When you create a POS system as a web app, you have the problem that not all the devices are controllable from your server.
In my case I have the following hardware connected to the workstation:
- Barcode scanner
- Receipt printer
The input hardware is easy, because a barcode scanner is seen as a keyboard, and both devices can communicate to the server through the web browser.
The tricky part is when a sale is completed, and the customer wants a receipt.
Since the printer is connected to the (dummy) workstation and not to the (remote) server, we cannot print the receipt directly.
The solution I used for this problem is ZeroMQ.
On the server I have a PHP process running which binds to 2 ZeroMQ sockets. One (ZMQ::SOCKET_PULL) is waiting for incoming print request from the web app, one (ZMQ::SOCKET_PUB) is publishing a print request to all subscribing workstations.
On the workstation (in my case a Windows laptop with an HP receipt printer connected to it) I have another PHP process running which is waiting for print jobs (on a ZMQ::SOCKET_SUB socket).
I can add the final piece of the puzzle to the web app, by opening a ZMQ::SOCKET_PUSH socket and sending the URL of the receipt page.
Now I have created a lightweight system for controlling the receipt printer, instead of using cronjobs (or scheduled tasks) to check for print jobs.
- I could remove the print-dispatcher part, and let the web app connect to the print receiver directly, but I prefer to have a stable part (the binding sockets) on the known server (so both connecting sockets know the host to connect to).
- The HTML page could be transported over ZeroMQ, but I like the extra request so the web app is sure the receipt is printed.