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Set the GrabzIt callback URL to an absolute, publicly reachable URL for a server-side handler. In the REST API, pass it as callback; in a client library, use that library’s callback argument. GrabzIt calls the handler after the capture finishes, and your application can use the returned capture id to retrieve the result. A localhost or 127.0.0.1 URL cannot receive the callback.
What the callback URL does
A callback URL is the endpoint GrabzIt contacts after it has finished processing a screenshot or HTML conversion. It lets your server learn that an asynchronous capture is ready without keeping the original request open. The callback carries capture details, including an id that can be used to retrieve the result. See GrabzIt’s REST Screenshot and HTML Conversion API and its Node.js callback handler documentation.
The callback is a server-to-server request. Keep your GrabzIt Application Key on your server; calling the REST API from browser-side code would expose it. GrabzIt also documents IP authorization to restrict which servers can access the API. See the REST API reference.
Set up a public callback endpoint
- Create a handler route. Deploy a server-side route with a stable URL, for example
https://example.com/grabzit/callback. It must be absolute and reachable from the public internet. - Pass that URL when you request a capture. For the REST API, provide the handler URL in the
callbackparameter. URL-encode parameter values. For a client library, use the callback parameter expected by that language’s documented method. - Read and validate the callback data. The official Node.js and Java handler documentation lists values including
id,filename,message,customId,format, andtargeterror. Useidto retrieve the capture; inspectmessageandtargeterrorwhen diagnosing an unsuccessful result. See the Java callback handler documentation and Node.js callback handler documentation. - Correlate the request if needed. Supply a
customidwith the REST request when you need to associate a callback with an application record. GrabzIt returns it ascustomIdwith the callback. Account for the difference in casing between the REST request parameter and documented callback field. - Retrieve and store the result. Use the callback’s capture ID with the result-retrieval method for the API or client library you chose, then update your own job state and make the image or PDF available to the part of your application that needs it.
Choose asynchronous callbacks or synchronous saving
| Approach | Endpoint requirement | Completion behavior | When it fits |
|---|---|---|---|
| Asynchronous callback | Requires an absolute, publicly reachable handler URL. | GrabzIt calls the handler after the capture completes; use the returned ID to retrieve the result. | Use when your application can process completion later and track a job. |
Synchronous SaveTo/save_to |
Does not require a public callback endpoint. | The documented library method saves synchronously instead of notifying a callback handler. | Use for local development or workflows that can wait for the save operation. |
GrabzIt documents PHP SaveTo for localhost and Node.js save_to as synchronous alternatives. Method names and argument casing differ among client libraries, so follow the documentation for your selected language rather than copying a method signature from another SDK. See the PHP API and Node.js technical documentation.
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Handle asynchronous display in a web page
A callback arrives after capture generation, not as part of the page’s initial request. If a user starts a capture and expects to see it in the same interface, record a unique correlation value such as customId, return a pending state to the browser, and let the browser check your own server for readiness. Once the callback has been processed and the result is available, your server can return the result URL or other display data. GrabzIt describes this callback-and-readiness pattern in its guide to displaying a screenshot with a callback handler.
Test the callback handler
- Make sure a capture already exists.
- Open GrabzIt Diagnostics and select a capture in the Out column.
- Choose Send to Callback Handler, enter the public handler URL, and optionally provide fields such as a Custom ID.
- Send the test, then check your server logs and job record to confirm the handler received and processed the callback.
This is GrabzIt’s documented callback test flow; it tests a callback for an existing capture, rather than creating a new capture as part of the test. See How to test a Callback Handler?.
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Troubleshoot callback URL failures
“You are trying to use a Callback URL that does not exist!”
Check that the URL is absolute, correctly spelled, deployed, and reachable from outside your network. A local development address such as localhost or 127.0.0.1 is not a valid public callback host. If a new domain has not propagated, GrabzIt’s troubleshooting guidance suggests temporarily using the server IP. See Callback URL troubleshooting.
The callback arrives but the application cannot match it to a request
Store a correlation value when creating the capture and send it as customid where supported. On callback, read customId and associate it with the stored job. Use the capture id for retrieving the result; do not treat the correlation value as a replacement for that ID.
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The page remains empty after the capture request
Do not expect the screenshot to be ready in the initial request when using an asynchronous callback. Keep the browser in a pending state, expose a readiness check through your application, and show the result only after the callback has been processed.
The callback indicates an error or no usable result appears
Inspect the documented callback fields, especially message and targeterror, and confirm that your handler uses the returned id with the correct result-retrieval method. For local work without a public handler, use the documented synchronous method for your language instead.
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