I'm calling a static Page method via javascript that takes between 5s and 10 min. I'd like to give my user the choice to either continue waiting for the request to complete or not, and use window.setTimeout() to check back every 30s.
I've tried both location.href = '/newpage.aspx' or firing a button's click handler (which does similar redirect) to redirect the user prior to completion of the page method, to no avail. I can immediately send user to a simple html page, but a redirect to any aspx page involving server-side appears to block. When the page method finally completes, the redirect does succeed.
Is this:
a browser issue? If all modern
browsers support at least 2
concurrent requests per domain, why
wouldn't this work?
a framework
limitation?
a desirable design
pattern or even possible? I've
checked, and after redirecting to an
HTML page, the original request continues processing (db updates, no problem).
Are Page method calls simply not asynchronous from a "concurrent HTTP request" perspective?
Any insight greatly appreciated!
It sounds like you're blocking on InProc Session, which is limited to one concurrent request per unique session. If you don't need access to the Session in your page method, you can disable it by modifying the WebMethod attribute:
[System.Web.Services.WebMethod(EnableSession=false)]
is there a way from a asp.net-page code behind with "Request.Redirect()" or another method to redirect to the last page (like Javascript history back)?
You can check the Request.UrlReferrer property, which will be set if the user has navigated to the given page from another one. This is nothing more than the HTTP Referrer header that a browser will set. This will be null if the user navigates to your page directly.
HTTP is stateless, so theres no way of being able to read the browsers history (on the server) in the same way that Javascript can (its client side).
However there are a couple of tricks you can use:
Javascript could write the URL into a textbox which gets submitted to the server
The last URL visited could be stored in session - which can be retreived on a later visit
If using the URL in session method, you'll probably want to code this into a HTTP handler (not module) and this will fire automatically on every request.
Obviously these will only work if the user has previously visited a page, and not directly.
What is AsyncPostBack, SyncPostBack and what is the difference between those methods?
An asynchronous postback is accomplished using javascript to send an XMLHttpRequest, known as AJAX, without leaving the web page you are on. A synchronous postback is a normal form post request from the web page, resulting in a complete request cycle and a (re)display of the same or a different web page. The difference is that with an asynchronous request the web page stays the same and the user can continue to interact with it while the request is taking place. This can make the interface seem more responsive to the user and improve the experience. In a synchronous, or full, postback the web browser must wait while the request is sent back and the new page is loaded. You also lose, if you don't maintain it on the server and reset it on the new page, any state on the original web page.
What whould be the best way to prevent multiple users on a page?
For example if a user is at the page "Home.aspx", no other users should be allowed to go there.
I'm using asp.net on the server and the js-frameword jQuery on the client side.
The easy part is only allowing one user to access a page. You can for example store a session id in an application variable to keep track of who's on the page.
The hard part is to know when the user leaves the page. The HTTP protocol only handles requests, so the server only knows when a user enters the page. There is no concept of "being on" a page in the protocol.
You can use the onunload event in client code to catch when a user goes somewhere else, however this will not always work. If the user loses the internet connection, there is no way to communicate back to the server that the user leaves the page. If the browser or computer crashes, there will naturally be no onunload event.
You can keep requesting data from the server, by for example reloading an image on the page. That way the server can know if the user is still on the page at certain intervals. However, if the user loses the internet connection, the server will think that the user has left, while the user thinks that he/she is still on the page.
Another problem is browser history and cache. A user might leave the page, then go back to the page again. You have to make sure that the page is not cached, or the browser will just use the cached page and the server has no idea that the user thinks that he/she is on the page again.
Agreed with Guffa, you cannot be sure that the browser is already on the page or not, you can only check if the browser is already connected to that page or not.
You can do a sort of "ping", but its more a trick than a 100% working solution and it requires javascript enabled.
I didn't do it but I should look at XMLHTTPRequest and onreadystatechange to handle this :
1) On page load, the browser (client) initiate a XMLHTTPRequest with the web site (server) then wait for callback with the onreadystatechange event.
2) The web site receive the request and "mark" the page as "in use" with the current DateTime.Now.
3) Then the web site sends the response.
4) The onreadystatechange event get the response and the event code re-request the server to re-initiate the 2 after 1 min.
5) If another client request the page, the server check the DateTime mark : if the mark is greater than 1min ago, it means the client didnt respond to the request and may not be on the page again.
Not sure why you would want to do this because it flies in the face of web usability. You could do a locking mechanism on each page in server side code (write user name, page and time to a DB), which is freed up when they go to another page. You would then check on a the page load event to find out if anyone currently has that page locked. However, and this is a big however - have you considered what happens if somebody just shuts their browser down or walks off and leaves it on a page. You would need to seriously consider a timeout to free up locks too. That would need to be a back ground service, either in global.asax as global code or a separate process.
Maybe use static variables to hold the ip of the first user to access the page and then check whether other requests come from the same ip, otherwise display a "no access" page.
make sure you use lock it:
Object thisLock = new Object();
lock (thisLock)
{
// access static variables
}
You should also use "Session_End" method in global.asax to remove the ip address in case the user leaves your website without pressing the logout button
What is difference between Server.Transfer and Response.Redirect?
What are advantages and disadvantages of each?
When is one appropriate over the other?
When is one not appropriate?
Response.Redirect simply sends a message (HTTP 302) down to the browser.
Server.Transfer happens without the browser knowing anything, the browser request a page, but the server returns the content of another.
Response.Redirect() will send you to a new page, update the address bar and add it to the Browser History. On your browser you can click back.
Server.Transfer() does not change the address bar. You cannot hit back.
I use Server.Transfer() when I don't want the user to see where I am going. Sometimes on a "loading" type page.
Otherwise I'll always use Response.Redirect().
To be Short: Response.Redirect simply tells the browser to visit another page. Server.Transfer helps reduce server requests, keeps the URL the same and, with a little bug-bashing, allows you to transfer the query string and form variables.
Something I found and agree with (source):
Server.Transfer is similar in that it sends the user to another page
with a statement such as Server.Transfer("WebForm2.aspx"). However,
the statement has a number of distinct advantages and disadvantages.
Firstly, transferring to another page using Server.Transfer
conserves server resources. Instead of telling the browser to
redirect, it simply changes the "focus" on the Web server and
transfers the request. This means you don't get quite as many HTTP
requests coming through, which therefore eases the pressure on your
Web server and makes your applications run faster.
But watch out: because the "transfer" process can work on only those
sites running on the server; you can't use Server.Transfer to send
the user to an external site. Only Response.Redirect can do that.
Secondly, Server.Transfer maintains the original URL in the browser.
This can really help streamline data entry techniques, although it may
make for confusion when debugging.
That's not all: The Server.Transfer method also has a second
parameter—"preserveForm". If you set this to True, using a statement
such as Server.Transfer("WebForm2.aspx", True), the existing query
string and any form variables will still be available to the page you
are transferring to.
For example, if your WebForm1.aspx has a TextBox control called
TextBox1 and you transferred to WebForm2.aspx with the preserveForm
parameter set to True, you'd be able to retrieve the value of the
original page TextBox control by referencing
Request.Form("TextBox1").
Response.Redirect() should be used when:
we want to redirect the request to some plain HTML pages on our server or to some other web server
we don't care about causing additional roundtrips to the server on each request
we do not need to preserve Query String and Form Variables from the original request
we want our users to be able to see the new redirected URL where he is redirected in his browser (and be able to bookmark it if its necessary)
Server.Transfer() should be used when:
we want to transfer current page request to another .aspx page on the same server
we want to preserve server resources and avoid the unnecessary roundtrips to the server
we want to preserve Query String and Form Variables (optionally)
we don't need to show the real URL where we redirected the request in the users Web Browser
Response.Redirect redirects page to another page after first page arrives to client. So client knows the redirection.
Server.Transfer quits current execution of the page. Client does not know the redirection. It allows you to transfer the query string and form variables.
So it depends to your needs to choose which is better.
"response.redirect" and "server.transfer" helps to transfer user from one page to other page while the page is executing. But the way they do this transfer / redirect is very different.
In case you are visual guy and would like see demonstration rather than theory I would suggest to see the below facebook video which explains the difference in a more demonstrative way.
https://www.facebook.com/photo.php?v=762186150488997
The main difference between them is who does the transfer. In "response.redirect" the transfer is done by the browser while in "server.transfer" it’s done by the server. Let us try to understand this statement in a more detail manner.
In "Server.Transfer" following is the sequence of how transfer happens:-
1.User sends a request to an ASP.NET page. In the below figure the request is sent to "WebForm1" and we would like to navigate to "Webform2".
2.Server starts executing "Webform1" and the life cycle of the page starts. But before the complete life cycle of the page is completed “Server.transfer” happens to "WebForm2".
3."Webform2" page object is created, full page life cycle is executed and output HTML response is then sent to the browser.
While in "Response.Redirect" following is the sequence of events for navigation:-
1.Client (browser) sends a request to a page. In the below figure the request is sent to "WebForm1" and we would like to navigate to "Webform2".
2.Life cycle of "Webform1" starts executing. But in between of the life cycle "Response.Redirect" happens.
3.Now rather than server doing a redirect , he sends a HTTP 302 command to the browser. This command tells the browser that he has to initiate a GET request to "Webform2.aspx" page.
4.Browser interprets the 302 command and sends a GET request for "Webform2.aspx".
In other words "Server.Transfer" is executed by the server while "Response.Redirect" is executed by thr browser. "Response.Redirect" needs to two requests to do a redirect of the page.
So when to use "Server.Transfer" and when to use "Response.Redirect" ?
Use "Server.Transfer" when you want to navigate pages which reside on the same server, use "Response.Redirect" when you want to navigate between pages which resides on different server and domain.
Below is a summary table of which chalks out differences and in which scenario to use.
The beauty of Server.Transfer is what you can do with it:
TextBox myTxt = (TextBox)this.Page.PreviousPage.FindControl("TextBoxID");
You can get anything from your previous page using the above method as long as you use Server.Transfer but not Response.Redirect
In addition to ScarletGarden's comment, you also need to consider the impact of search engines and your redirect. Has this page moved permanently? Temporarily? It makes a difference.
see: Response.Redirect vs. "301 Moved Permanently":
We've all used Response.Redirect at
one time or another. It's the quick
and easy way to get visitors pointed
in the right direction if they somehow
end up in the wrong place. But did you
know that Response.Redirect sends an
HTTP response status code of "302
Found" when you might really want to
send "301 Moved Permanently"?
The distinction seems small, but in
certain cases it can actually make a
big difference. For example, if you
use a "301 Moved Permanently" response
code, most search engines will remove
the outdated link from their index and
replace it with the new one. If you
use "302 Found", they'll continue
returning to the old page...
There are many differences as specified above. Apart from above all, there is one more difference. Response.Redirect() can be used to redirect user to any page which is not part of the application but Server.Transfer() can only be used to redirect user within the application.
//This will work.
Response.Redirect("http://www.google.com");
//This will not work.
Server.Transfer("http://www.google.com");
Transfer is entirely server-side. Client address bar stays constant. Some complexity about the transfer of context between requests. Flushing and restarting page handlers can be expensive so do your transfer early in the pipeline e.g. in an HttpModule during BeginRequest. Read the MSDN docs carefully, and test and understand the new values of HttpContext.Request - especially in Postback scenarios. We usually use Server.Transfer for error scenarios.
Redirect terminates the request with a 302 status and client-side roundtrip response with and internally eats an exception (minor server perf hit - depends how many you do a day) Client then navigates to new address. Browser address bar & history updates etc. Client pays the cost of an extra roundtrip - cost varies depending on latency. In our business we redirect a lot we wrote our own module to avoid the exception cost.
Response.Redirect is more costly since it adds an extra trip to the server to figure out where to go.
Server.Transfer is more efficient however it can be a little mis-leading to the user since the Url doesn't physically change.
In my experience, the difference in performance has not been significant enough to use the latter approach
Server.Transfer doesn't change the URL in the client browser, so effectively the browser does not know you changed to another server-side handler. Response.Redirect tells the browser to move to a different page, so the url in the titlebar changes.
Server.Transfer is slightly faster since it avoids one roundtrip to the server, but the non-change of url may be either good or bad for you, depending on what you're trying to do.
Response.Redirect: tells the browser that the requested page can be found at a new location. The browser then initiates another request to the new page loading its contents in the browser. This results in two requests by the browser.
Server.Transfer: It transfers execution from the first page to the second page on the server. As far as the browser client is concerned, it made one request and the initial page is the one responding with content.
The benefit of this approach is one less round trip to the server from the client browser. Also, any posted form variables and query string parameters are available to the second page as well.
Just more details about Transfer(), it's actually is Server.Execute() + Response.End(), its source code is below (from Mono/.net 4.0):
public void Transfer (string path, bool preserveForm)
{
this.Execute (path, null, preserveForm, true);
this.context.Response.End ();
}
and for Execute(), what it is to run is the handler of the given path, see
ASP.NET does not verify that the current user is authorized to view the resource delivered by the Execute method. Although the ASP.NET authorization and authentication logic runs before the original resource handler is called, ASP.NET directly calls the handler indicated by the Execute method and does not rerun authentication and authorization logic for the new resource. If your application's security policy requires clients to have appropriate authorization to access the resource, the application should force reauthorization or provide a custom access-control mechanism.
You can force reauthorization by using the Redirect method instead of the Execute method. Redirect performs a client-side redirect in which the browser requests the new resource. Because this redirect is a new request entering the system, it is subjected to all the authentication and authorization logic of both Internet Information Services (IIS) and ASP.NET security policy.
-from MSDN
Response.Redirect involves an extra round trip and updates the address bar.
Server.Transfer does not cause the address bar to change, the server responds to the request with content from another page
e.g.
Response.Redirect:-
On the client the browser requests a page http://InitiallyRequestedPage.aspx
On the server responds to the request with 302 passing the redirect address http://AnotherPage.aspx.
On the client the browser makes a second request to the address http://AnotherPage.aspx.
On the server responds with content from http://AnotherPage.aspx
Server.Transfer:-
On the client browser requests a page http://InitiallyRequestedPage.aspx
On the server Server.Transfer to http://AnotherPage.aspx
On the server the response is made to the request for http://InitiallyRequestedPage.aspx passing back content from http://AnotherPage.aspx
Response.Redirect
Pros:-
RESTful - It changes the address bar, the address can be used to record changes of state inbetween requests.
Cons:-
Slow - There is an extra round-trip between the client and server. This can be expensive when there is substantial latency between the client and the server.
Server.Transfer
Pros:-
Quick.
Cons:-
State lost - If you're using Server.Transfer to change the state of the application in response to post backs, if the page is then reloaded that state will be lost, as the address bar will be the same as it was on the first request.
Response.Redirect
Response.Redirect() will send you to a new page, update the address bar and add it to the Browser History. On your browser you can click back.
It redirects the request to some plain HTML pages on our server or to some other web server.
It causes additional roundtrips to the server on each request.
It doesn’t preserve Query String and Form Variables from the original request.
It enables to see the new redirected URL where it is redirected in the browser (and be able to bookmark it if it’s necessary).
Response. Redirect simply sends a message down to the (HTTP 302) browser.
Server.Transfer
Server.Transfer() does not change the address bar, we cannot hit back.One should use Server.Transfer() when he/she doesn’t want the user to see where he is going. Sometime on a "loading" type page.
It transfers current page request to another .aspx page on the same server.
It preserves server resources and avoids the unnecessary roundtrips to the server.
It preserves Query String and Form Variables (optionally).
It doesn’t show the real URL where it redirects the request in the users Web Browser.
Server.Transfer happens without the browser knowing anything, the browser request a page, but the server returns the content of another.