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ColdFusion Event Gateway / Web Service Gave Me Trouble August 17, 2006 1:21 pm

Posted by Doug McCaughan in : ColdFusion, Daily Life, Programming, Technology
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My programming love is ColdFusion. I have been working with it since early 1998 with the release of version 3. Version 8 will release next year. A feature of CFMX7 that I have not had the opportunity to use is the Event Gateway to turn a CFC into a web service.

ColdFusion event gateways are ColdFusion MX elements that let ColdFusion MX react to or generate external events or messages in an asynchronous manner. Event gateways let a ColdFusion application handle information that does not come through an HTTP request. For example, you can use event gateways to handle instant messages, short messages from mobile devices, or messages sent to a TCP/IP port. [Source]

The Project

I have a project that uses XML feeds to update inventory status from a wholesaler to the reseller. Unfortunately, do to problems at the wholesaler end, one of the feeds is unavailable and will remain so. The solution is to use a query of the database to obtain IDs of the active products then use CFHTTP to get the information from the wholesaler’s website (site scraping). Since there are roughly 1500 active products at any one time, making 1500 CFHTTP calls at once causes timeouts as the browser gives up before the webserver can finish its task. Since this update will be done only occasionally, 1500 requests to the wholesaler is not a big deal.

The Solution

The solution to the problem is to implement the CFHTTP request as a CFC, create a gateway instance with a 5 minute timeout, and call the gateway instance with small groups of product ids from a .cfm page. The concept works beautifully since the Event Gateway is asynchronous and decoupled from the browser which initiated the instance.

The Problem

I choose the .cfm to manage grouping the product IDs so that there would only be one query instead of 150 queries (~1500 products in groups of 10) as would happen if the query were within the CFC. So the code builds a list of IDs within a structure (as required by the Event Gateway)

The onIncomingMessage method must take a CFEvent structure that contains input information in its Data field, and processes the contents of the Data field as needed. [Source]

The loop that calls the gateway instance builds the list, calls the instance, clears the list, builds the next list, calls the instance, and so forth. Apparently I am able to alter the datastructure so quickly that the second list replaces the first list before the gateway instance fires so the CFC ends up processing only the information from the latter lists. More simply, the CFC processes whatever data is in the structure at the time the gateway event fires from the queue not the state of the structure at the time the gateway event was called by the ColdFusion application. Set up a gateway instance and try the following code. This is the CFC to which the gateway instance will be associated.


<cfcomponent displayname="Test Gateway" hint="Logs the idlist passed into the CFEvent.Data structure.">
   <cffunction name="onIncomingMessage" access="remote">
      <cfargument name="CFEvent" type="struct" required="yes">
         text="Gateway test complete for ids #CFEvent.Data.idlist#."

The calling CF application is show below


   datarange = structNew();
   tmplist = "8,1008,1019,1020,1021,1022,1024,1025,1026,1027,1030";

   datarange.idlist = duplicate(tmplist);
   datarange.datasource = "adult";

   status = SendGatewayMessage("TestGateway", datarange);

   tmplist = "8,20,19,18,17,16";

   datarange.idlist = duplicate(tmplist);
   status = SendGatewayMessage("TestGateway", datarange);

You would expect the log file to have two different lines.

Gateway test complete for ids 8,1008,1019,1020,1021,1022,1024,1025,1026,1027,1030
Gateway test complete for ids 8,20,19,18,17,16

but instead it has two identical lines.

Gateway test complete for ids 8,20,19,18,17,16
Gateway test complete for ids 8,20,19,18,17,16

The Solution

I have not tested the solution yet but I am betting that I either

  1. have to let the CFC call the gateway instance after it has processed a batch of ids thus making this a synchronous process to itself but still asynchronous to the browser or
  2. Create a unique structure for each group of IDs to be processed

I prefer the latter. Update: Testing is complete and solution #2 works great!

Other Resources

About event gateways
Abobe’s (fomerly Macromedia formerly Allaire) overview
Deploying an event gateway
Macrodobellaire’s livedocs for initial setup of an event gateway
Using the CFML event gateway for asynchronous CFCs
This was the most helpful livedoc with critical information non-chalantly hidden within such as "Using the CFML event gateway for asynchronous CFCs"
Livedocs for SendGatewayMessage
The Asynchronous CFML Gateway – A Real-World Example of a Great New Technology
Matthew Woodward comes through in this ColdFusion Developer’s Journal article
Understanding Asynchronous Processing by Ben Forta
It wouldn’t be ColdFusion unless Uncle Ben said something.
Concurrency Library
It is very difficult to mention asychronous CFML without someone mentioning Sean Corfield’s work.
XML Path Language (XPath)
XPath is a language for addressing parts of an XML document, designed to be used by both XSLT and XPointer.
XPath Axes
XML Lessons at W3Schools

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