Topic 4: Using Data Queues

Understanding Data Queues in RPG

Data queues are essential structures used in RPG for managing and processing data in a multi-user environment. They allow programs to send and receive messages asynchronously, making them crucial for applications that require real-time data processing.

What is a Data Queue?

A data queue is a collection of messages that can be accessed by one or more programs. The messages in a data queue can be of fixed or varying lengths, and they can hold any type of data, such as strings or numeric values. Data queues are particularly useful in scenarios where multiple programs need to communicate with each other without direct interaction.

Creating a Data Queue

To create a data queue in RPG, you will typically use the CRTDTAQ command. Here’s an example of how to create a data queue named MYDATAQ:

`rpg // RPGLE code snippet DCL-F MYDATAQ DISK;

// Create data queue command EXECUTE 'CRTDTAQ DATA(MYDATAQ)'; `

In this example, MYDATAQ is created as a data queue that can be accessed by other programs.

Sending Messages to a Data Queue

Messages can be sent to a data queue using the SNDDTAQ command. Below is an example of how to send a message to the MYDATAQ data queue:

`rpg // RPGLE code snippet DCL-S message VARCHAR(100); message = 'Hello, this is a test message'; EXECUTE 'SNDDTAQ DTAQ(MYDATAQ) MSG(' + message + ')'; `

In this example, a message is created and sent to the MYDATAQ data queue. The message can be any string that you need to process later.

Receiving Messages from a Data Queue

To retrieve messages from a data queue, you can use the RCVDTAQ command. Below is an example of how to receive a message from the MYDATAQ data queue:

`rpg // RPGLE code snippet DCL-S receivedMsg VARCHAR(100); EXECUTE 'RCVDTAQ DTAQ(MYDATAQ) MSG(' + receivedMsg + ')'; `

In this example, we receive a message from the MYDATAQ data queue and store it in the variable receivedMsg for further processing.

Practical Example: Simple Messaging Application

Let’s consider a simple example where two programs communicate using a data queue: 1. Sender Program: This program sends messages to the data queue. 2. Receiver Program: This program receives messages from the data queue and processes them.

Sender Program Example:

`rpg // RPGLE Sender Program DCL-F MYDATAQ DISK; DCL-S message VARCHAR(100);

message = 'Message from Sender'; EXECUTE 'SNDDTAQ DTAQ(MYDATAQ) MSG(' + message + ')'; `

Receiver Program Example:

`rpg // RPGLE Receiver Program DCL-F MYDATAQ DISK; DCL-S receivedMsg VARCHAR(100);

EXECUTE 'RCVDTAQ DTAQ(MYDATAQ) MSG(' + receivedMsg + ')'; // Process the received message // For example, display it using a simple print statement DSPLY receivedMsg; `

In this simple messaging application, the sender program sends a message, and the receiver program retrieves and displays it.

Conclusion

Data queues are a powerful feature in RPG that facilitate communication between programs. By using data queues, you can build responsive applications that handle data efficiently in a multi-user environment. Understanding how to create, send, and receive messages in data queues is crucial for developers working with RPG.