Send heartbeat responses

[uellenberg]

This sends heartbeat responses so that mission control can detect disconnection and reconnect automatically.

[loukylor]

Services would probably work better here (rather than a pub and sub) since you can send back a response after receiving a request. You can look at #72 or the arm code to see an example of services. Here's the relevant ROS docs

[uellenberg]

Services would probably work better here (rather than a pub and sub) since you can send back a response after receiving a request. You can look at #72 or the arm code to see an example of services. Here's the relevant ROS docs

@loukylor I'll take a look at it tomorrow. Ideally, this is just a temporary solution for making mission control able to reconnect to the websocket automatically. What I really want to do is use websocket ping/pong control frames, since they'll end up using significantly less data. So, I'll probably modify all this code anyway - I just have to figure out how to listen to pings/pongs through rosbridge_server (which may require modifying it).

[loukylor]

Ideally we don't have to modify rosbridge_server as that'd greatly increase the effort to maintain it. Is the ack really taking up that much bandwidth in the first place? If it isn't, we can just stick to pub/subs or services.

[adamseth2]

Personally, I think sub/pub right now is fine, and can always overhaul it as this is a nice feature. I am also not too worried about bandwidth for this one as it is just sending the time. Right now though, it doesn't work for me for some reason though.

[uellenberg]

Nevermind, I've learned that for some reason browsers don't expose ping/pong control frames, so we won't be able to use them. I'll look into services - one issue with pub/sub is that ping/pong isn't directed towards a single client, which potentially breaks things if we ever want to support multiple clients.

[uellenberg]

Services would probably work better here (rather than a pub and sub) since you can send back a response after receiving a request. You can look at #72 or the arm code to see an example of services. Here's the relevant ROS docs

@loukylor ROSLib services don't appear to support CBOR, and ends up sending more data anyway. I agree that a service model makes more sense, but I'm not sure what real benefits it brings. Switching to a service will just make the message length longer, and I'm not sure whether that's a good idea for something that's sent as frequently as a heartbeat.

[loukylor]

While I still disagree that services may affect bandwidth, since a single service call per second would not even be on the order of 100bytes per second, since this is already implemented I am okay with merging this. I will ensure formatting is right and review functionality then approve.

[loukylor]

Has this been tested?

[uellenberg]

Has this been tested?

@loukylor Yes, Adam and I tested it yesterday.