- Find the stock.
- Package it.
- Print the shipping label, cut out and attach it.
- Take the order to the post box.
A lot of that is beyond our means to automate, but we did get a Brother QL-570 label printer to speed up step 3 quite a lot. That prints and cuts adhesive labels that look more professional than what we were doing before, and costs less per label.
The next idea was to automate the printing of the shipping labels, so that the printer kicks in each time there is a new order without us having to do anything else.
Getting print jobs on a queue
The first step was getting a queue for shipping label print jobs. A queue seemed like a natural way to set this up, as it de-couples the printing from the server managing the ordering system, both in implementation and in timing.
Laravel has a great queue abstraction that made it easy to use a Beanstalkd queue on the local development server and an SQS queue in production. That combines nicely with Laravel’s asynchronous queued events, letting us drop a print job on to the queue for each new order.
We also added a print button to trigger the printing of the address for a particular order, and a generic printing form that allows printing of any address, which is useful for eBay orders.
This set up was also quite easy to test in Laravel, giving us confidence that the implementation hadn’t broken anything else in the ordering system, and that it would queue up the jobs correctly in production.
Consuming print jobs
A small Python script listens for print jobs on either Beanstalkd or SQS.
This made it easy to do trial runs with the local development server. The only tricky thing was making the Beanstalkd queue available to a process outside the Vagrant box, which turned out to require an SSH tunnel.
Communicating with the label printer
To print out the jobs on the label printer, we’re using a Raspberry Pi to listen to the print queue and send them to the label printer. This runs Ubuntu Mate and uses the Python script above to consume jobs and print them.
The communication with the Brother label printer takes a few steps.
First a search and replace is done on an SVG template file to get a representation of the address label.
That SVG is then rendered to a bitmap, as that’s the format required by the Brother printer command to send to the printer.
It turned out to be too much hassle to use the printing package directly in code, as it’s designed for command line use. In the end it was more pragmatic to just make shell commands to it from the Python script. These commands first convert the bitmap into the format for the printer, and then send that over the cable to the printer on
The final step was to have monit start the printer script on the Raspberry Pi and keep it running.
Fun order notifications
With this working, the shipping labels collect underneath the printer ready to be stuck on to the packaged orders. A nice side-effect is that if we’re in the room when an order comes in, we get a fun order notification in the form of the shipping label getting printed and cut by the label printer.