Certificate PDFs in Kotlin

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

The Kotlin implementation

OkHttp is the client most projects here already have. The render happens in a Spring controller returning an InputStreamResource, and you expose the response as an InputStreamResource.

@GetMapping("/certificate/{id}.pdf", produces = [MediaType.APPLICATION_PDF_VALUE])
fun certificatePdf(@PathVariable id: String): ResponseEntity<InputStreamResource> {
    val html = renderCertificate(id)

    val body = mapper.writeValueAsString(
        mapOf(
            "html" to html,
            "options" to mapOf("format" to "A4", "printBackground" to true)
        )
    ).toRequestBody("application/json".toMediaType())

    val request = Request.Builder()
        .url("https://api.pdfpipe.xyz/v1/pdf")
        .header("Authorization", "Bearer " + System.getenv("PDFPIPE_KEY"))
        .post(body)
        .build()

    // Read the body exactly once. Logging it here would consume it.
    val response = client.newCall(request).execute()
    val stream = response.body!!.byteStream()

    return ResponseEntity.ok(InputStreamResource(stream))
}

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 Kotlin

OkHttp response bodies must be closed exactly once. Reading one twice throws IllegalStateException, which is easy to trigger when logging the body before returning it.

What people try first

Most Kotlin projects reach for iText on the JVM, with the same AGPL licensing question that reaches Java. 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 Kotlin documents, and the same certificate in other stacks.