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We have started looking at how the Neutron advanced services being defined
and developed right now can be used within the Neutron policy framework we
are building. Furthermore, we have been looking at a new model for the
policy framework as of the past couple of weeks. So, I have been trying to
see how the services will fit in (or can be utilized by) the policy work in
general and with the new contract-based model we are considering in
particular. Some of the I like to discuss here are specific to the use of
service chains with the group policy work but some are generic and related
to service chaining itself.

If I understand it correctly, the proposed service chaining model requires
the creation of the services in the chain without specifying their
insertion contexts. Then, the service chain is created with specifying the
services in the chain, a particular provider (which is specific to the
chain being built) and possibly source and destination insertion contexts.

1- This fits ok with the policy model we had developed earlier where the
policy would get defined between a source and a destination policy endpoint
group. The chain could be instantiated at the time the policy gets defined.
(More questions on the instantiation below marked as 1.a and 1.b.) How
would that work in a contract based model for policy? At the time a
contract is defined, it's producers and consumers are not defined yet.
Would we postpone the instantiation of the service chain to the time a
contract gets a producer and at least a consumer?

1.a- It seems to me, it would be helpful if not necessary to be able to
define a chain without instantiating the chain. If I understand it
correctly, in the current service chaining model, when the chain is
created, the source/destination contexts are used (whether they are
specified explicitly or implicitly) and the chain of services become
operational. We may want to be able to define the chain and postpone its
creation to a later point in time.

1.b-Is it really possible to stand up a service without knowing its
insertion context (explicitly defined or implicitly defined) in all cases?
For certain cases this will be ok but for others, depending on the
insertion context or other factors such as the requirements of other
services in the chain we may need to for example instantiate the service
(e.g. create a VM) at a specific location that is not known when the
service is created. If that may be the case, would it make sense to not
instantiate the services of a chain at any level (rather than instantiating
them and mark them as not operational or not routing traffic to them)
before the chain is created? (This leads to question 3 below.)

2- With one producer and multiple consumers, do we instantiate a chain
(meaning the chain and the services in the chain become operational) for
each consumer? If not, how do we deal with using the same
source/destination insertion context pair for the provider and all of the
consumers?

3- For the service chain creation, I am sure there are good reasons for
requiring a specific provider for a given chain of services but wouldn't it
be possible to have a generic "chain" provider which would instantiate each
service in the chain using the required provider for each service (e.g.,
firewall or loadbalancer service) and with setting the insertion contexts
for each service such that the chain gets constructed as well? I am sure I
am ignoring some practical requirements but is it worth rethinking the
current approach?

Best,

Mohammad
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