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GET TOPIC Functional Keyword

Feature Since Version Notes
GET TOPIC >v1.4.6 Retrieves data from a topic

1. Overview

  • Description: Instructs the broker to retrieve the current value stored (either internally via KEEP TOPIC or the last retained message published via PUBLISH TOPIC) for a specified MQTT topic.

2. Signature

  • Syntax: 
    GET TOPIC "<topicPath>"

3. Parameters

  • "<topicPath>": The topic path (string) from which to retrieve the value. Can include wildcards (+, #) in some contexts, though typically used with specific paths.

4. Return Value

  • Description: Returns the value stored at the specified topic path. If the topic does not exist or has no stored value, it returns EMPTY.

5. Usage Examples

Basic Example

Retrieve a configuration setting and use it in a publish command.

DEFINE ACTION ApplyThreshold
ON TOPIC "sensors/temperature" DO
    SET "threshold" = GET TOPIC "config/temp_threshold"
    IF PAYLOAD > {threshold} THEN
        PUBLISH TOPIC "alerts/temp" WITH "Temperature exceeded threshold!"
    ENDIF

Intermediate Example

Check if a device is enabled before processing its data.

DEFINE ACTION ProcessIfEnabled
ON TOPIC "device/A/data" DO
    // Assumes "config/device/A/enabled" stores "true" or "false"
    IF GET TOPIC "config/device/A/enabled" == "true" THEN
        PUBLISH TOPIC "processed/device/A" WITH (PAYLOAD * 0.5)
    ENDIF

Advanced Example (Counter)

Increment a counter stored internally.

DEFINE ACTION IncrementCounter
ON TOPIC "events/trigger" DO
    SET "currentCount" = GET TOPIC "internal/counter"
    IF {currentCount} == EMPTY THEN
        SET "newCount" = 1
    ELSE
        SET "newCount" = {currentCount} + 1
    ENDIF
    KEEP TOPIC "internal/counter" WITH {newCount} // Store updated count
    PUBLISH TOPIC "public/counter" WITH {newCount} // Optionally publish it

6. Notes & Additional Information

  • GET TOPIC is fundamental for accessing stored state or configuration within LOT.
  • Always consider checking the return value against EMPTY if there's a possibility the topic might not have a value yet.