Ticket PDFs in FastAPI

Why tickets become PDFs

A ticket has to work at a gate with no signal, on a phone that may be offline, or on paper. Anything requiring a live request fails at exactly the wrong moment. That is the difference between a document and a view of a database, and it is why event organisers and travel operators keep asking for this.

What a ticket has to carry

Before any of the code below matters, the template has to produce a document that is actually a ticket. These are the fields that make it one, and the ones a reader or an auditor will look for first.

• the scannable code, which is the ticket; everything else is decoration
• the event or journey, with date and time including timezone
• the seat, zone or class where the entitlement is specific
• the holder name where the ticket is non-transferable
• the terms of entry, which are what a gate refusal is justified by

Turning your Jinja2Templates template into a document

You already have the markup. Templates.get_template("invoice.html").render(context) gives you it as a string, which is the only input the render needs. TemplateResponse is for returning HTML to a browser. When the HTML is going to a renderer you want the string, so call the underlying Jinja template directly rather than unwrapping a Response you never send.

The FastAPI implementation

httpx is the client most projects here already have. The render happens in an async endpoint returning StreamingResponse, and you return StreamingResponse over the httpx byte iterator.

# routers/tickets.py
import httpx
from fastapi import APIRouter, Depends
from fastapi.responses import StreamingResponse
from fastapi.templating import Jinja2Templates

router = APIRouter()
templates = Jinja2Templates(directory="templates")


@router.get("/tickets/{ticket_id}.pdf")
async def ticket_pdf(ticket_id: int, db=Depends(get_db)):
    ticket = await db.get_ticket(ticket_id)

    # The template directly, not TemplateResponse: we want the string, and
    # TemplateResponse is for HTML you are actually sending to a browser.
    html = templates.get_template("ticket.html").render(ticket=ticket)

    client = httpx.AsyncClient(timeout=60.0)
    request = client.build_request(
        "POST",
        "https://api.pdfpipe.xyz/v1/pdf",
        headers={"Authorization": f"Bearer {settings.pdfpipe_key}"},
        json={"html": html, "options": {"format": "A4", "printBackground": True}},
    )
    upstream = await client.send(request, stream=True)
    upstream.raise_for_status()

    return StreamingResponse(
        upstream.aiter_bytes(),
        media_type="application/pdf",
        background=BackgroundTask(upstream.aclose),
    )

The CSS that makes a ticket page correctly

The layout problem specific to this document is that a barcode or QR code must render at a size and contrast a scanner can actually read after printing. These rules handle it.

/* The code has to survive printing. Keep it black on white at a
   size a scanner can resolve, and never let it break across a page. */
.barcode {
  break-inside: avoid;
  image-rendering: pixelated;
  min-width: 45mm;
  filter: none;
}

@page { size: A6 landscape; margin: 5mm; }

What goes wrong in FastAPI

Calling a synchronous HTTP client inside an async def endpoint blocks the event loop for the whole render, which stalls every other request on that worker. Either use httpx.AsyncClient, or declare the endpoint def rather than async def so it runs in the threadpool.

What people try first

Most FastAPI projects reach for WeasyPrint called from a background task, which moves the memory cost off the request but not out of the process. That works until it is running on more than one machine, at which point the browser becomes the thing you operate rather than the thing you use.

Where the ticket lives afterwards

Rendering is the short part. A ticket is worthless the moment the event ends, so nothing needs to survive it except the record that it was scanned. That record lives in your database, not in the PDF.

Getting the document right

• Check the code itself. A ticket that will not scan is worthless no matter how it looks.
• Generation is triggered by a booking being confirmed, so size the timeout for that path rather than for a health check.
• Volume arrives in bursts, so queue the render rather than doing it inside the request that triggered it.
• Backgrounds are painted by default here, so a design that uses colour needs nothing set. Only an explicit print_background of false turns them off.

Frequently asked

Related

Other FastAPI documents, and the same ticket in other stacks.