6. Working with Instances
Instances are a way for an application to represent unique objects within a Topic, by specifying one or more key fields that form a unique identifier for the instance. Examples include identifying unique commercial flights within a “Flight Status” Topic or a unique sensor measuring the temperature in a “Temperature” Topic.
Modeling data using instances can provide several benefits to a system, including:
- An application can represent dynamic behavior of objects that come and go in a system, such as aircraft that may fly within range of a radar system and then fly out of range. See 6.1 Instance States for more details.
- Many QoS policies are applied per instance. For example, the 7.5.12 HISTORY QosPolicy depth is applied per instance. This allows an application to specify: “Keep the last N samples for every instance this DataReader receives.” See 6.3.1 QoS Policies that are Applied per Instance for more examples.
- An application can use DataReader methods such as read_instance() and take_instance() to process all the samples for an instance at once.
- ContentFilteredTopics are more efficient when filtering instances. Using ContentFilteredTopics in combination with instances is a great way to allow applications to take advantage of writer-side filtering to only subscribe to logical subsets of a Topic by specifying the instances that they are interested in.
Instances are defined by key fields that make up a unique identifier of the object being represented. Key fields are similar to primary keys in a database—each unique combination of key field values represents a unique instance. Key fields are specified using the @key directive, as shown in 2.4 DDS Samples, Instances, and Keys.
Table 6.1 Example Keys and Instances
|
Instance |
Key |
Data type |
Sample |
|---|---|---|---|
|
Commercial flight being tracked |
Airline name and flight number, such as: Airline: "United Airlines" Flight number: 901 |
string airline short flight_num float lat float long |
UA, 901, 37.7749, 122.4194 UA, 901, 37.7748, 122.4195 |
|
Sensor that is sending data, such as an individual temperature sensor |
Unique identifier of that sensor, such as: "Floor-08-South" |
string sensor_id long temperature |
Floor-08-South, 78 Floor-08-South, 79 |
|
Car being monitored |
Vehicle identification number (VIN) of the car |
string VIN float lat float long |
JH4DA9370MS016526, 37.7749, 122.4194 JH4DA9370MS016526, 37.7748, 122.4195 |
See the following sections:
See also more details on instances from the DataWriter and DataReader perspectives:
- For the DataWriter: 7.3.14 Managing Instances (Working with Keyed Data Types)
- For the DataReader: 8.3.8 Accessing and Managing Instances (Working with Keyed Data Types)
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