[pulseaudio-discuss] RFC: Public API for managing nodes

[Tanu Kaskinen]

On Wed, 2013-07-17 at 09:27 +0200, David Henningsson wrote:
> On 07/16/2013 03:20 PM, Tanu Kaskinen wrote:
> > Yes, automatic use of the combine module is planned (the combining
> > functionality would move to the core, because what's the point of having
> > code in a module if the core directly depends on that code).
> 
> Okay, so combine stuff would then not have its own node or set of nodes, 
> but essentially be as integrated as mixing is today?

Yes. Or well, I wouldn't say that it's as integrated as mixing, since it
would still create a separate combine sink, which is a bit ugly, but as
you said, the sink and the intermediate sink inputs wouldn't show up as
nodes. I think it would make sense to make combining really as
integrated as mixing, but that's not a high priority.

> >>      - Can there be more than one edge between the same nodes? E g, one
> >> default connection and one explicit connection? Or how does this work?
> >
> > Yes, there can be both an explicit and a default connection between two
> > nodes, or it can be also thought as being one connection that is both
> > explicit and default. I don't know what is a better way to think about
> > it, but perhaps it doesn't matter anyway.
> 
> I would prefer the latter way. I would then see "default" and "explicit" 
> as a property of the edge.

I agree about that, at a conceptual level. Would you perhaps like to
have it this way also at the code level, i.e. have connection objects
with "default" and "explicit" fields instead of storing the information
in the nodes?

> >>      - Can edges have properties, e g, a volume?
> >
> > I don't know. Currently I'm not aware of a requirement to have
> > properties on the connections. My current thinking regarding volume
> > specifically is that we shouldn't attach volume to nodes: let's keep the
> > nodes for routing only.
> 
> Ok, that makes sense.
> 
> >>    * As for possible node operations, a move operation would be the same
> >> as a "batched remove + add" operation. Maybe therefore the batch
> >> solution would be better, i e, the client API inputs an array of node
> >> operations?
> >
> > I don't like the idea that if a client wants to move a stream, it has to
> > break the operation down to "remove + add", and then the server tries to
> > guess what the client really meant. Having a "move" operations keeps the
> > client intention obvious.
> 
> Fair point, although the move can be on both sides, whereas one type of 
> move would be "I want node A to take data from C instead of B" and the 
> other type would be "I want node A to output data to node C instead of B".

These both are served by the move operation.

> Or perhaps you want to swap, so that if you're currently on "A -> B and 
> C -> D" and you want "A -> D and B -> C" and you want to do all of this 
> atomically. The number of operations you want to do might grow out of 
> hand unless you have a "batch mode".

The core doesn't currently have a concept of atomic swapping, so I don't
see the need for a separate swap command, but the core has a concept of
atomic moving.

If you think applications need to be able to swap connections in one go
(even though the server will execute the commands sequentially), I'm not
opposed at all to having a batch mode.

> >
> >>    * The operations on changing the default connections does not make
> >> sense to me. If the definition for default connections are those made
> >> automatically by the routing system, if you change them then you broke
> >> the definition...?
> >
> > What operations do you mean? Moving or removing a default connection is
> > not supported as such, but if the client tries that anyway, we can
> > implicitly convert the connection to an explicit one and disable default
> > connections, or we can require the client to do these operations
> > explicitly, but I think the latter would be too inconvenient for the
> > client.
> 
> Ok, I think I didn't read the proposal well enough. Having done that, I 
> understand that you're suggesting a global switch "default connections 
> on/off" only. Or is it a per-node switch?

It is a per-node switch.

> I have another idea that might be worth considering: how about that the 
> "explicit" layer can both enable and disable connections? So that there 
> could be a default connection between A and B, but there is also some 
> sort of explicit override that disables it. This would be more flexible 
> than a more global on/off switch.

I'm not sure what you mean. Do you perhaps mean that the default
connection on/off switch should be per-node (which it already is in my
proposal), or that it should be per-connection (so that if there are
multiple default from node A, it's possible to disable only a subset of
those)?

I didn't make make it possible to disable individual default
connections, because I had a feeling that it would have very messy
semantics. If default connection from A to B is disabled, what is the
routing code supposed to do when conditions change and the default
routing is re-evaluated? Can it ever reactivate the connection between A
and B again? Is the per-connection disabling handled as a blacklist of
connections that must never be automatically activated?

-- 
Tanu