A screenshot or PDF webhook is an HTTP POST that a rendering provider sends to your public endpoint after a job finishes. To receive events reliably, submit the request with the provider’s webhook_url (and async=true when required), expose an HTTPS endpoint that accepts POST, verify the provider’s HMAC-SHA256 signature against the raw request body, acknowledge quickly with a 2xx response, and process the provider-specific payload idempotently.
The exact parameter names, signature header, payload fields, file-retention rules and current availability differ by provider. The workflow below covers ScreenshotOne, ScreenshotMAX, Screenshot API and Doppio patterns without assuming that one provider’s payload can be used for another.
How the callback flow works
- Your application sends a screenshot or PDF render request and includes a callback URL such as
webhook_url. Doppio places its callback configuration underdoppio.webhook. - If the API supports asynchronous execution, add its documented async flag. ScreenshotOne and ScreenshotMAX document
async=true; ScreenshotMAX returns202 Acceptedwhile rendering continues. - The provider renders the page or PDF in the background.
- When the job completes, the provider sends an HTTP
POSTto your callback URL. The body and result fields are provider-specific. - Your endpoint validates the signature, stores or queues the event, returns a fast
2xxresponse, and performs slower work outside the request thread.
A webhook is a notification, not a live connection. Your endpoint must already be reachable from the public internet when the provider attempts delivery.
Requirements for a production webhook endpoint
Public HTTP reachability
Use a public HTTPS URL, for example https://example.com/webhooks/render. ScreenshotMAX states that the URL must be publicly accessible over HTTPS or HTTP, accept POST, and return a 2xx status. Prefer HTTPS so the payload and signature cannot be observed in transit. Localhost addresses and private network hostnames will not work unless you expose them through a secure tunnel or deploy the receiver.
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Fast acknowledgement
Read and authenticate the request, durably enqueue the event, and respond with 200 or another appropriate 2xx quickly. Do not wait for image downloads, PDF conversion, database-heavy work or downstream API calls before acknowledging. A timeout or non-2xx response can cause retries, duplicate events or a delivery marked as failed.
Raw-body access
Signature verification requires the exact bytes sent by the provider. Capture the raw body before JSON parsing, whitespace normalization or re-serialization. A parsed-and-stringified object can produce a different byte sequence and therefore a different HMAC.
Secret management
Store the webhook secret in an environment variable or secret manager. Never put it in source control, a URL, a client-side bundle or logs. Use a separate secret for each environment when the provider allows it.
Node.js receiver with HMAC verification
The following Express endpoint demonstrates the essential order: obtain raw bytes, read the provider’s signature header, compute HMAC-SHA256, compare in constant time, deduplicate by the provider’s identifier, enqueue the event, and acknowledge. Replace the header name and signing-string construction with the exact rules in your provider’s documentation; providers use different signature headers and canonical formats.
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import crypto from 'node:crypto';
const app = express();
const secret = process.env.WEBHOOK_SECRET;
if (!secret) throw new Error('WEBHOOK_SECRET is required');
// Keep this route raw. Do not run express.json() before it.
app.post('/webhooks/render', express.raw({ type: '*/*', limit: '2mb' }), async (req, res) => {
const raw = req.body; // Buffer, unchanged
const supplied = req.get('X-Provider-Signature'); // Use your provider's header
if (!supplied) return res.status(401).send('missing signature');
// Adapt the message format (raw, timestamp.raw, prefix + raw, etc.)
// to the provider's documented HMAC-SHA256 scheme.
const expected = crypto.createHmac('sha256', secret).update(raw).digest('hex');
const a = Buffer.from(supplied, 'utf8');
const b = Buffer.from(expected, 'utf8');
if (a.length !== b.length || !crypto.timingSafeEqual(a, b)) {
return res.status(401).send('invalid signature');
}
let event;
try {
event = JSON.parse(raw.toString('utf8'));
} catch {
return res.status(400).send('invalid JSON');
}
// Choose the identifier documented by your provider.
const eventId = event.id ?? event.render_id ?? event.reference;
if (!eventId) return res.status(400).send('missing event identifier');
// Replace with an atomic INSERT/queue operation with a unique eventId.
// If it already exists, acknowledge it without processing twice.
await enqueueIfNew({ eventId, event });
return res.sendStatus(200);
});
app.listen(process.env.PORT || 3000, () => console.log('webhook receiver ready'));In a real deployment, make enqueueIfNew an atomic database insert or queue operation. The unique key prevents a retry from generating a second screenshot record, email or billing action.
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Python receiver example
Flask must also preserve the raw bytes. Configure your WSGI server and reverse proxy to pass the request body unchanged.
import hashlib
import hmac
import json
import os
from flask import Flask, request, abort
app = Flask(__name__)
SECRET = os.environ["WEBHOOK_SECRET"].encode()
@app.post("/webhooks/render")
def render_webhook():
raw = request.get_data(cache=False, as_text=False)
supplied = request.headers.get("X-Provider-Signature")
if not supplied:
abort(401)
expected = hmac.new(SECRET, raw, hashlib.sha256).hexdigest()
if not hmac.compare_digest(supplied, expected):
abort(401)
try:
event = json.loads(raw)
except ValueError:
abort(400)
event_id = event.get("id") or event.get("render_id") or event.get("reference")
if not event_id:
abort(400)
# Atomically persist or enqueue event_id and event here.
enqueue_if_new(event_id, event)
return ("", 204)Do not replace request.get_data() with a parsed JSON object before computing the digest. As with Node, substitute the provider’s actual signature header and canonical message format.
Submitting an asynchronous render request
Generic cURL shape
Use the provider’s documented endpoint and authentication, then include the callback and async parameters. The names below reflect the documented ScreenshotOne and ScreenshotMAX pattern; consult the selected provider for its exact endpoint and required fields.
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curl -G 'https://api.example.test/render'
-H 'Authorization: Bearer YOUR_API_KEY'
--data-urlencode 'url=https://example.com'
--data-urlencode 'webhook_url=https://your-domain.example/webhooks/render'
--data-urlencode 'async=true'ScreenshotOne documents webhook_url and async=true; ScreenshotMAX documents the same callback parameter and a 202 Accepted asynchronous response. Doppio’s example nests the callback under its doppio.webhook configuration instead of using this flat shape.
Python request pattern
import requests
payload = {
"url": "https://example.com",
"webhook_url": "https://your-domain.example/webhooks/render",
"async": "true",
}
r = requests.post(
"https://api.example.test/render",
headers={"Authorization": "Bearer YOUR_API_KEY"},
json=payload,
timeout=30,
)
r.raise_for_status()
print(r.status_code, r.text)Node.js request pattern
const response = await fetch('https://api.example.test/render', {
method: 'POST',
headers: {
'Authorization': 'Bearer YOUR_API_KEY',
'Content-Type': 'application/json'
},
body: JSON.stringify({
url: 'https://example.com',
webhook_url: 'https://your-domain.example/webhooks/render',
async: true
})
});
if (!response.ok) throw new Error(`${response.status}: ${await response.text()}`);Verify signatures correctly
- Read the raw request bytes.
- Read the signature header specified by the provider.
- Construct the exact signing message specified by that provider. Some schemes sign only the body; others include a timestamp or prefix.
- Compute HMAC-SHA256 with the webhook secret.
- Compare using a constant-time comparison function.
- Reject missing, malformed or mismatched signatures with
401(or the status required by the provider). - Only after verification parse JSON and act on the event.
ScreenshotOne, ScreenshotMAX and Screenshot API all document HMAC-SHA256 authentication, but their headers and signing details are provider-specific. Do not assume that a signature generated for one service can validate another. If a timestamp is included, enforce a short freshness window to reduce replay risk and retain the event identifier for deduplication.
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Payloads, files and retention
Design your parser around the provider you actually use. ScreenshotOne shows a screenshot_url and a storage location. ScreenshotMAX shows id, file, expires and created. Screenshot API shows render_id, success, a URL, content type, timing, size, error and timestamp. Treat unknown fields as optional so a provider can add fields without breaking your receiver.
If the payload contains an expiration time, fetch or copy the file before it expires. ScreenshotMAX’s expires field makes that deadline explicit. A storage location supplied by ScreenshotOne may be preferable to downloading through the callback request itself. Never assume a result URL is permanent; persist the bytes or move them to storage you control when your retention policy requires it.
Idempotency, retries and ordering
Make delivery safe to repeat
Providers can retry after a timeout, network failure or non-2xx response. Use the documented request, render or event identifier as a unique database key. ScreenshotMAX exposes id, Screenshot API exposes render_id, and ScreenshotOne exposes a render/reference concept. If an identifier is absent, derive a carefully scoped idempotency key from the provider’s documented fields rather than from the entire mutable payload.
Acknowledge before long work
Persist the verified event, return 204 or 200, and let a worker download the file, update your application and notify users. If processing fails later, retry the job internally without forcing the provider to resend the callback.
Do not depend on delivery order
Store event timestamps and job state. A completion event may arrive after a retry or after another event for the same render. Apply state transitions conditionally and keep the raw verified payload for diagnosis.
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Deployment and security checklist
- Expose a stable public HTTPS route with a valid certificate.
- Allow inbound
POSTtraffic at the proxy, load balancer and application firewall. - Set a request-size limit that accommodates the provider’s JSON body.
- Preserve the body bytes and relevant headers through every proxy.
- Keep the callback secret outside source control and rotate it according to your provider’s procedure.
- Log request IDs, verification outcomes and processing latency, but redact secrets and sensitive page data.
- Use a queue or durable database before returning success.
- Implement deduplication with a unique constraint.
- Restrict accepted methods to
POSTand reject unexpected content types when appropriate. - Apply rate limits and maximum body sizes without blocking legitimate provider retries.
Troubleshooting webhook failures
The provider cannot connect
Check DNS, the TLS certificate chain, firewall rules, proxy routing and whether the URL is publicly reachable rather than bound to localhost or a private address. Confirm that the callback path and method are correct.
The provider reports a non-2xx response
Inspect application and proxy logs for the status code. Common causes are authentication middleware applied to the webhook route, a missing signature header, JSON parsing before raw-body capture, or an exception during slow processing. Authenticate and enqueue first, then return a quick success.
Every signature is invalid
Confirm the correct secret, header name, encoding and canonical message. Compare the digest against the untouched raw bytes, not parsed JSON. Check whether the provider prefixes the digest (for example, a scheme label) or includes a timestamp. Do not invent a different HMAC input.
Events are processed twice
Retries are normal. Add a unique constraint for the provider’s event, render or request identifier and treat an existing key as an already accepted event.
The file URL no longer works
Inspect expiration metadata and download the file promptly. ScreenshotMAX includes expires; other providers may expose storage locations or different retention behavior. Move required files into your own durable storage.
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Async callbacks return 503
Availability can depend on the deployment. Screenshot API’s current documentation says async callbacks return 503 without charging a credit on the cited deployment, despite documenting the intended callback protocol. Do not build a production dependency on that deployment until its status changes; use a supported synchronous or polling workflow, or another provider.
Choosing a provider for webhook-based rendering
Compare the callback parameter, whether async mode is explicit, acknowledgement rules, signature header and secret handling, payload identifiers, file retention and current deployment status. Doppio’s official example is concise for teams that want an asynchronous screenshot endpoint with a POST callback. ScreenshotOne and ScreenshotMAX document the complete callback workflow, including HMAC verification and asynchronous execution. Screenshot API documents the protocol but currently reports the deployment-specific 503 limitation described above.
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Operational testing before launch
- Deploy the endpoint to a public staging URL.
- Send a test render with a unique identifier.
- Confirm the proxy preserves the body and signature header.
- Verify that an invalid signature is rejected and creates no job.
- Return a forced 500 once and confirm your retry/deduplication behavior.
- Test an expired or unavailable result URL and ensure the worker reports it without blocking acknowledgements.
- Check logs and metrics for delivery latency, verification failures, duplicate events and queue depth.
Frequently Asked Questions
Can a webhook endpoint be localhost?
Not directly. The provider must reach it from the public internet; deploy it publicly or use a secure tunnel for development.
Should I parse JSON before checking the signature?
No. Capture the raw request body, verify the provider’s HMAC-SHA256 signature, and parse JSON only after verification.
What status should the endpoint return?
Return a quick 2xx after the verified event is durably accepted or queued. The provider’s documentation may specify 200, 202 or another 2xx.
Are webhook payload fields interchangeable between providers?
No. ScreenshotOne, ScreenshotMAX and Screenshot API expose different identifiers and result fields, so implement a provider-specific adapter.
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