Keyed data

Concept

Topics come in two flavors: those whose associated data type has specified some fields as defining the key, and those whose associated data type has not.If the data type has some fields that act as a key, the Topic essentially defines a collection of data instances whose values can be independently maintained.

Since the key fields are contained within the data structure, Connext could examine the key fields each time it needs to determine which data instance is being modified. However, for performance and semantic reasons, it is better for your application to declare all the data-instances it intends to modify, prior to actually writing any samples. This is known as registration.

Example Description

This example illustrates how to use keyed data. In particular, in this example a single DataWriter creates three instances differentiated by the fiel <code>, which is the field used as key.

These three instances has the following behavior:

• The first instance is continuously written.
• At t=5 we start writing the second and third instances.
• At t=10 we unregister the second instance, meaning that this DataWriter no longer has information to send. By default, unregistering an instance disposes it, but we change this in the datawriter QoS.
• At t=15 we dispose the third instance.

In this example, a single DataReader receives the data sent by the DataWriter. Here we show how the reader can access instance state information. This requires only a few modifications in the on_data_available handler.

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