Aynchronous web server calls in Silverlight and ma

问题:

I've read that Silverlight 2.0 imposes by design an asynchronous model when communicating with the web server. I haven't had a chance to experiment with Silverlight, but I assume that it uses a thread-pool to manage threads like in the .NET Framework.

Now, since some browsers, most notably Internet Explorer, have an hard-coded limit of maximum two concurrent HTTP connections that can be made on the web server, what happens if I make a bunch of asynchronous requests from Silverlight?

Does Silverlight bypass this limitation in the web browser and open as many HTTP connections as there are threads available, or do the asynchronous requests queue up and wait for one of the two connections to become available?

回答1:

In IE (haven't tested others) Silverlight is restricted to 2 connections at a time.

The behavior in Silverlight is to simply not make the request. So if you make 5 Async web service requests right in a row, the first 2 will happen, the other three won't. No exception is thrown that i've seen...

Fiddler is a big help here :)

回答2:

Create a messaging manager interface for your client. Any outgoing request are posted to a queue that this manager processes against. It would serially process queued messages (i.e., when the call back of the last message sent to the server is invoked, can then safely proceed to process the next queued message).

You can consume the other connection resource by keeping a Comet connection open to the server. The server would push any return messages to the client via this Comet connection. You'll need to stamp out-going messages with a unique number that can be embedded as a property on in-coming messages - so that results can be correlated to request. The messaging manager would dispatch a result message to the appropriate handler for that result.

Essentially you end up using two connection resources to establish bi-directional messaging. But there is no artificial limit on the number of requesters on the client (though request will get serially transmitted to the server). The act of sending is always fast, though, because you don't wait for any result to be computed - you just need to deliver the message reliably to the server and return. Results come back asynchronously on the other Comet connection.

We do something along these lines with our Flex client apps in conjunction to Adobe BlazeDS running in our Tomcat web server:

A Flex-based asynchronous stack

回答3:

Firefox is also limited to two connections, in addition to IE as stated already.

Note that the limit is per hostname.

If you add entries to your hosts file, or use dns aliases you can get more connections. For example in testing, add lines like '127.0.0.1 test1' to your hosts file, and then you can open two connections to http://localhost and two more to http://test1

回答4:

I guess, being a .NET application Silverlight 2 has an independent from browser limit.
I would assume It is maxconnection attribute in Machine.config as mentioned in http://support.microsoft.com/kb/828219

回答5:

Firstly the Machine.config file would not be used as the Silverlight control is sandboxed with its own version of the CoreCLR.

I believe that the Silverlight control actually makes use of the underlying browser to make the asynchronous HTTP requests. This is most likely the case considering how the Silverlight control can't gain access to SOAP fault information as the SOAP specification requires that the server returns an HTTP 500 response code and the Silverlight control doesn't get that from the browser hosting the control.

This post here serves to confirm this.

As to the limit of concurrent HTTP connections, I believe IE5 and later limit the number of connections to the same site based on HTTP protocol version - HTTP/1.0 it limits to 4 connections and HTTP/1.1 to 3 connections. Most of the time the web server will limit the number of connections to 2 per client, queueing or discarding the remainder.