Convert Markdown to PDF 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.
• A Markdown file. report.md next to the program, up to 1 MB. A sample is further down.

The program

The program reads report.md, posts it as markdown with bookmarks from its headings, waits for the result when the API answers 202, stops with the API’s message when something is wrong, and saves the PDF.

require "json"
require "net/http"

body = {
  markdown: File.read("report.md"),
  settings: {
    pdf: { outline: true },
  },
}
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)

# not done within 60 seconds: the answer is 202 and the render goes on in the background;
# its result answers the same way until the render has finished
while response.code == "202"
  sleep 2
  response = Net::HTTP.get_response(URI("https://api-eu.formfeed.dev/v1/renders/#{render["id"]}/result"), auth)
  render = JSON.parse(response.body)
end

# 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)

File.binwrite("report.pdf", Net::HTTP.get(URI(render["download_url"])))
puts "#{render["id"]} #{render["page_count"]} page(s), #{render["units"]} unit(s)"

Run it

It prints the render’s id, the page count and the units it cost, and report.pdf lies next to the program.

export FORMFEED_API_KEY=ff_test_...
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.

A Markdown file with a title

The front matter gives the PDF its title and author; without one the first heading is the title. A line \newpage starts a new page, and a link to a heading works inside the PDF:

---
title: Release notes 2.4
author: Fennlor Studio GmbH
---

# Release notes 2.4

## What is new

- Faster previews, see [the numbers](#numbers)
- Footnotes in the export[^1]
- [x] Tested on every plan

## Numbers

| Step    | Before | After  |
| :------ | -----: | -----: |
| Preview | 840 ms | 310 ms |
| Export  | 2.1 s  | 1.4 s  |

\newpage

## Thanks

To everyone who reported a problem.

[^1]: From version 2.4 on.

What the settings control

• Look. settings.markdown.theme is document, which styles the text, in the brand kit’s fonts and colour where the workspace has one, or none; settings.markdown.css comes after it.
• Raw HTML. Shown as text by default. settings.markdown.html: "allow" writes it as it stands, for Markdown you wrote yourself, never for text from your users or a model.
• Line breaks. settings.markdown.breaks: true makes a single line break a line break, for text from a chat.
• Paper, header and footer. paper, margin, header.html and footer.html work as for HTML; elements with the classes pageNumber and totalPages show the page numbers.

Placeholders in the Markdown

Add "engine": "jinja2" (or liquid, handlebars) and a data object, and {{ … }} in the text is filled before it is read as Markdown, with all helpers. The Markdown guide has the rest.

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 Markdown to PDF API.

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