Question

I have this situation….
Client-initiated SOAP 1.1 communication between one server and let’s say, tens of thousands of clients. Clients are external, coming in through our firewall, authenticated by certificate, https, etc.. They can be anywhere, and usually have their own firewalls, NAT routers, etc… They’re truely external, not just remote corporate offices. They could be in a corporate/campus network, DSL/Cable, even Dialup.

Client uses Delphi (2005 + SOAP fixes from 2007), and the server is C#, but from an architecture/design standpoint, that shouldn’t matter.

Currently, clients push new data to the server and pull new data from the server on 15-minute polling loop. The server currently does not push data – the client hits the “messagecount” method, to see if there is new data to pull. If 0, it sleeps for another 15 min and checks again.

We’re trying to get that down to 7 seconds.

If this were an internal app, with one or just a few dozen clients, we’d write a cilent “listener” soap service, and would push data to it. But since they’re external, sit behind their own firewalls, and sometimes private networks behind NAT routers, this is not practical.

So we’re left with polling on a much quicker loop. 10K clients, each checking their messagecount every 10 seconds, is going to be 1000/sec messages that will mostly just waste bandwidth, server, firewall, and authenticator resources.

So I’m trying to design something better than what would amount to a self-inflicted DoS attack.

I don’t think it’s practical to have the server send soap messages to the client (push) as this would require too much configuration at the client end. But I think there are alternatives that I don’t know about. Such as:

1) Is there a way for the client to make a request for GetMessageCount() via Soap 1.1, and get the response, and then perhaps, “stay on the line” for perhaps 5-10 minutes to get additional responses in case new data arrives? i.e the server says “0”, then a minute later in response to some SQL trigger (the server is C# on Sql Server, btw), knows that this client is still “on the line” and sends the updated message count of “5”?

2) Is there some other protocol that we could use to “ping” the client, using information gathered from their last GetMessageCount() request?

3) I don’t even know. I guess I’m looking for some magic protocol where the client can send a GetMessageCount() request, which would include info for “oh by the way, in case the answer changes in the next hour, ping me at this address…”.

Also, I’m assuming that any of these “keep the line open” schemes would seriously impact the server sizing, as it would need to keep many thousands of connections open, simultaneously. That would likely impact the firewalls too, I think.

Is there anything out there like that? Or am I pretty much stuck with polling?

TIA,
Chris

UPDATE 4/30/2010:
Having demonstrated that having 7-second notification is neither easy nor cheap, especially without going outside of the corporate standard of HTTPS/SOAP/Firewalls, we’re probably going to pitch a two-phase solution. Phase1 will have the clients poll “on-demand” with the GetMessageCount being performed through SOAP, nothing fancy here. There will be a “refresh” button to pull new data (which is reasonable here, as the user will usually have reason to suspect that new data is ready, i.e. they just changed the fabric color in the online system, so they know to click REFRESH before viewing the shipping manifest on the desktop, and now they see the color in the description.) (This is NOT really a garment/fashion app, but you get the idea).
The notion of having the two aps always be in sync, with real-time updates pushed from the host, is still on the table, using the technologies discussed here. But I expect that it will be pushed off for another release, as we can deliver 85% of the functionality without having to do this. However, I hope that we get to do a Proof Of Concept, and can demonstrate that it’ll work. I’ll come back and post future updates. Thanks for everyone’s help on this.

Answer 1

The two big parties on multi-tier development in Delphi are components4developers (with their kbmMW product described in the answer by Mark Robinson) and RemObjects with their product RemObjects SDK (they have a nice example that might be similar to what you want: Push notifications for iPhone).

In your complex environment, multi-cast UDP might not cut it, but from a overhead perspective it is unbeatable.

If a connection is open, it can be used in a bi-directional way (this is also used by .NET remoting and WCF), but has additional overhead.

You will need to find a balance between keeping connections live (locking resources), and creating new connections (costing time and latency).

–jeroen