Topic wildcards

Overview

In MQTT, wildcards provide a powerful mechanism for subscribing to multiple topics simultaneously. When a client subscribes to a topic, it can either subscribe to the exact topic of a published message or utilise wildcards to broaden its subscription.

How are wildcards used?

Clients can subscribe to a wildcard topic to receive messages from multiple matching topics.
Wildcards can reduce overhead by eliminating the need to subscribe to each topic individually.
Wildcards are used when there is uncertainty about the topics that publishing clients will use.

Single level wildcard

Taking advantage of MQTT wildcard capabilities is through the use of a custom event parser. This section will walk you through the process.

Example

Events will be received from various rooms within a house using the follow example topic structure:

sensors/kitchen/temperaturehumidity
sensors/livingroom/temperaturehumidity
sensors/hallway/temperaturehumidity
sensors/bedroom1/temperaturehumidity
sensors/bedroom2/temperaturehumidity

And the subscription will use the single level wildcard method '+':

sensors/+/temperaturehumidity

We shall use the '+' MQTT wildcard to recieve all sensor data from each connected room.

mqttConsumer:
  broker: tcp://127.0.0.1:1883
  topic: sensors/groundfloor/+/temperaturehumidity

  clientId: tempHumidyManagementProcessor
  qos: 1

  deserializer:
    parser: com.fractalworks.mqtt.example.TemperatureHumiditySensorParser
    compressed: false
    batch: false

This custom parser

public class TemperatureHumiditySensorParser implements StreamEventParser<List<StreamEvent>> {

    private String ROOM = "room";
    private ObjectMapper objectMapper;

    public TemperatureHumiditySensorParser() {
        // REQUIRED
    }

    @Override
    public void initialise() throws StreamsException {
        objectMapper = new ObjectMapper();
        objectMapper.configure(SerializationFeature.INDENT_OUTPUT, false);
        objectMapper.configure(SerializationFeature.ORDER_MAP_ENTRIES_BY_KEYS, true);
        objectMapper.setSerializationInclusion(JsonInclude.Include.NON_EMPTY);
    }

    @Override
    public List<StreamEvent> translate(Object payload) throws TranslationException {
        TemperatureHumidityEvent event = parseSingleEvent(payload);
        if(event == null){
            throw new TranslationException("Expected TemperatureHumidityEvent type to parse.");
        }
        return Collections.singletonList(event.toStreamEvent());
    }

    @Override
    public List<StreamEvent> translate(Object payload, String source) throws TranslationException {
        // First parse the event to the standard data type
        List<StreamEvent> events = translate(payload);

        // Now decorate the parsed events with room identifier
        String[] tokens = source.split("/");
        if(tokens.length != 4){
            // Now decorate the event with the source information
            events.forEach(event -> {
                event.addValue(ROOM, tokens[3]);
            });
        }
        return events;
    }

    /**
     * Parse object to a single TemperatureHumidityEvent
     *
     * @param obj
     * @return
     */
    private TemperatureHumidityEvent parseSingleEvent(final Object obj) {
        TemperatureHumidityEvent event = null;
        try {
            if (obj instanceof String str) {
                event = objectMapper.readValue(str, TemperatureHumidityEvent.class);
            } else if (obj instanceof byte[] byteArray) {
                event = objectMapper.readValue(byteArray, TemperatureHumidityEvent.class);
            }
        } catch (IOException e) {
            // Consume exception
        }
        return event;
    }

    /**
     * Parse object to multiple stream events
     *
     * @param obj
     * @return
     */
    private List<TemperatureHumidityEvent> parseMultipleEvents(final Object obj) {
        List<TemperatureHumidityEvent> events = null;
        try {
            if (obj instanceof String str) {
                events = Arrays.asList(objectMapper.readValue(str, TemperatureHumidityEvent[].class));
            } else if (obj instanceof byte[] byteArray) {
                events = Arrays.asList(objectMapper.readValue(byteArray, TemperatureHumidityEvent[].class));
            }
        } catch (IOException e) {
            // Consume exception
        }
        return events;
    }
}

Multi level wildcard

Nothing really changes much when using this wildcard except that you may want to change how you parse the topic string.

Example

This example would now receive all sensor information from a house. The parser would need to coded such that events can be handled holistically within the target stream processor

// Some code
mqttConsumer:
  broker: tcp://127.0.0.1:1883
  topic: sensors/#

  clientId: tempHumidyManagementProcessor
  qos: 1

  deserializer:
    parser: com.fractalworks.mqtt.example.TemperatureHumiditySensorParser
    compressed: false
    batch: false

Notes

See the great documentation HiveMQ has produced that covers this subject.

PreviousMQTT NextSession management

Last updated 9 months ago

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public class TemperatureHumiditySensorParser implements StreamEventParser<List<StreamEvent>> { private String ROOM = "room"; private ObjectMapper objectMapper; public TemperatureHumiditySensorParser() { // REQUIRED } @Override public void initialise() throws StreamsException { objectMapper = new ObjectMapper(); objectMapper.configure(SerializationFeature.INDENT_OUTPUT, false); objectMapper.configure(SerializationFeature.ORDER_MAP_ENTRIES_BY_KEYS, true); objectMapper.setSerializationInclusion(JsonInclude.Include.NON_EMPTY); } @Override public List<StreamEvent> translate(Object payload) throws TranslationException { TemperatureHumidityEvent event = parseSingleEvent(payload); if(event == null){ throw new TranslationException("Expected TemperatureHumidityEvent type to parse."); } return Collections.singletonList(event.toStreamEvent()); } @Override public List<StreamEvent> translate(Object payload, String source) throws TranslationException { // First parse the event to the standard data type List<StreamEvent> events = translate(payload); // Now decorate the parsed events with room identifier String[] tokens = source.split("/"); if(tokens.length != 4){ // Now decorate the event with the source information events.forEach(event -> { event.addValue(ROOM, tokens[3]); }); } return events; } /** * Parse object to a single TemperatureHumidityEvent * * @param obj * @return */ private TemperatureHumidityEvent parseSingleEvent(final Object obj) { TemperatureHumidityEvent event = null; try { if (obj instanceof String str) { event = objectMapper.readValue(str, TemperatureHumidityEvent.class); } else if (obj instanceof byte[] byteArray) { event = objectMapper.readValue(byteArray, TemperatureHumidityEvent.class); } } catch (IOException e) { // Consume exception } return event; } /** * Parse object to multiple stream events * * @param obj * @return */ private List<TemperatureHumidityEvent> parseMultipleEvents(final Object obj) { List<TemperatureHumidityEvent> events = null; try { if (obj instanceof String str) { events = Arrays.asList(objectMapper.readValue(str, TemperatureHumidityEvent[].class)); } else if (obj instanceof byte[] byteArray) { events = Arrays.asList(objectMapper.readValue(byteArray, TemperatureHumidityEvent[].class)); } } catch (IOException e) { // Consume exception } return events; } }