What is a proxy page?

Question

I am new to the Javascript API for ArcGIS. I am trying to get some of the samples working using my own data and ArcGIS Services. I note that many of the samples have this line of code:
//identify proxy page to use if the toJson payload to the geometry service is greater than 2000 characters. 
//If this null or not available the buffer operation will not work.  Otherwise it will do a http post to the proxy. 
esriConfig.defaults.io.proxyUrl = "/arcgisserver/apis/javascript/proxy/proxy.ashx"; 
esriConfig.defaults.io.alwaysUseProxy = false;

I have absoluteluy no clue what any of this means! What is a proxy page? If I am using my own data can I just use the proxyURL provided by ESRI in the sample? If not how do I create my own one?

I tried googling the first comment "identify proxy page to use......." but all that came up in google was the same line from the code samples....

derekswingley1 · Answer

I do have a control panel which allows me to create 'Virtual Folders", I have used this to create seperate ASP.NET web applications within sub folders of the website. Could I simply put the proxy page in a new "virtual folder" of the website?

I think that should work. I would set it up and give it a shot. Don't be afraid dive in and start trying. Those directions are meant to be as simple as possible to get you up and running when you're working on a local dev machine. When you're in production, it's up to you as to where you put things.

GarethMann · Answer

As long as your hosting provider allows you to run server-side code, you can use a proxy page.

OK, but the issue is with step 4 in the instructions that you linked to under where it says "ASP.NET", it says to use IIS to create a web application in the "default website" directory of the web server. In our shared hosting account we do not have access to IIS on the web server or the default web site directory.

I do have a control panel which allows me to create 'Virtual Folders", I have used this to create seperate ASP.NET web applications within sub folders of the website. Could I simply put the proxy page in a new "virtual folder" of the website?

derekswingley1 · Answer

...this is saying that the proxy page must reside on the same machine as the one hosting the webpage with the javascript code. Is this correct?

Yes, your proxy must be on the same machine as your html/css/js files. 
For the proxyURL can I not just specify a IP Address to another web server?

You cannot do this. One of the main reasons to use a proxy is so that you can access cross domain resources. If you could use a proxy hosted on another server (AKA a cross domain resource), you wouldn't need a proxy.

Directly asking them if you can set up a proxy page is probably not the best way to phrase the question. Most hosting providers probably don't want to hear that you'd like to use their server as an intermediary to grab content from other sites.

One thing to clarify is that the proxy is server side code to fetch content from a URL. As long as your hosting provider allows you to run server-side code, you can use a proxy page. There are example proxy pages available in PHP, .NET and Java. To use those, you need to specify which domains your proxy can access. This is explained in the comments in each proxy page.