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On 02/05/2014 09:10 AM, Henry Gessau wrote:
> 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?

Correct. This thread's initial transaction has already committed update
of binding:host_id if that is what's triggering this thread to bind. If
another thread committed a change to binding:host_id while this thread
was binding, that is covered in 4c.

> 
>> 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?

I had considered that, but that would then involve two separate
transacitons - 1st the port update to unbind and then the port delete.
Some other thread might rebind the port between the two, so a retry loop
would be needed. This loop is not needed if the same transaction unbinds
and deletes the port.

-Bob

> 
>>
>> 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
> 
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