Invoice PDFs in Gin

Why invoices become PDFs

An invoice is a legal record of a demand for payment, and in most jurisdictions it has to be retained in a form that cannot change after issue. A page that re-renders from live data is not that. That is the difference between a document and a view of a database, and it is why finance teams and anyone billing customers keep asking for this.

What a invoice has to carry

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

• an invoice number that is sequential and never reused
• the issue date, and separately the payment due date
• supplier and customer legal names with addresses, not trading names
• tax registration numbers for both parties where the jurisdiction requires them
• per-line quantity, unit price, tax rate and line total
• the tax subtotal broken out by rate, then the gross total

Turning your html/template template into a document

You already have the markup. Tmpl.ExecuteTemplate(&buf, "invoice.html", data) gives you it as a string, which is the only input the render needs. html/template escapes by context, which is what you want, but it means any pre-built HTML fragment you interpolate needs the template.HTML type or it arrives on the page as visible angle brackets.

The Gin implementation

net/http ships with the runtime, so this adds no dependency. The render happens in a Gin handler using DataFromReader, and you c.DataFromReader, passing the upstream body straight through.

// handlers/invoice.go
package handlers

import (
	"bytes"
	"encoding/json"
	"html/template"
	"net/http"
	"os"
	"time"

	"github.com/gin-gonic/gin"
)

var tmpl = template.Must(template.ParseGlob("templates/*.html"))

func InvoicePDF(c *gin.Context) {
	invoice, err := store.FindInvoice(c.Param("id"))
	if err != nil {
		c.AbortWithStatus(http.StatusNotFound)
		return
	}

	var html bytes.Buffer
	if err := tmpl.ExecuteTemplate(&html, "invoice.html", invoice); err != nil {
		c.AbortWithError(http.StatusInternalServerError, err)
		return
	}

	payload, _ := json.Marshal(map[string]any{
		"html":    html.String(),
		"options": map[string]any{"format": "A4", "printBackground": true},
	})

	req, _ := http.NewRequestWithContext(c, http.MethodPost, "https://api.pdfpipe.xyz/v1/pdf", bytes.NewReader(payload))
	req.Header.Set("Authorization", "Bearer "+os.Getenv("PDFPIPE_KEY"))
	req.Header.Set("Content-Type", "application/json")

	client := &http.Client{Timeout: 60 * time.Second}
	resp, err := client.Do(req)
	if err != nil {
		c.AbortWithError(http.StatusBadGateway, err)
		return
	}
	// Without this the connection leaks, and the service just gets slower for days.
	defer resp.Body.Close()

	c.DataFromReader(http.StatusOK, resp.ContentLength, "application/pdf", resp.Body,
		map[string]string{"Content-Disposition": `attachment; filename="invoice.pdf"`})
}

The CSS that makes a invoice page correctly

The layout problem specific to this document is that line items are unbounded, so the table has to break across pages while the totals block stays whole and the column headers repeat. These rules handle it.

/* Line items run past one page. Repeat the header, keep the
   totals block whole, and never strand a single row. */
thead { display: table-header-group; }
tfoot { display: table-footer-group; }
tr    { break-inside: avoid; }

.totals {
  break-inside: avoid;
  break-before: auto;
}

@page {
  size: A4;
  margin: 18mm 16mm 22mm;
}

What goes wrong in Gin

Not closing resp.Body leaks a connection per request. The symptom is not an error, it is a service that gets slower over days and recovers on restart, which is the hardest kind of bug to attribute.

What people try first

Most Gin projects reach for chromedp, which drives a real Chrome you install, supervise, and periodically clean zombie processes out of. 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 invoice lives afterwards

Rendering is the short part. Invoices are kept for the statutory retention period, which in most jurisdictions runs to six years or more. That means the file has to be stored, not regenerated, and the stored copy has to be the one that was sent.

Getting the document right

• Check the total. A rounding difference between what you display and what you charge is the kind of error that surfaces during an audit rather than on the day.
• Generation is triggered by an order completing, or a billing cycle closing, 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 Gin documents, and the same invoice in other stacks.