Certificate PDFs in FastAPI

Why certificates become PDFs

A certificate is meant to be shown to a third party who has no relationship with the issuing system, and often printed. A link that could rot is not a credential. That is the difference between a document and a view of a database, and it is why training providers, course platforms and event organisers keep asking for this.

What a certificate has to carry

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

• the recipient full name exactly as they gave it
• the achievement, named precisely enough to be checked
• the issue date, and an expiry where the credential lapses
• a unique certificate ID that a verifier can look up
• the issuing body and an authorised signature or seal

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/certificates.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("/certificates/{certificate_id}.pdf")
async def certificate_pdf(certificate_id: int, db=Depends(get_db)):
    certificate = await db.get_certificate(certificate_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("certificate.html").render(certificate=certificate)

    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 certificate page correctly

The layout problem specific to this document is that the design is fixed and usually landscape, so it must fit exactly one page regardless of how long the recipient's name is. These rules handle it.

/* Exactly one landscape page, whatever the name's length. */
@page {
  size: A4 landscape;
  margin: 0;
}

.certificate {
  width: 297mm;
  height: 210mm;
  break-after: avoid;
  display: grid;
  place-items: center;
}

/* Long names shrink rather than wrap onto a second line. */
.recipient {
  font-size: clamp(28px, 5vw, 54px);
  text-wrap: balance;
}

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 certificate lives afterwards

Rendering is the short part. The recipient keeps it indefinitely and may present it years later to an employer who will want to verify it. That is the argument for the ID field: the PDF is the artefact, but the ID is what makes it checkable after your system has changed.

Getting the document right

• Check the recipient's name, which must render correctly for every alphabet your learners actually use.
• Generation is triggered by a course or assessment being completed, 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 certificate in other stacks.