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Raise an event from the web service to a client

Is this possible?

The client and web service were both created using VB.NET 2008 Framework 3.5

Our client application does all of its transactions with the server via a web service. We have a new messaging function which needs to be implemented and in order to receive new messages we need to either poll the server at regular intervals in order to check for new messages, or we need the web server to be able to tell the client application when there is a new message.

The second option is clearly ideal. So the question is how can it be done?
1 Solution
You cannot do your preferred option with web services. A services sits there waiting to be consumed by a client, and has no way making a connection to the client. In order to acheive this you would need to modify the client to accept incoming tcp connections, and then have the server know the address of each client and initiate a tcp connection from the code-behind to the client when required, and then any firewalls in between are going to be a problem.

The only reasonable way is to have the client poll for new messages.
tfslnAuthor Commented:
So how do instant message applications work? Or any other application which display notifications like that? Do they constantly poll servers to check for new messages/items?

Having the server know the address of each client is not a problem, because i could send that information to the server each time a user logs in. I guess the only problem would be firewalls like you say.
IM applications tend to use their own protocol and not http (even if they use tcp 80). Their connections tend to stay open so that data can be pushed back and forth, unlike http which was designed as a stateless protocol: the connections were supposed to be closed after a page was requested. Only with http 1.1 were connections defaulted to remain open as opening connections tend to be expensive.
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