The MQTT modules (MQTT Transmission, MQTT Distributor and MQTT Engine) all provide status tags within Ignition that can be used to monitor the overall health of the MQTT pipeline.

MQTT Engine

The tags automatically created for MQTT Engine are documented here

Engine Info - Edge Nodes

Use the tags below to verify the expected number of Edge Nodes have come online and remain online:

Use the tags below to identify the Sparkplug ID and last timestamp for the Offline and Online nodes

onlineEdgeNodes = system.dataset.toPyDataSet(system.tag.readBlocking("[MQTT Engine]Engine Info/Edge Nodes/Online Nodes")[0].value)     
print "Online Sparkplug EdgeNodes: " + str(system.tag.readBlocking("[MQTT Engine]Engine Info/Edge Nodes/NodesOnline")[0].value)
if system.tag.readBlocking("[MQTT Engine]Engine Info/Edge Nodes/NodesOnline")[0].value > 0:
    for row in onlineEdgeNodes:
        data = []
        data.append(["Sparkplug EdgeNode Descriptor", row[0]])
        data.append(["Last Connect Date", row["Date"]])
        print data
         
offlineEdgeNodes = system.dataset.toPyDataSet(system.tag.readBlocking("[MQTT Engine]Engine Info/Edge Nodes/Offline Nodes")[0].value)     
print "Offline Sparkplug EdgeNodes: " + str(system.tag.readBlocking("[MQTT Engine]Engine Info/Edge Nodes/NodesOffline")[0].value)
if system.tag.readBlocking("[MQTT Engine]Engine Info/Edge Nodes/NodesOffline")[0].value > 0:
    for row in offlineEdgeNodes:
        data = []
        data.append(["Sparkplug EdgeNode Descriptor", row[0]])
        data.append(["Last Connect Date", row["Date"]])
        print data

Engine Info - MQTT Clients

Edge Node - Node Info Tags

For each connected Edge Node, a Node Control and Node Info folder containing tags is created along with a Device Info folder for each connected device.

Using these tags we can look into the health of each Edge Node connection: 

Reasons for rebirth requests are:

• Triggered by user
  • A rebirth request for the Edge Node was manually triggered at MQTT Engine
    
• Message sequence number error

  • The message sequence number received was not in order 

    Common cause:

    • In a Sparkplug compliant system, the combination of Group ID and Edge Node ID (the Sparkplug Edge Node Descriptor) that identifies the Edge Node must be unique. If there are two or more Transmitters with the same Sparkplug Edge Node Descriptor, data from these two transmitters will sent with the same topic resulting in the next message sequence number expected by the MQTT client being incorrect. As a result, the MQTT Client will mark the data as stale and request a rebirth from the transmitter. If you have multiple MQTT Clients subscribing to the namespace, this will also likely create a firestorm of rebirth requests across the system.
• Received a message for an edge node that is offline

  • An NDATA message was received from an Edge Node that is marked as Offline at MQTT Engine.
    

    Common causes:

    • A BIRTH message was not published by the Edge Node or was not received by MQTT Engine after marking the Edge Node Offline
    • There are two or more Transmitters with the same Sparkplug Edge Node Descriptor. A DEATH message has been received for one Edge Node marking the Edge Node Offline and subsequent data is received from another Edge Node with teh same Sparkplug Edge Node Descriptor.
• Reordering sequence numbers
  

  • The message sequence number received was not in order after waiting the specified number of milliseconds after receiving an out of order message for the expected message to arrive.
    

    Common cause:

    • The reordering timeout at MQTT Engine, configured to support clustered MQTT servers which do not support guaranteed in order delivery of QoS 0 messages, is not long enough for MQTT Engine to receive a message with the next sequence number. 
• Unable to set Edge Node online
  • Explanation?
• Failed to find metric name from alias
  • The alias in the DATA message did not match any alias previously published in the BIRTH message
• UDT tag doesn't exist
  • UDT tag received in DATA message not found in previously published BIRTH messages
• Unknown metric
  • Explanation?
• DDATA before BIRTH
  • An DDATA message was received from an Edge Node Device that is marked as Offline at MQTT Engine
    

    Common causes:

    • A DBIRTH message was not published by the Edge Node or was not received by MQTT Engine after marking the Edge Node Device Offline