barcode-server

barcode-server

A simple daemon to read barcodes from USB Barcode Scanners and expose them to other service using HTTP calls, a websocket API or MQTT.

Features

• Autodetect Barcode Scanner devices on the fly
• Request Server information via REST API
• Subscribe to barcode events using
  • Websocket API
• Push barcode events using
  • HTTP requests
  • MQTT messages
• Get statistics via Prometheus exporter

How to use

Device Access Permissions

Ensure the user running this application is in the correct group for accessing input devices (usually input), like this:

sudo usermod -a -G input myusername

Configuration

barcode-server uses container-app-conf to provide configuration via a YAML or TOML file as well as ENV variables. Have a look at the documentation about it.

The config file is searched for in the following locations (in this order):

• ./
• ~/.config/
• ~/

See barcode_server.yaml for an example in this repo.

Native

# create venv
python -m venv ./venv
# enter venv
source ./venv/bin/activate
# install barcode-server
pip install barcode-server
# exit venv
deactivate

# print help
./venv/bin/barcode-server -h

This will give you an overview of all available commands:

> ./venv/bin/barcode-server -h
Usage: barcode-server [OPTIONS] COMMAND [ARGS]...

Options:
  --version   Show the version and exit.
  -h, --help  Show this message and exit.

Commands:
  config  Print the current configuration of barcode-server
  run     Run the barcode-server

Docker

The docker image will by default be launched with the run command, making it easier for production deployment. When starting the docker container, make sure to pass through input devices as well as the configuration file:

docker run -it --rm \
  --name barcode \
  --device=/dev/input \
  -v "/home/markus/.config/barcode_server.yaml:/app/barcode_server.yaml" \
  -e PUID=0 \
  -e PGID=0 \
  markusressel/barcode-server

Note: Although barcode-server will continuously try to detect new devices, even when passing through /dev/input like shown above, new devices can not be detected due to the way docker works. If you need to detect devices in real-time, you have to use the native approach.

The user and group id that should be used within the container can be specified using the PUID and PGID environment variables.

To override the default command, simply specify command arguments directly:

docker run -it --rm \
  ...
  markusressel/barcode-server --help

Webserver

By default the webserver will listen to 127.0.0.1 on port 9654.

Authorization

When specified in the config, an API token is required to authorize clients, which must be passed using a X-Auth-Token header when connecting. Since barcode-scanner doesn't rely on any persistence, the token is specified in the configuration file and can not be changed on runtime.

Rest API

barcode-server provides a simple REST API to get some basic information. This API can not be used to retrieve barcode events. To do that you have to use one of the approaches described below.

Endpoint	Description
/devices	A list of all currently detected devices.

Websocket API

In addition to the REST API barcode-server also exposes a websocket at /, which can be used to get realtime barcode scan events.

To connect to it, you have to provide

• a Client-ID header with a UUID (v4)
• (optional) an empty Drop-Event-Queue header, to ignore events that happened between connections
• (optional) an X-Auth-Token header, to authorize the client

Messages received on this websocket are JSON formatted strings with the following format:

{
  "id": "33cb5677-3d0b-4faf-9dc4-d19a8ee7d8a1",
  "serverId": "cash-register-1",
  "date": "2020-08-03T10:00:00+00:00",
  "device": {
    "name": "BARCODE SCANNER BARCODE SCANNER",
    "path": "/dev/input/event3",
    "vendorId": "ffff",
    "productId": "0035"
  },
  "barcode": "4250168519463"
}

To test the connection you can use f.ex. websocat:

> websocat - autoreconnect:ws://127.0.0.1:9654 --text --header "Client-ID:dc1f14fc-a7a6-4102-af60-2b6e0dcf744c" --header "Drop-Event-Queue:" --header "X-Auth-Token:EmUSqjXGfnQwn5wn6CpzJRZgoazMTRbMNgH7CXwkQG7Ph7stex"
{"date":"2020-12-20T19:35:04.769739","device":{"name":"BARCODE SCANNER BARCODE SCANNER","path":"/dev/input/event3","vendorId":65535,"productId":53},"barcode":"D-t38409355843o52230Lm54784"}
{"date":"2020-12-20T19:35:06.237408","device":{"name":"BARCODE SCANNER BARCODE SCANNER","path":"/dev/input/event3","vendorId":65535,"productId":53},"barcode":"4250168519463"}

HTTP Request

When configured, you can let barcode-scanner issue a HTTP request (defaults to POST) when a barcode is scanned, which provides the ability to push barcode events to a server that is unaware of any client. The body of the request will contain the same JSON as in the websocket API example.

To do this simply add the following section to your config:

barcode_server:
  [ ... ]
  http:
    url: "https://my.domain.com/barcode"

Have a look at the example config for more options.

MQTT Publish

When configured, you can let barcode-scanner publish barcode events to a MQTT broker. The payload of the message will contain the same JSON as in the websocket API example.

To do this simply add the following section to your config:

barcode_server:
  [ ... ]
  mqtt:
    host: "my.mqtt.broker"

Have a look at the example config for more options.

Statistics

barcode-server exposes a prometheus exporter (defaults to port 8000) to give some statistical insight. A brief overview of (most) available metrics:

Name	Type	Description
websocket_client_count	Gauge	Number of currently connected websocket clients
devices_count	Gauge	Number of currently detected devices
scan_count	Gauge	Number of times a scan has been detected
device_detection_processing_seconds	Summary	Time spent detecting devices
rest_endpoint_processing_seconds	Summary	Time spent in a rest command handler
notifier_processing_seconds	Summary	Time spent in a notifier

FAQ

Can I lock the Barcode Scanner to this application?

Yes. Most barcode readers normally work like a keyboard, resulting in their input being evaluated by the system, which can clutter up your TTY or other open programs. barcode-server will try to grab input devices, making it the sole recipient of all incoming input events from those devices, which should prevent the device from cluttering your TTY.

If, for some reason, this does not work for you, try this:

Create a file /etc/udev/rules.d/10-barcode.rules:

SUBSYSTEM=="input", ACTION=="add", ATTRS{idVendor}=="xxxx", ATTRS{idProduct}=="yyyy", RUN+="/bin/sh -c 'echo remove > /sys$env{DEVPATH}/uevent'"
SUBSYSTEM=="input", ACTION=="add", ATTRS{idVendor}=="xxxx", ATTRS{idProduct}=="yyyy", DEVPATH=="*:1.0/*", KERNEL=="event*", RUN+="/bin/sh -c 'ln -sf /dev/input/$kernel /dev/input/barcode_scanner'"

Replace the idVendor and idProduct values with the values of your barcode reader (a 4 digit hex value with leading zeros). You can find them in the log output of barcode-reader or using lsusb with the wireless receiver attached to your computer.

Reload udev rules using:

udevadm control --reload

then remove and reinsert the wireless receiver. You should now have a symlink in /dev/input/barcode_scanner:

ls -lha /dev/input/barcode_scanner

which can be used in the device_paths section of the barcode-server config.

Source: This and That

Contributing

GitHub is for social coding: if you want to write code, I encourage contributions through pull requests from forks of this repository. Create GitHub tickets for bugs and new features and comment on the ones that you are interested in.

License

barcode-server is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program.  If not, see <https://www.gnu.org/licenses/>.