使用Google IoT Core网关与Pub/Sub时subFolder为空的问题排查

The root cause here is that you’re manually constructing the full absolute MQTT topic path instead of using the relative topic format expected by the Google IoT Core device client SDK. When you bypass the SDK’s built-in topic handling by hardcoding the full path, the SDK can’t parse out the subFolder value from the topic structure.

Here’s what’s happening: #

Your current code builds an absolute topic like /devices/mb.{device_id}/events/{attribute}, but the IoT Core SDK is designed to work with relative topics (e.g., events/{attribute}) when publishing events—especially when a gateway is acting on behalf of another device. Using the full absolute path skips the SDK’s logic that extracts the {attribute} segment to populate the subFolder field in the resulting Pub/Sub message.

Fix: Use relative topics and specify the target device ID #

Modify your report method to use a relative topic path and explicitly pass the target device ID to the publish call. This lets the SDK handle constructing the correct full topic and automatically populates the subFolder field with your attribute value.

Modified code example:

def report(self, device_id, attribute, value):
    # Use a relative topic path (SDK will handle the full device path)
    topic = f"events/{attribute}"
    timestamp = datetime.utcnow().timestamp()
    # Build the full device ID matching what's shown in your Pub/Sub message
    full_device_id = f"mb.{device_id}"
    # Explicitly specify the device ID so the gateway publishes on its behalf
    client.publish(topic, json.dumps({"v": value, "ts": timestamp}), device_id=full_device_id)

Why this works: #

When you use the relative topic events/{subfolder} and provide the device_id parameter, the IoT Core SDK automatically constructs the full valid topic path (/devices/{full_device_id}/events/{attribute}) and extracts the {attribute} segment to populate the subFolder attribute in the Pub/Sub message. This aligns with the behavior described in the IoT Core documentation, where subFolder is derived directly from the topic segment following /events/.

After making this change, your Pub/Sub messages should have the subFolder field set to the attribute value you’re reporting (like motion or temperature).

内容的提问来源于stack exchange，提问作者Matthew Brown