A backend for the small web
Project description
Pyriodic Backend
Backend for the small web
The aim of this project is to make the simplest possible "backend" for static HTML websites.
Pyriod Backend is not a backend in the traditional sense, it does not handle the request-response cycle. What it allows is to periodically (hence the name) update certain parts of the website with data. A usecase would be showing the weather, or the server load. The updating is being done on registering functions that are linked to specific tag ids in the provided HTML document. The registered functions can do whatever, as long as they return a string of text.
Pyriodic Backend does not have or require any specific server software like
gunicorn. The only requirements are Python and cron, and both can be found in
even the most minimal Linux distros or containers.
This is a very early alpha version, there might be breaking changes in the future
Usage example
-
Create a virtual environment and install the package
python3 -m venv venv source /venv/bin/activate pip install pyriodic-backend -
Create a new Python file
main.pyimport os from pyriodic_backend.entities import HTMLFile from pyriodic_backend.pyriodic_backend import PyriodicBackend def get_temperature(): # here would be gathering data from sensors return "21.2" def get_humidity(): # here would be gathering data from sensors return "73%" def get_load(): load = os.getloadavg() return ", ".join([str(round(x, 2)) for x in load]) index_file = HTMLFile("/var/www/html/index.html") pyriodic_backend = PyriodicBackend() pyriodic_backend.register(html_file=index_file, tag_id="temperature", func=get_temperature, interval=1) pyriodic_backend.register(html_file=index_file, tag_id="humidity", func=get_humidity, interval=4) pyriodic_backend.register(html_file=index_file, tag_id="cpu_load", func=get_load, interval=1) pyriodic_backend.run()
And the corresponding HTML file:
<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <meta name="viewport" content="width=device-width, initial-scale=1.0"> <title>Document</title> </head> <body> The current temperature is <span id="temperature"></span> The current humidity is <span id="humidity"></span> The CPU load is <span id="cpu_load"></span> </body> </html>
Explanation
The first step is to write the functions that will update the specific tags
in the HTML document based on their ids. Then it is required to create
HTMLFile entities with absolute links to the files that need to be updated.
One or more files can be updated in a single run. After that, the functions
need to be linked to the files by using the .register() method. Finally
the last thing is to .run() the update.
Keeping State
The script itself is stateless unless you create your own means to retain data between script executions.
Pyriodic Backend keeps its own log of when a function was run for the last time, to allow intervals longer than one minute. Currently this is being handled by a JSON file created in the venv, which keeps the function signatures and their last run time. In the future I will add more possible ways to keep track of function execution, maybe Redis or something similar. Suggestions are very welcome.
Custom location of the JSON file
By default, the JSON file is created in the same directory as the installed library within the Python virtual environment. The file is not meant to be viewable or editable by humans. But if you want to store the database JSON file in a custom location, you can do it with:
from pyriodic_backend.backend import FileJSONBackend
pyriodic_backend = PyriodicBackend(backend=FileJSONBackend(file_path="/home/user/customdb.json"))
The location needs to be writable for the user running the script.
System setup of Pyriodic Backend
Pyriodic Backend requires cron to work. Cron is preinstalled in most, if not all, Linux distributions.
The preferred way is to run Pyriodic Backend every minute, and define the actual intervals of functions execution in code.
To run PB in cron, open the cron editing tool in the terminal:
crontab -e
At the bottom of the file add the proper cron stanza, it will be very similar to this:
* * * * * /home/user/my-project/venv/bin/python /home/user/my-project/main.py
The five stars at the beginning define that the script will be run every minute. Cron requires that both the Python executable and the script file be provided with absolute paths. You will need to adjust the paths to match your setup. The easiest way to check if the paths are correct is to paste the line into the terminal (without the asterisks) and see if it works.
In result, the script will be run every minute. The interval argument when
registering a method defines how many minutes must pass between each
function call. In the example above, the get_humidity method will be run
every 4 minutes. One minute is the default.
File permissions when updating HTML files
HTML files served by typical HTTP servers like Nginx or Lighttpd usually
live in system directories like /var/www/html and are owned by the root
or www-data users. The simplest way to allow PB to update them is to add
it to the root user cron:
sudo crontab -e
This way the whole script will be run as root, which should allow the files to be updated.
Adding logging
Pyriodic Backend has support for logging. To enable logging, at the
beginning of the main.py file, import the logging module and configure the
logger using logging.basicConfig() as shown below:
import logging
logger = logging.getLogger(__name__)
logging.basicConfig(
format="%(asctime)s %(message)s",
filename="/home/user/my-project/pyriodic_backend.log",
encoding="utf-8",
level=logging.DEBUG,
)
Allowing partial updates
At the defalt settings, if any of the methods for a given file, raises an Exception, the whole file is not modified. This is to prevent incosistently updated pages. To enable partial updates, set it in the PyriodicBackend invocation:
pyriodic_backend = PyriodicBackend(allow_partial=True)
Contributions
Contributions are most welcome! And feel free to raise an issue with any feedback or bug report.
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