ColdFusion 10: Using filterCriteria in WebSockets for subscribing and publishing

Yesterday Ben Nadel asked me a question on Twitter, on using filterCriteria when publishing a message on a web socket channel from server side. The method ‘wspublish’ allows you to perform a server side push to a client who has subscribed to a channel. It takes three parameters – channelName, message and filterCriteria. I always assumed that the values present in subscriberInfo and publisherInfo can be compared in Channel Listener functions (canSendMessage, beforePublish etc) before the message can be received by the client. Although this technique is available, what I found after having a discussion with a fellow developer (Awdhesh), is that I can specify simple conditions when subscribing or publishing.

So what does it mean to subscribe to a channel with conditions? And more importantly why do I need it? Consider a scenario where you are subscribing to a particular company stock and would like to know its stock value in real time. The server on the other hand keeps updating the stock values for all the companies and you might end up receiving unnecessary data. Instead you can specify a filter criteria while subscribing to a channel and only the messages that satisfy the criteria will be delivered. Here’s how you do it:

socket.subscribe("myChannel", {name: 'Sagar', age: 25, selector: "stock eq 'ADBE'"}, myChannelHandler);

The second parameter is a JSON string containing some key-value pairs. One of the keys is the 'selector' where the filter criteria is specified. Here I’m declaring a condition ‘stock eq ADBE’ i.e. I’m interested in receiving stock quotes for ADOBE only.

The publisher can send a message on the channel by specifying the same as the third parameter:

<cfset wspublish("myChannel", 50, {stock=’ADBE’})>

The second parameter is the message (value of the stock) and the third parameter is a struct that specifies the key ‘stock’ with its value set to ‘ADBE’. When this message is published on the channel it will be received by the client since it matches the filter criteria specified by the client at the time of subscribing to the channel. If a message is published on the channel with the third parameter set to {stock=’MSFT’} then the same wouldn’t be sent to the client who has subscribe to ADBE stocks.

Similarly a filter criteria can be specified at the time of publishing as well. In the same context of stocks; say you want only those clients whose age is greater than 21 to receive the stock update. You can do that by specifying the filter criteria while publishing the message on a channel:

<cfset wspublish("myChannel", 50, {stock=’ADBE’, selector="age GT 21"}})>

Again, the filter criteria ‘age GT 21’ is specified as a value to the key ‘selector’. If you observe the subscribe method, it has ‘name’ and ‘age’ as keys specified as third parameter. Before publishing the message to a subscriber the value of the key ‘age’ is checked and if it satisfies the condition (age GT 21) then the message (stock value) will be published and received by the subscriber.

Demo:
I have created a sample demo that you can download it here. Open publisher.cfm, subscriber-ADBE.cfm, subscriber-MSFT.cfm on three browser instances. Click on the subscribe button to subscribe to the channels. In publisher.cfm select the stock from the drop down list and provide a value and click ‘Publish’. You’ll see that the subscribers (ADBE and MSFT) would receive values only for those stock quotes that they have subscribed to.