Report PDFs in Gin

Why reports become PDFs

A report is a snapshot of numbers at a moment in time. Its whole value is that it does not change when the underlying dashboard does. That is the difference between a document and a view of a database, and it is why analytics, operations and account teams keep asking for this.

What a report has to carry

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

• the reporting period as an explicit start and end date
• the generation timestamp, distinct from the period
• the data source or filter set, so a reader can tell what was excluded
• each metric with its unit, since a bare number is not a figure
• comparison against the prior period where the report claims a trend

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/report.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 ReportPDF(c *gin.Context) {
	report, err := store.FindReport(c.Param("id"))
	if err != nil {
		c.AbortWithStatus(http.StatusNotFound)
		return
	}

	var html bytes.Buffer
	if err := tmpl.ExecuteTemplate(&html, "report.html", report); 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="report.pdf"`})
}

The CSS that makes a report page correctly

The layout problem specific to this document is that charts and tables must not split mid-element, and a report long enough to matter will page-break somewhere awkward unless break rules are set. These rules handle it.

/* Charts and tables must not split. Sections start on a fresh
   page so a heading never ends up alone at the foot of one. */
.chart, figure, table { break-inside: avoid; }
h2 { break-after: avoid; }
section { break-before: page; }
section:first-of-type { break-before: auto; }

@page { size: A4; margin: 20mm 18mm; }

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 report lives afterwards

Rendering is the short part. Reports pile up faster than any other document here because they are produced on a schedule whether or not anyone reads them. Most teams keep twelve months and expire the rest, which makes storage lifecycle a design decision rather than an afterthought.

Getting the document right

• Check the period the figures cover, because a report without an unambiguous date range is worse than no report.
• Generation is triggered by a schedule, usually month end or week end, so size the timeout for that path rather than for a health check.
• This is a batch shape: render the run through the batch endpoint rather than firing thousands of individual requests.
• 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 report in other stacks.