Adding subscription IDs to ROSBridge-published messages

[Ian Sherman]

I would like the ROSBridge server to return subscription ids along with messages. This would allow me, on the client side, to associate received messages with the components that requested the subscription. Then a component, could, for example, unsubscribe after it receives N messages.

After digging into the ROSBridge source I see the following:

We have a capabilities.subscribe.Subscription object, which in its own words "Keeps track of the client's multiple calls to subscribe." When a client requests a subscription to a topic it has previously subscribed to, with different parameters or not, the Subscription object associated with that topic tells the internal.subscribers.SubscriberManager to add a callback to its internal.subscribers.MultiSubscriber associated with that topic. The MultiSubscriber will, in turn, add the callback that propagated its way through thus far to the rospy.Subscriber that actually interacts with the ROS master.

Which brings me to my first question. On line 127 of internal.subscribers [1] we see the MultiSubscriber maintains a dict of client_ids to single callback functions. Shouldn't this be a dict of client_ids to lists of callback functions? In its current implementation, the dict would only keep track of the most recently added callback. In practice this is invisible at the moment, since it's actually the rospy.Subscriber that maintains a list of callbacks, but it could have consequences in the future.

The callback that's propagated to rospy.Subscriber is, at its core, the publish function from the capabilities.subscribe.Subscribe class that has been curried and wrapped a few times. Somehow, we need to bind the subscription id to this callback function so the id is actually returned with the JSON object back to the client. Where should this happen?

The subscription ID (aka sid) currently makes it as far as the subscribe function of the capabilities.subscribe.Subscription object. Unfortunately all the subscriptions managed by a capabilities.subscribe.Subscription object share the same callback (wrapped in a internal.subscription_modifiers.MessageHandler that is responsible for throttling the callback rate). I suspect that what we really want is the MessageHandler to maintain a list of callbacks, each with a different sid, so it can publish to each sid independently.

Does anyone have any thoughts on this approach? 

Anyone that can explain the thinking behind the current design?

On line 252 of rosbridge_library.capabilities.subscribe I see "# TODO: fragmentation, proper ids" [2] -- what does "proper ids" refer to?

Does including the sid have implications for fragmentation messages, which also use the "id" field?

One consequence I can see is that the same message with the same frame may be sent to the client multiple times, once for each sid.

To inform the discussion, right now the ROSBridge Protocol reads: 

"If a client has multiple subscriptions to the same topic, then messages are sent at the lowest throttle_rate, with the lowest fragmentation size, and highest queue_length. It is recommended that the client provides IDs for its subscriptions, to enable rosbridge to effectively choose the appropriate fragmentation size and publishing rate."

I'm assuming that the second sentence there means that without unique ids, ROSBridge will clobber the subscription parameters with whatever the most recent subscription request was, rather than choosing the "lowest common denominator" of all subscription requests. Because it's not clear how sids are used otherwise. 

Many thanks. Happy to move this to a Github issue conversation if that's preferred.

Sincerely,

Ian

[1] https://github.com/RobotWebTools/rosbridge_suite/blob/hydro-devel/rosbridge_library/src/rosbridge_library/internal/subscribers.py#L127

[2] https://github.com/RobotWebTools/rosbridge_suite/blob/hydro-devel/rosbridge_library/src/rosbridge_library/capabilities/subscribe.py#L252

--

Ian Sherman

Lead Engineer

BOT&DOLLY

150 Mississippi Street San Francisco CA 94107

o. 415.701.9700

www.botndolly.com

[Chad Jenkins]

Hi Ian,

First, congrats  :)

Thanks for this bringing this issue forward, and provide very thorough description.  The issue of handling multiple subscriptions has been a topic of active discussion in the design of rosbridge for some time.  So, this will be a good discussion to have.

As you mentioned, it would be helpful move this to an issue on the rosbridge_suite repo.  I expect Russell Toris et al. will have thoughts and eagerness to move this forward.

Chad
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