Add an 'event' command

[danielballan]

Suggestion by @tacaswell which I am just recording:

Add an 'event' command so that

Msg('create')
Msg('read', obj1)
Msg('read', obj2)
Msg('save')

can be accomplished by

Msg('event', [obj1, obj2])

There is really no reason one should need to interleave other commands between these; there is no situation we can think of where 'event' is not sufficient. [edit: see next comment] This is a nice simplification.

[danielballan]

Having just written that, I have just thought of one such situation where we need 'create' / 'save' -- the case where we want a plan to be able to see a reading before the Event is created and decide whether to 'save' or 'drop'. I therefore amend my previous statement: the proposed 'event' command is a nice simplification for the common case, reducing the number of messages per plan, but 'create' and 'save' still have an important use.

[prjemian]

Will a description document be generated that relates to this event document? Will there be a 1-to-1 relationship for such description documents when an event is generated this way?

[danielballan]

Yes, the behavior of the RunEngine would be precisely identical in these two modes. It simplify removes the overhead of processing multiple Msgs.

[prjemian]

I think I understand. Consider this scenario:

Msg('event', [obj1, obj2])
Msg('event', [obj1, obj2])

generates two descriptor documents, essentially duplicates but for UUID.

If so, can some smarts be added in RE to see if a descriptor has already been generated for the same signal list: [obj1, obj2]?

[danielballan]

Yes, in fact those smarts already exist:

Msg('create')
Msg('read', obj1)
Msg('read', obj2)
Msg('save')  # in the RunEngine._save coroutine, the RunEngine builds and emits a new Descriptor and then the Event
Msg('create')
Msg('read', obj1)
Msg('read', obj2)
Msg('save')  # in the RunEngine._save coroutine, the RunEngine finds that it already has emitted an appropriate Descriptor and merely emits the Event

I would apply the same logic to

Msg('event', [obj1, obj2])  # emit Descriptor, Event
Msg('event', [obj1, obj2])  # emit Event