see
> this as creation of a new unbound port, with PortContext properties as
> shown. If a value for binding:host_id was supplied, binding will occur
> afterwards as described in 4 below.
> 
> PortContext.original: None
> PortContext.original_bound_segment: None
> PortContext.original_bound_driver: None
> PortContext.current['binding:host_id']: supplied value or None
> PortContext.current['binding:vif_type']: 'unbound'
> PortContext.bound_segment: None
> PortContext.bound_driver: None
> 
> 3b) Similarly, in a port update operation on a previously unbound port,
> all drivers' port_update_precommit() and port_update_postcommit()
> methods will be called, with PortContext properies as shown. If a value
> for binding:host_id was supplied, binding will occur afterwards as
> described in 4 below.
> 
> PortContext.original['binding:host_id']: previous value or None
> PortContext.original['binding:vif_type']: 'unbound' or 'binding_failed'
> PortContext.original_bound_segment: None
> PortContext.original_bound_driver: None
> PortContext.current['binding:host_id']: current value or None
> PortContext.current['binding:vif_type']: 'unbound'
> PortContext.bound_segment: None
> PortContext.bound_driver: None
> 
> 3c) In a port update operation on a previously bound port that does not
> trigger unbinding or rebinding, all drivers' update_port_precommit() and
> update_port_postcommit() methods will be called with PortContext
> properties reflecting unchanged binding states as shown.
> 
> PortContext.original['binding:host_id']: previous value
> PortContext.original['binding:vif_type']: previous value
> PortContext.original_bound_segment: previous value
> PortContext.original_bound_driver: previous value
> PortContext.current['binding:host_id']: previous value
> PortContext.current['binding:vif_type']: previous value
> PortContext.bound_segment: previous value
> PortContext.bound_driver: previous value
> 
> 3d) In a the port update operation on a previously bound port that does
> trigger unbinding or rebinding, all drivers' update_port_precommit() and
> update_port_postcommit() methods will be called with PortContext
> properties reflecting the previously bound and currently unbound binding
> states as shown. If a value for binding:host_id was supplied, binding
> will occur afterwards as described in 4 below.
> 
> PortContext.original['binding:host_id']: previous value
> PortContext.original['binding:vif_type']: previous value
> PortContext.original_bound_segment: previous value
> PortContext.original_bound_driver: previous value
> PortContext.current['binding:host_id']: new or current value
> PortContext.current['binding:vif_type']: 'unbound'
> PortContext.bound_segment: None
> PortContext.bound_driver: None
> 
> 4) If a port create or update operation triggers binding or rebinding,
> it is attempted after the initial transaction is processed and committed
> as described in 3 above. The binding process itself is just as before,
> except it happens after and outside the transaction. Since binding now
> occurs outside the transaction, its possible that multiple threads or
> processes could concurrently attempt to bind the same port, although
> this is should be a rare occurrence. Rather than trying to prevent this
> with some sort of distributed lock or complicated state machine,
> concurrent attempts to bind are allowed to proceed in parallel. When a
> thread completes its attempt to bind (either successfully or
> unsuccessfully) it then performs a second transaction to update the DB
> with the result of its binding attempt. When doing so, it checks to see
> if some other thread has already committed relevant changes to the port
> between the two transactions. There are three possible cases:
> 
> 4a) If the thread's binding attempt succeeded, and no other thread has
> committed either a new binding or changes that invalidate this thread's
> new binding between the two transactions, the thread commits its own
> binding results, calling all drivers' update_port_precommit() and
> update_port_postcommit() methods with PortContext properties reflecting
> the new binding as shown. It then returns the updated port dictionary to
> the caller.
> 
> PortContext.original['binding:host_id']: previous value
> PortContext.original['binding:vif_type']: 'unbound'
> PortContext.original_bound_segment: None
> PortContext.original_bound_driver: None
> PortContext.current['binding:host_id']: previous value

Are you not expecting/allowing the host_id to change in this scenario? Why?

> PortContext.current['binding:vif_type']: new value
> PortContext.bound_segment: new value
> PortContext.bound_driver: new value
> 
> 4b) If the thread's binding attempt either succeeded or failed, but some
> other thread has committed a new successful binding between the two
> transactions, the thread returns a port dictionary with attributes based
> on the DB state from the new transaction, including the other thread's
> binding and any other port state changes. No further calls to mechanism
> drivers are needed here since they are the responsibility of the other
> thread that bound the port.
> 
> 4c) If some other thread committed changes to the port's
> binding-relevant state but has not committed a successful binding, then
> this thread attempts to bind again using that updated state, repeating 4.
> 
> 5) Port deletion no longer does anything special to unbind the port. All
> drivers' delete_port_precommit() and delete_port_postcommit() methods
> are called with PortContext properties reflecting the binding state
> before deletion as shown.
> 
> PortContext.original: None
> PortContext.original_bound_segment: None
> PortContext.original_bound_driver: None
> PortContext.current['binding:host_id']: previous value or None
> PortContext.current['binding:vif_type']: previous value
> PortContext.bound_segment: previous value
> PortContext.bound_driver: previous value

Could this part of the port deletion also be done by port update?

> 
> 6) In order to ensure successful bindings are created and returned
> whenever possible, the get port and get ports operations also attempt to
> bind the port as in 4 above when binding:host_id is available but there
> is no existing successful binding in the DB.
> 
> 7) We can either eliminate MechanismDriver.unbind_port(), or call it on
> the previously bound driver within the transaction in 3d and 5 above. If
> we do keep it, the old binding state must be consistently reflected in
> the PortContext as either current or original state, TBD. Since all
> drivers see unbinding as a port update where current_bound_segment is
> None and original_bound_segment is not None, calling unbind_port() seems
> redundant.
> 
> 8) If bindings shouldn't spontaneously become invalid, maybe we can
> eliminate MechanismDriver.validate_bound_port().
> 
> 
> I've provided a lot of details, and the above may seem complicated. But
> I think its actually much more consistent and predictable than the
> current port binding code, and implementation should be straightforward.
> 
> -Bob