Firmware update fails over-the-air when using MQTT with ThingWorx Edge - device disconnects mid-transfer

IoT PTC ThingWorx
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1

We’re experiencing inconsistent OTA firmware updates for our ThingWorx Edge devices using MQTT protocol. Updates work fine on stable WiFi but fail frequently in field deployments with intermittent connectivity.

Current setup uses MQTT QoS 0 with default keep-alive (60s). When pushing 2MB firmware files, the transfer often stops midway with no error logs on the Edge side. Device appears to disconnect and reconnect during large transfers.


MQTT Connection: keepAlive=60, cleanSession=true
QoS Level: 0 (fire and forget)
Transfer size: ~2048KB chunked at 512 bytes

We’ve noticed session persistence issues - when the device reconnects after brief network drops, it doesn’t resume from where it left off. Is there a recommended MQTT configuration for reliable chunked firmware transfers over unreliable networks? How should we handle session persistence to enable transfer resumption?

2

We had similar issues last year. Another thing to check: are you implementing any transfer state tracking on the Edge device? We added a simple checkpoint mechanism that saves the last successfully received chunk number to local storage. On reconnect, the device requests resumption from that checkpoint. Without this, even with persistent sessions, you’re relying entirely on MQTT broker buffering which has limits.

3

QoS 0 is your main issue here. For firmware updates, you absolutely need QoS 1 (at least once delivery) to ensure message acknowledgment. Also, 512-byte chunks are quite small - this creates excessive overhead with poor network conditions. Try 4KB-8KB chunks instead.

4

Thanks Sara. I increased chunk size to 4KB and switched to QoS 1. Still seeing disconnections though. The keep-alive of 60 seconds might be too aggressive for our cellular connections. Should I increase it? Also, what about cleanSession=true - doesn’t that prevent resumption?

5

Carlos, that checkpoint approach sounds promising. Are you storing it in a file or using the Edge SDK’s persistence features? Also, how do you signal the resume point back to the platform?

6

We use a local JSON file on the Edge device that tracks: firmware_version, total_chunks, last_received_chunk, checksum_validation. On reconnect, the Edge device publishes a resume request with the last_received_chunk value. The ThingWorx service picks up from there. Simple but effective for our rural deployments.

7

Yes, cleanSession=true wipes the session state on reconnect. For OTA updates, you need cleanSession=false so the broker maintains session state during brief disconnects. This allows your device to pick up where it left off. Combine this with a longer keep-alive interval - I’d suggest 180-300 seconds for cellular connections. The broker will buffer messages during short outages. Just be aware this increases broker memory usage, so monitor your ThingWorx platform resources.

8

Let me provide a comprehensive solution covering all three critical aspects:

MQTT Keep-Alive and QoS Configuration: Switch to QoS 1 (at-least-once delivery) for all firmware transfer messages. This ensures acknowledgment and automatic retry. Set cleanSession=false to maintain session state across disconnections. For cellular/unreliable networks, increase keep-alive to 240-300 seconds:


mqttConfig.setKeepAlive(240);
mqttConfig.setCleanSession(false);
mqttConfig.setQos(1);
mqttConfig.setConnectionTimeout(30);

Chunked Firmware Transfer: Increase chunk size to 4KB-8KB to reduce overhead. Implement exponential backoff for retries. Each chunk should include: sequence_number, total_chunks, checksum, and firmware_version metadata. The platform should track transfer progress per device.


chunkSize = 4096; // 4KB chunks
chunkMetadata = {seq: chunkNum, total: totalChunks,
                 checksum: md5(chunkData), version: fwVersion};

Edge Device Session Persistence: Implement stateful transfer tracking on both Edge and platform sides. The Edge device should persist transfer state locally (last_chunk_received, firmware_metadata, partial_checksum) and request resumption on reconnect:


// Edge-side persistence
var transferState = {
  firmwareId: "fw_v2.1.0",
  lastChunkReceived: 45,
  totalChunks: 512,
  partialChecksum: "a3f2...",
  timestamp: Date.now()
};
FileSystem.writeJSON("fw_transfer.json", transferState);

Platform-Side Implementation: On ThingWorx platform, create a FirmwareTransferService that maintains device transfer sessions. When device reconnects, check if there’s an incomplete transfer and resume from the last acknowledged chunk. Implement a timeout mechanism (e.g., 24 hours) to clean up stale sessions.

Additional Recommendations:

  • Enable MQTT last will and testament (LWT) to detect unexpected disconnections
  • Implement integrity verification: validate full firmware checksum after complete transfer before applying update
  • Use ThingWorx Edge SDK’s connection status events to trigger transfer pause/resume logic
  • Monitor MQTT broker queue depth - set maxQueueSize appropriately for your device fleet
  • Consider implementing bandwidth throttling during peak hours to avoid network congestion
  • Add device-side logging for transfer diagnostics: chunk receive times, reconnection events, checksum failures

Testing Strategy: Simulate poor network conditions using tools like tc (traffic control) on Linux to add latency/packet loss. Test scenarios: 5-second disconnections, 30-second outages, gradual signal degradation. Verify transfer resumes correctly in each case.

This approach has proven reliable for our deployments across 2,000+ Edge devices in remote locations with cellular connectivity. Average firmware update success rate improved from 67% to 98% after implementing these changes.