This tutorial will introduce using Broker Agent Messaging (BAM) to run background tasks written in PHP.
In the past, PHP developers who wanted to run tasks in the background
were limited to cron jobs or using the system() function to
launch the tasks manually.
This tutorial introduces PHP developers to using Broker Agent Messaging (BAM) to queue tasks to be handled by a background agent, also written in PHP. The application will initiate tasks to be handled in the background on a frontend script. This script is a normal PHP web page. The tasks in this case are items to add to an RSS feed. The background agent then takes the tasks and executes them to add the items to the RSS document. We are only using RSS modification as an example. In general, this functionality might be used to execute tasks that take a long time, but which you don't want to have the user wait for while the page is reloading. Further examples include payment processing, heavy computation, or any other slow process.
Let's go ahead and dive into the code of the application. The application consists of two PHP scripts: a frontend page and a backend agent. We'll go over exactly what we mean by agent toward the end of the tutorial, but for now it suffices to know that it is code that will handle the tasks assigned by the frontend. Let's look at the frontend first, since that will be most familiar to existing PHP developers.
The frontend script will look very familiar to any PHP developer. It's a
normal page that's executed when the user browses to it. The only difference
is that it uses some functions specific to BAM. The page consists mainly of
a form with a button and a frame containing the RSS feed. The button is a
submit button that simply calls back to the same page, POSTing a value of
"add". When this value is present, the PHP code sends 5 messages to the
background process using the bam_send_message() function.
There are two things to take notice of in the calls to
bam_send_message():
The backend script will be new to most developers. It is written in PHP, but has a slightly unusual execution pattern. Let's look at the script at the high level first.
This backend script implements an event handler called an BAM "agent". These agents can listen for and send certain kinds of communications. For now, we'll focus on messages. Messages are simple, unidirection communications that include a to address, a from address, and a value.
When a message is received by the agent, the script is executed from top to
bottom. When the bam_dispatch() function is called, it dispatches
the message to the event handler function, bam_message(). It
passes the to and from addresses as well as the value of the message to this
function. Thus in this case, all of our task handling code goes into
bam_message().
Now let's take a look at the contents of the function:
This function just does some basic XML manipulations to add new items to the RSS feed. The main thing to notice is that we use the value of the message sent to construct the description of the RSS item.