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Use the 24-hour render cache to cut costs in Ruby with Net::HTTP

Every response carries an X-Cache header, and it is worth wiring into your logs on day one: HIT means the bytes came from the 24-hour cache — served in milliseconds and not counted against your quota. MISS means a real browser rendered the page and one render was metered.

Below is a complete, runnable Ruby program using Net::HTTP, no third-party dependency needed. It reads your API key from the SNAPDOK_KEY environment variable — free keys take about thirty seconds and need no card.

require "json"
require "net/http"
require "uri"

uri = URI("https://snapdok.io/v1/render")

res = Net::HTTP.start(uri.host, uri.port, use_ssl: true, read_timeout: 90) do |http|
  req = Net::HTTP::Post.new(uri)
  req["Authorization"] = "Bearer #{ENV.fetch("SNAPDOK_KEY")}"
  req["Content-Type"] = "application/json"
  req.body = JSON.generate({
    url: "https://your-app.com/report/42",
    format: "png",
    width: 1280,
})
  http.request(req)
end

raise "snapdok error #{res.code}: #{res.body}" unless res.is_a?(Net::HTTPSuccess)

File.binwrite("shot.png", res.body)
puts "#{res["X-Cache"]}  # HIT = free, MISS = rendered and metered"

The cache key is the normalised request body: URL plus every rendering parameter. Same URL at a different width, format or scale is a different entry; alias spellings (device_scale_factor vs device_scale) are folded together first, so they share an entry. Failed renders are never cached and never metered.

Two design consequences. First, idempotent retries are safe: a queue worker that re-fires the same job inside a day costs nothing extra. Second, if you need a fresh render of a page that just changed under the same URL, vary the request — the pragmatic trick is a throwaway query parameter on the target URL (?v=deploy-id), which changes the fingerprint and forces a MISS. Rate limiting still applies to HITs (it protects the endpoint, not the renderer), so keep an eye on X-RateLimit-Remaining in tight loops.

Same endpoint, one more trick: if the page you are rendering has a form on it, adding "pdf_forms": true to a PDF render brings it back with real, fillable AcroForm fields — a PDF people can type into, not a picture of one. How that works.

Notes for Net::HTTP

Stdlib only — no gems. The traps Net::HTTP is famous for, all handled in the sample: TLS is not automatic (set use_ssl: true or you will POST in the clear to port 80 and get a redirect); the default read_timeout of 60 s is close enough to real render times that raising it is wise; and the response body is a binary-ready String, but write it with File.binwrite — plain File.write on Windows would mangle line-ending bytes in the PDF. Check is_a?(Net::HTTPSuccess): Net::HTTP never raises on HTTP error statuses.

Response headers worth reading

Header	Meaning
`X-Cache`	HIT or MISS.
`X-Quota-Remaining`	Metered renders left this month.
`X-RateLimit-Remaining`	Requests left in the current second.

Use the 24-hour render cache to cut costs in Ruby with Net::HTTP

Notes for Net::HTTP

Response headers worth reading

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