This is a problem I've thought about previously, and every time I've seen someone try to pull it off they generally experience problems with latency, particularly with day trading type models.

It seams to me that rather than TSD acting as a central hub, and trying to sort out the tangled mess of what signals are firing, and which subscribers should get them, you could potentially harness the distributed computing power of all of the parties involved just by thinking about the architecture in a slightly different way.

For example, provide the signal provider with an application that intermitantly determines who is subscribing to their signals, who's joined the service, who's cancelled etc. This info would only need to be updated once a day at the very most. The signal provider then knows EXACTLY how many subscribers they actually have (There's a potential for a fraudulent service not to pass on all revenues to the signal supplier, e.g. 1000 punters sign up, but the provider is only paid for 500)

Then when the signal provider opens, closes or modifies a trade, this information can be broadcast to specific subscribers, as well as TSD who retains a centralised record of the transaction. Implimentation should be easy enough to achieve with multi user instant messaging type technologies.

Each signal provider becomes responsible for broadcasting to his own subscribers, and the application could also send notification by bulk SMS, email, Instant Messanger etc

Anyone fancy building this ?