Closed Loop System

What Is a Closed Loop System?

Running a closed-loop system lets your server know whether the picker acknowledged the pick-to-light task. This feedback loop is essential for maintaining accurate inventory and ensuring every pick is verified.

Modern devices support barcodes and QR-Codes that are a natural choice for transaction confirmations. In fact, the encoded data can be customized for each individual pick.

Callback Confirmations

To track each pick, include a unique transaction ID with the command. In the REST API, this field is called a nonce. When the picker responds, Voodoo sends that same ID back to your server so you know exactly which pick was confirmed.

REST API Example

import requests, json

session = requests.Session()
url = "https://www.voodoodevices.com/api/"

# Login
x = session.post(url + "user/login/",
    json={'username': 'yourusername', 'password': 'yourpassword'})
z = json.loads(x.text)

# Send command with nonce for closed-loop tracking
session.post(
    url + "device/D4F660:AFA0CB/",
    headers={'referer': url, 'x-csrf-token': z['token']},
    json={
        'command': 'flash',
        'line1': 'Jessica',
        'line2': 'Pick 3',
        'line3': 'SKU:001256',
        'nonce': 'transactionID001',
        'seconds': 120
    }
)

QueryString Example

https://www.voodoodevices.com/api/D4F660:AFA0CB/pick/Jessica Pick 3/SKU:001256/140,c5,4/20/transactionID001

Acknowledgement State Model

After you send a command with a transaction ID, your server can receive one of three results:

Callback URL Configuration

The prefix of the response URL (e.g., http://your.domain.com:8000/voodoodevices/) is a string you choose and enter in www.voodoodevices.com on the Settings page. The URL can be on any port, use HTTP or HTTPS and use GET or POST methods.

Receiving the Callback

Your server needs an endpoint that Voodoo can reach. Below is a minimal Python example that listens on port 8000 and prints every callback it receives — a useful starting point before you wire the result into your own order or inventory system.

from http.server import BaseHTTPRequestHandler, HTTPServer
import json

# Voodoo calls back using either GET or POST, depending on
# the method you configured on the Settings page.
#
# GET example — result encoded in the URL path:
#   GET /voodoodevices/transactionID001/ack
#
# POST example — result sent as a JSON body:
#   {
#     "acknack":  "ACK",
#     "location": "A1-SHELF002",
#     "deviceid": "D4F660",
#     "nonce":    "transactionID001",
#     "qty":      3
#   }

class CallbackHandler(BaseHTTPRequestHandler):

    def do_GET(self):
        print(f"GET  {self.path}")
        self.send_response(200)
        self.end_headers()

    def do_POST(self):
        length = int(self.headers.get("Content-Length", 0))
        body = self.rfile.read(length)
        data = json.loads(body)
        print(f"POST {self.path}")
        print(json.dumps(data, indent=2))
        self.send_response(200)
        self.end_headers()

if __name__ == "__main__":
    server = HTTPServer(("0.0.0.0", 8000), CallbackHandler)
    print("Listening on port 8000 ...")
    server.serve_forever()

Common Patterns

Here are common integration patterns for closed-loop systems:

• Simple confirmation: Use the transaction ID to mark a pick as complete in your WMS when an "ack" is received.
• Timeout handling: Re-send the command or escalate to a supervisor when a "nack" is received.
• Barcode verification: Display a unique barcode on the device for each pick. The picker scans the barcode to confirm, and a kill command is sent to the device. No button push needed!
• Quantity adjustment: Use multi-button devices to let pickers adjust quantity before acknowledgement. The final quantity is sent along with the ack/nack callback.

Summary Flow