Async PDF rendering with a verified webhook in Ruby

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.
• Ruby. You need Ruby 3; the program uses net/http and json from the standard library.
• 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.

require "json"
require "net/http"

data = JSON.parse(File.read("data.json"))
body = {
  template: "revenue-report",
  data: data,
  mode: "async",
  webhook_url: "https://example.com/hooks/formfeed",
  webhook_secret: ENV.fetch("FORMFEED_WEBHOOK_SECRET"),
}
auth = { "Authorization" => "Bearer #{ENV.fetch("FORMFEED_API_KEY")}" }

response = Net::HTTP.post(
  URI("https://api-eu.formfeed.dev/v1/renders"),
  JSON.generate(body),
  auth.merge("Content-Type" => "application/json")
)
render = JSON.parse(response.body)

# every error is a problem document (RFC 9457) with a title and a detail
abort "#{render["title"]}: #{render["detail"]}" unless response.is_a?(Net::HTTPSuccess)

puts "#{render["id"]} #{render["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)
ruby render.rb

What is particular about Ruby

• Hashes become JSON. Symbol keys are fine: JSON.generate writes them as strings, and the API’s snake_case names are idiomatic Ruby anyway.
• abort with the problem. A problem document carries a title and a detail; the program prints both and exits with status 1.
• In Rails. Put the call in a job (Active Job, Sidekiq), not in the request cycle: a synchronous render answers when the PDF is done, which can take up to 60 seconds before the API answers 202 and finishes 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.

# gem install sinatra rackup puma && ruby hook.rb
require "json"
require "openssl"
require "sinatra"

set :bind, "0.0.0.0"
set :port, 8080
SECRET = ENV.fetch("FORMFEED_WEBHOOK_SECRET")

post "/hooks/formfeed" do
  # the signature covers the bytes on the wire: read the raw body, never re-serialised JSON
  body = request.body.read

  # Webhook-Signature: t=<unix seconds>,v1=<hex HMAC-SHA256 of "t.body">
  parts = request.env.fetch("HTTP_WEBHOOK_SIGNATURE", "").split(",").select { |part| part.include?("=") }
  signature = parts.to_h { |part| part.strip.split("=", 2) }
  timestamp = signature["t"].to_i
  expected = OpenSSL::HMAC.hexdigest("SHA256", SECRET, "#{timestamp}.#{body}")
  given = signature["v1"].to_s.downcase

  fresh = (Time.now.to_i - timestamp).abs <= 300
  halt 401 unless fresh && given.bytesize == expected.bytesize && OpenSSL.fixed_length_secure_compare(expected, given)

  event = JSON.parse(body)
  case event["type"]
  when "render.completed" then puts "done #{event["data"]["id"]} #{event["data"]["download_url"]}"
  when "render.failed" then puts "failed #{event["data"]["id"]}"
  end
  status 204
end

Start the receiver

gem install sinatra rackup puma
FORMFEED_WEBHOOK_SECRET=… ruby hook.rb

Verifying in Ruby

• Constant time. OpenSSL.fixed_length_secure_compare needs equal lengths, hence the bytesize check before it. In Rails, ActiveSupport::SecurityUtils.secure_compare does both.
• Raw body in Rails. Use request.raw_post. params is the parsed JSON, which no longer has the bytes the signature covers.

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 Ruby guides

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

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
• Go
• C# (.NET)
• Java