A customer needed a solution to the following problem. There are multiple web POS systems that need to print to a receipt printer. The web server is not in the same place as the POS terminals, but they are connected to a network.

To solve the problem I used ZeroMQ to connect the web server to the receipt printers. The current solution consists of four parts.

1. Web app
2. Web server (connects a PUSH socket to a PULL socket)
3. ZeroMQ device (a multiplexer receives from a PULL socket and sends these to a PUB socket)
4. Computer with receipt printer (a sink that receives on a SUB socket subscribed to its own address)

First a user in a browser clicks the button to print a receipt.

This command is converted into a data packet formatted for the receipt printer.

This data packet is send to a small program that connects the web server and the point of sale systems. The first frame of the data packet contains the name of the POS system that should print the receipt.

The command to print the receipt contains the address of the terminal that requested to print.

The two small programs are written in Go. The multiplexer program opens two sockets and reads from one socket and sends the other socket. Program 4 opens a socket and reads from it. It sends the packets that it receives to WinAPI printer functions.

The nice thing of the solution is that the multiplexer program doesn’t need to know how many printers are connected. So the customer can just connect a sink program for every new terminal that is added.