Title: Patterns in the Web

Author:

Date: 03 October 2003 13:16:00 +01:00 or Fri, 03 October 2003 13:16:00 +01:00

Summary:

During the development of a an internet based GIS system we came across the problem of maintaining session state on two separate server entities...

Body:

During the development of a an internet based GIS system we came across the problem of maintaining session state on two separate server entities - the website (the application itself) and the mapping server through which the mapping component (a Java Applet embedded in the application) communicates. With our initial architecture, there was the possibility for the website session to timeout while the mapping session remained active. The consequence of this was the website session timeout had to be set to as high as possible a value to minimise the occurrence of this situation, which is a problem in our system due to the fact we are restricting clients to a certain number of concurrent logins and using the ASP SessionEnd event to log a user out.

A graceful solution to force both sessions to end was needed.

The solution came in the shape of the GOF PROXY pattern, which although it is a very simple and well known idea, its application is not immediately obvious in an IIS/ASP environment.

In summary the Java Applet makes requests to the proxy (an ASP page) which forwards the request on to the appropriate CGI programme on the mapping server and conversely feeds the response back to the applet. If the web session times out we can prevent requests going through the proxy. We can also return some text representing an error to the applet, which can close down gracefully.

This solution also has the benefit of allowing us to simplify general error handling in the applet itself. Any errors in making requests to the mapping server can be trapped inside the proxy page and fed to the applet in a more friendly or standardised way.

The final benefit is that we can implement simple load balancing (there are potentially several mapping servers) inside the proxy by using the ASP Application variable to store the address of the next server to use.

The technique is of course not limited to Java applets - any TCP/IP aware component can use it.

The design and code for the proxy is shown below in the style of a GOF pattern.

PROXY

Intent

Provide a surrogate or placeholder for another object to access.

Also Known As

Surrogate
Ambassador

Motivation

It may be a requirement to hive off some heavy processing from the main web server without losing the advantages of having a single web server as a point of contact.

Applicability

A forwarding proxy provides a web page to a client while forwarding the processing for that request to a different web server.
A protecting proxy provides a web page that includes logic to dictate when to allow access. (This could include changing port numbers).

Structure

Collaborations

Proxy forwards requests onto the real subject when appropriate, depending on the kind of proxy.

Consequences

The PROXY pattern introduces a level of indirection when accessing an object. In a web environment this can have large (time) overheads.

Sample Code

Ensure that the response is not cached and that all output is buffered. Note that it is very important that the <% be the first lines in the file, especially when binary data may be proxied, as any leading data may corrupt what you are trying to proxy.

<%
  Response.Buffer = True
  Response.CacheControl = "Private"
  Response.Expires = 0

Dimension all variables.

on error resume next

dim xmlHttp
set xmlHttp
     = Server.CreateObject(
           "Msxml2.ServerXMLHTTP")

Construct the url to proxy. This sample only proxies GET parameters, but could be easily modified to include POST.

dim url
url = "http://a.n.other.webserver.com/scripts/gcis.dll"
if(len(Request.ServerVariables(
                      "QUERY_STRING")) <> 0) then
  url = url + "?" + Request.ServerVariables(
                      "QUERY_STRING")
end if

Get the request.

xmlHttp.open "GET", url, False
xmlHttp.send

If no error and the status of the request was OK.

if Err.number = 0 and xmlHttp.Status = 200 then

Depending upon the data type; for textual data use;

  ' plain text response
  Response.ContentType = "text/plain"
  Response.write xmlHttp.responseText

For binary data (commented out for this example as the data is text);

  ' binary data (e.g. gif image)
  ' Response.ContentType = "image/gif"
  ' Response.binarywrite xmlHttp.responseBody

Otherwise handle any errors (you can respond with html, or even do a Server.Transfer operation).

  else
    Response.write "Error"
  end if

Tidy up

  set xmlHttp = Nothing

Send the response and ignore any other instructions on this page. This is especially important when sending binary data as any further data may corrupt what you are attempting to proxy.

  Response.End
%>

Related Patterns

ADAPTER
DECORATOR

Notes:

More fields may be available via dynamicdata ..