Building a Background PDF Generation App Using Python Celery and Template Data
In today’s digital age, businesses often require the generation of PDF files with dynamic data for various purposes such as reports, invoices, or certificates. However, generating these PDF files synchronously within a web application can lead to performance issues and user experience degradation. To solve this, we can create a background app that asynchronously generates PDF files with data from templates. Here, we will build such an app using Celery and Flask, two popular Python frameworks.
Step-by-Step Implementation
Project Structure
pdf_generator/
├── app.py
├── celery_config.py
├── tasks.py
└── templates/
└── index.html
Step 1: Setting Up the Environment
pip install Flask Celery redis Jinja2 reportlab
Step 2: Defining Celery Configuration
In celery_config.py, define the Celery configuration, including the broker and backend URLs.
from celery import Celery
celery_app = Celery(
'tasks',
broker='redis://localhost:6379/0',
backend='redis://localhost:6379/0',
)
Step 3: Defining Celery Tasks
In tasks.py, define the Celery task to generate PDF files. We’ll use the ReportLab library to create PDFs with dynamic data.
from celery_config import celery_app
from reportlab.lib.pagesizes import letter
from reportlab.pdfgen import canvas
@celery_app.task
def generate_pdf():
# Generate a PDF file
filename = 'example.pdf'
c = canvas.Canvas(filename, pagesize=letter)
c.drawString(100, 750, "Hello, this is a PDF generated by Celery!")
c.save()
return filename
Step 3: Creating the Flask Application
In app.py, create a Flask application and configure it to use Celery. Define routes to trigger PDF generation tasks.
from flask import Flask, render_template
from tasks import generate_pdf
app = Flask(__name__)
# Load Celery configuration
app.config.update(
CELERY_BROKER_URL='redis://localhost:6379/0',
CELERY_RESULT_BACKEND='redis://localhost:6379/0'
)
# Initialize Celery
from celery import Celery
celery = Celery(app.name, broker=app.config['CELERY_BROKER_URL'])
celery.conf.update(app.config)
@app.route('/')
def index():
return render_template('index.html')
@app.route('/generate-pdf')
def trigger_generate_pdf():
result = generate_pdf.delay()
return f'PDF generation task queued! Task ID: {result.id}'
if __name__ == '__main__':
app.run(debug=True)
Step 4: Implementing the PDF Generation Logic
Create a template HTML file (index.html) in the templates directory. This file will contain the structure of the PDF file. Use placeholders for dynamic data.
<!DOCTYPE html>
<html>
<head>
<title>PDF Generator</title>
</head>
<body>
<h1>Welcome to the PDF Generator!</h1>
<p>Click the button below to generate a PDF file.</p>
<form action="/generate-pdf" method="get">
<button type="submit">Generate PDF</button>
</form>
</body>
</html>
Step 5: Run the application
- Start a Redis server.
- Run the Celery worker by executing celery -A celery_config.celery_app worker –loglevel=info.
- Run the Flask application by executing python app.py.
Access the Flask application in browser at http://127.0.0.1:5000 and click the “Generate PDF” button to trigger the Celery task to generate a PDF file.
Interpret the Result
- “PDF generation task queued!”: This part of the output confirms that the task to generate a PDF file has been queued for execution.
- “Task ID: Each task in Celery is assigned a unique identifier (UUID) known as the task ID. This ID is used to track the progress and status of the task. In this case, “dc86e58c-abcb-47fe-8f83-10f0590a9773” is the task ID assigned to the PDF generation task.
- Overall, this output indicates that the PDF generation process has been initiated and is awaiting execution by the Celery worker. Once the task is picked up by the worker, it will generate the PDF file asynchronously in the background.