Building Data Agents Using the Messaging Functions

Building Data Agents Using the Messaging Functions

What Is Message Queuing

Message queuing is a communication tool that allows applications to reliably interconnect. The queue acts as a holding container for messages as they are sent between applications. The applications send messages to and read messages from queues to communicate back and forth. An application writes a message to a queue, which will then be received and processed by another application at some point determined by the receiving application. This type of communication is designed for asynchronous use where the applications involved are not waiting for an immediate response from the other end.

What are Data Agents

Data agents are applications written in the Recital scripting language that use the message queue functions to manage the transmitted data. Typically a data agent runs in the background reading data from the message queue and processing it. Any Recital application can open a message queue and transmit data to the Data Agent.

Building a Simple Data Agent

In this simple Data Agent example we will open a queue called recital_queue which sends XML encoded messages.

First we need to open a the queue

mqdes=mqopen("/recital_queue")
// Check for error
if (mqdes < 0)
    debug(strerror(error()))
    return
endif

Then we need to wait in a loop processing incoming data.

do while .t.
    // Read each message 
    do while (mqcurmsgs(mqdes) > 0)
        // Read the next message
        mstr=mqreceive(mqdes)
        // Decode the xml message
        obj1 = xml_decode(mstr)
        // TODO... process the message
    enddo
    // wait 1 second before retrying 
    sleep 1
 enddo

When we have finished we can close the queue

 mqclose(mqdes)