add ability to create sessions by ref

[d-bohls]

This came from #211

Should this be deferred to Unplanned?
Is there a use case that requires it?
Are there cases where this would be heavily used if available?

[jashnani]

@lilesj and I discussed this at our last grooming meeting and realized that this is somewhat of an important feature that's missing from the Python API. After the 0.3 release, we'll have various database objects (Clusters, Frames, Signals, ECUs, etc.) that can be created on the fly; but for referencing them or using them in a session, we still have to use strings. If the business logic around those database objects grows or if we have a lot of database objects to begin with, the more strings (and/or longer strings) we'll need to use. For this reason and for parity with the XNET C API, we should consider allowing sessions to be created using references.

This is going to be similar to nixnet-python/nixnet_examples/can_dynamic_database_creation.py except we'll be passing in db objects to the session. I'm still thinking through the underlying design and user facing stuff for this, but if you have ideas, feel free to discuss those with me.

[epage]

Current places this has impact:

• FrameInStreamSession(intf_name, database_name, cluster_name)
• FrameOutStreamSession(intf_name, database_name, cluster_name)
• FrameInQueuedSession(intf_name, database_name, cluster_name, frame)
• FrameOutQueuedSession(intf_name, database_name, cluster_name, frame)
• FrameInSinglePointSession(intf_name, database_name, cluster_name, frame)
• FrameOutSinglePointSession(intf_name, database_name, cluster_name, frame)
• SignalInSinglePointSession(intf_name, database_name, cluster_name, signals)
• SignalOutSinglePointSession(intf_name, database_name, cluster_name, signals)
• SignalConversionSinglePointSession(database_name, cluster_name, signals)

Currently these exclusively call nxCreateSession. nxCreateSessionByRef is the additional function that will need to be used.

• Presumably, using ByRef removes all the non-leaf database fields from the __init__s, leaving cluster_name, frame, or signals.
• This will only work by exclusively using names or references.
• How do database and session lifetimes work when using ByRef?
• Which approach would be considered the priority use case that we should optimize the API UX around?
• Do we expose the database handle or does each session's __init__ access a protected member of a database?

[d-bohls]

@lilesj As I understand the issue, this would be an enhancement and add some flexibility for users. Would adding this functionality allow users to solve some problem that they currently cannot solve? Can you provides some examples to help me understand the importance of this issue?

[lilesj]

Lets say that a user programmatically creates a database for testing purposes. The database creation code will almost always be in a different file, class, function, etc. In this scenario, how will a user consume the created database to create their sessions? Will they have to dig into the database to find the string properties so they can create their sessions? The examples are straightforward since they all use string globals but this isn't a realistic representation of a customer use case.