Async PDF rendering with a verified webhook in Go

What you need

• An API key. Create one in the app under API keys and export it as FORMFEED_API_KEY. A test key (ff_test_…) is right for trying things out. See the quick start.
• Go. You need Go 1.21 or newer; the program uses the standard library only.
• The template. revenue-report from the report example, and its data.json next to the program.
• A secret. 16 to 200 random characters in FORMFEED_WEBHOOK_SECRET, known to both programs. It signs the deliveries of this render, retries included.
• A public URL. Replace https://example.com/hooks/formfeed with your endpoint. While developing, formfeed listen forwards deliveries to localhost without a tunnel.

The program

The request names the template, switches to mode: "async" and says where to report. The answer comes back immediately with the status queued.

package main

import (
	"bytes"
	"encoding/json"
	"fmt"
	"net/http"
	"os"
)

const endpoint = "https://api-eu.formfeed.dev/v1/renders"

// a render or a problem document: both are JSON objects, so one struct decodes either
type render struct {
	ID     string `json:"id"`
	Status string `json:"status"`
	Title  string `json:"title"`
	Detail string `json:"detail"`
}

// send makes one call with the key and decodes the answer
func send(method, url string, body []byte) (int, render) {
	request, _ := http.NewRequest(method, url, bytes.NewReader(body))
	request.Header.Set("Authorization", "Bearer "+os.Getenv("FORMFEED_API_KEY"))
	request.Header.Set("Content-Type", "application/json")
	response, err := http.DefaultClient.Do(request)
	if err != nil {
		panic(err)
	}
	defer response.Body.Close()
	var answer render
	json.NewDecoder(response.Body).Decode(&answer)
	return response.StatusCode, answer
}

func main() {
	data, err := os.ReadFile("data.json")
	if err != nil {
		panic(err)
	}
	body, _ := json.Marshal(map[string]any{
		"template":       "revenue-report",
		"data":           json.RawMessage(data),
		"mode":           "async",
		"webhook_url":    "https://example.com/hooks/formfeed",
		"webhook_secret": os.Getenv("FORMFEED_WEBHOOK_SECRET"),
	})

	status, result := send("POST", endpoint, body)
	// every error is a problem document (RFC 9457) with a title and a detail
	if status >= 400 {
		fmt.Fprintf(os.Stderr, "%s: %s\n", result.Title, result.Detail)
		os.Exit(1)
	}
	fmt.Println(result.ID, result.Status) // queued: the webhook reports the rest
}

Run it

It prints the render’s id and queued. A few seconds later your endpoint receives render.completed with the full render, its download_url included, or render.failed with the reason.

export FORMFEED_API_KEY=ff_test_...
export FORMFEED_WEBHOOK_SECRET=$(openssl rand -hex 24)
go run main.go

What is particular about Go

• json.RawMessage. The sample data goes into the body as it is on disk, without being decoded into Go types first, so the program works for any template.
• One struct for both answers. A render and a problem document are both JSON objects, so a single struct with title and detail beside the render fields decodes either.
• Timeouts. http.DefaultClient has none. In a service, give the client a Timeout a little above 60 seconds: a synchronous render answers when the file is done, or after 60 seconds with 202 while it goes on in the background.

The receiver

It listens on port 8080, reads the raw body, checks the signature and the age of the timestamp, and only then looks at the JSON. A delivery that fails the check gets 401.

package main

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"fmt"
	"io"
	"net/http"
	"os"
	"strconv"
	"strings"
	"time"
)

var secret = []byte(os.Getenv("FORMFEED_WEBHOOK_SECRET"))

// valid checks "Webhook-Signature: t=<unix seconds>,v1=<hex HMAC-SHA256 of t.body>".
func valid(header string, body []byte) bool {
	parts := map[string]string{}
	for _, part := range strings.Split(header, ",") {
		if key, value, ok := strings.Cut(strings.TrimSpace(part), "="); ok {
			parts[key] = value
		}
	}
	timestamp, err := strconv.ParseInt(parts["t"], 10, 64)
	if err != nil {
		return false
	}
	if age := time.Now().Unix() - timestamp; age > 300 || age < -300 {
		return false
	}
	mac := hmac.New(sha256.New, secret)
	fmt.Fprintf(mac, "%d.%s", timestamp, body)
	given, err := hex.DecodeString(parts["v1"])
	return err == nil && hmac.Equal(mac.Sum(nil), given)
}

func main() {
	http.HandleFunc("/hooks/formfeed", func(w http.ResponseWriter, r *http.Request) {
		// the signature covers the bytes on the wire: read the raw body, never re-serialised JSON
		body, _ := io.ReadAll(r.Body)
		if !valid(r.Header.Get("Webhook-Signature"), body) {
			w.WriteHeader(http.StatusUnauthorized)
			return
		}

		var event struct {
			Type string `json:"type"`
			Data struct {
				ID          string `json:"id"`
				DownloadURL string `json:"download_url"`
			} `json:"data"`
		}
		json.Unmarshal(body, &event)
		switch event.Type {
		case "render.completed":
			fmt.Println("done", event.Data.ID, event.Data.DownloadURL)
		case "render.failed":
			fmt.Println("failed", event.Data.ID)
		}
		w.WriteHeader(http.StatusNoContent)
	})
	http.ListenAndServe(":8080", nil)
}

Start the receiver

FORMFEED_WEBHOOK_SECRET=… go run hook.go

Verifying in Go

• hmac.Equal. It compares the decoded signature bytes in constant time, which also makes upper-case hex harmless.
• Read once. io.ReadAll(r.Body) gives the exact bytes; decode the JSON from that slice afterwards, not from the request body a second time.

What arrives

A POST with Webhook-Signature: t=<unix seconds>,v1=<hex>, where the hex is HMAC-SHA256 over t, a dot and the raw body, keyed with your secret exactly as you have it, as UTF-8 bytes: the webhook_secret you sent, or a registered endpoint’s whole whsec_… string, prefix included. Every attempt is signed anew with its own t. Webhook-Id stays the same across retries, so it is your deduplication key. data is the render as GET /renders/{id} returns it:

{
  "id": "evt_5b0c8e6f2d4a4c3e9b1a7f6e5d4c3b2a",
  "type": "render.completed",
  "created_at": "2026-09-18T10:42:00Z",
  "workspace_id": "3a6f…",
  "data": {
    "id": "rnd_7da09ab6cd9946b798c187c2a9350852",
    "status": "succeeded",
    "download_url": "https://cdn.formfeed.dev/o/eu/…/document.pdf?exp=…&sig=…",
    "page_count": 2,
    "units": 1,
    …
  }
}

Retries, and what your endpoint owes

• Answer fast. A delivery succeeds with a 2xx within 10 seconds. Store the event and do the work afterwards.
• Retries. Anything else, a redirect included, is retried after 1 minute, 5 minutes, 30 minutes, 2 hours and 12 hours: six attempts in all, over about 14 hours and 40 minutes. The same event can therefore arrive twice: deduplicate by Webhook-Id.
• Without a secret. A webhook_url without webhook_secret is delivered unsigned. For signed deliveries of every render, register an endpoint once in the app or through POST /webhooks; it gets its own secret.
• Try it locally. formfeed listen --forward-to http://localhost:8080/hooks/formfeed relays the events of your test renders to your machine, signed with the session’s own secret rather than webhook_secret: start the receiver with that one, which formfeed listen --print-secret prints (run it before the session, since it ends a running one). formfeed trigger render.completed --template revenue-report makes a real test render whose event you watch arrive. The key needs the webhook:listen scope. See developing webhooks locally.

When the API says no

• One shape for every error. Errors are problem documents (RFC 9457) with type, title, status, detail, a stable code and the request_id to quote to support; the type URL leads to a page about that error. The list is in the error reference.
• The usual ones. 401 unauthorized for a missing or revoked key, 404 template_not_found for a wrong template name, 422 template_runtime_error when the template fails on the data, 429 rate_limited above the plan’s requests per second, 402 quota_exceeded when the Free plan’s units are used up (a paid plan bills extra units up to its hard cap).
• Retry safely. Send an Idempotency-Key header with the POST, and repeat the request with the same key when you do not know whether the first attempt arrived: for 24 hours the key returns the first answer instead of rendering and billing again. The answer is stored once the first attempt has been answered, so do not retry while it may still run; and problems below 500 are stored too, so a corrected request needs a new key.

The job around this program

This guide is one program. What the job needs around it, from the template to the limits and the price, is on the page Report PDF generation.

More Go guides

• Generate a PDF invoice from JSON in Go
• Create a ZUGFeRD / Factur-X e-invoice in Go
• Convert HTML to PDF in Go
• Generate Open Graph images in Go
• Generate PDFs from a CSV in Go
• Merge PDFs and add a password in Go

FAQ

Can I poll instead of receiving a webhook?

Yes: GET /renders/{id} until the status is succeeded or failed; the SDKs have waitFor and wait_for for it. A webhook saves the requests and reports sooner. See async and batch.

Why must the raw body be used?

The signature covers the bytes on the wire. Parsing and re-serialising JSON may reorder keys, change number formats or escape characters differently, and the HMAC of that text no longer matches.

What is the five-minute tolerance for?

It stops a captured delivery from being replayed later. It also means the clock of your server must be roughly right.

Async render with a webhook in other languages

• Node.js (TypeScript)
• Python
• PHP
• C# (.NET)
• Java
• Ruby