Pyriodic Backend

Backend for the static web

The aim of this project is to make a simple to use 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. A usecase would be showing the weather, or the server load. The updating is being done by 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 to change the tag's text, or a dictionary of attributes to change the tag's style.

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

1. Create a virtual environment and install the package

   python3 -m venv venv
   source /venv/bin/activate
   pip install pyriodic-backend
   

2. Create a new Python file main.py

   from datetime import datetime
   import os
   from pyriodic_backend.entities import HTMLFile
   from pyriodic_backend.generics import get_cpu_load
   from pyriodic_backend.pyriodic_backend import PyriodicBackend
   
   def change_bg_color():
       hour = datetime.now().hour
       minute = datetime.now().minute
       return {
           "color": "white",
           "background-color": f"rgb({hour * 5}, {minute * 2}, {minute // 5})",
       }
   
   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%"
   
   index_file = HTMLFile("/var/www/html/index.html")
   
   pyriodic_backend = PyriodicBackend()
   
   pyriodic_backend.register_styling(
       html_file=index_file, tag_id="colourful-box", func=change_bg_color, interval=1
   )
   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_cpu_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>
       <div id="colourful-box">
           Thix box changes its background color!
       </div>
       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>
   

Now, when running the script by doing

python main.py

The HTML file will change, the colourful div will get a style attribute, and the temperature, humidity and cpu_load spans will have text values.

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() or .register_styling() method. Finally the last thing is to .run() the update.

Methods and Styling Methods

Pyriodic Backend allows to update the page in two different ways: update the tag's text content or its style.

To update the tag's text content, create a method that returns a string and register it using the .register() method. To update the tag's styling, create a method that returns a dictionary of CSS styles, and use the .register_styling() method.

Generic Methods

There are three predefined methods currently available for common usecases:

get_cpu_load

Returns the last 1 minute, 5 minutes, and 15 minutes CPU load of the host as a comma separate list, for example "0.1, 0.3, 0.45".

get_uptime

Returns the uptime of the host in days, hours, minutes and seconds, for example "1d 0:50:54".

get_ram_usage

Returns the percentage of used RAM on the host, for example "54%".

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.