Receipt PDFs in Kotlin

Why receipts become PDFs

A receipt is proof a payment happened, and customers attach it to expense claims. It has to survive being forwarded, printed and filed by someone who never had an account with you. That is the difference between a document and a view of a database, and it is why ecommerce and point-of-sale teams keep asking for this.

What a receipt has to carry

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

• the payment reference from the processor, which is what a dispute is traced by
• the amount actually captured, which is not always the amount ordered
• the payment method, usually the card brand and last four digits
• the timestamp with a timezone, since expense systems reconcile by date
• the merchant legal name, which may differ from the brand on the site

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("/receipt/{id}.pdf", produces = [MediaType.APPLICATION_PDF_VALUE])
fun receiptPdf(@PathVariable id: String): ResponseEntity<InputStreamResource> {
    val html = renderReceipt(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 receipt page correctly

The layout problem specific to this document is that receipts are short and usually want a narrow page, so the default A4 canvas leaves an expanse of white unless the page size is set deliberately. These rules handle it.

/* A receipt on A4 is mostly white space. Set the page to the
   content, and let it grow only as far as the items require. */
@page {
  size: 80mm 200mm;
  margin: 6mm;
}

body { font-size: 11px; }

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 receipt lives afterwards

Rendering is the short part. The customer keeps it, not you, and they submit it to an expense system months later. It has to be readable by software that never saw your site, which is why the amount and reference belong in selectable text rather than inside an image.

Getting the document right

• Check the amount paid and the payment reference, which is what an expense system matches against.
• Generation is triggered by a successful payment webhook, 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 receipt in other stacks.