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Callback

A callback is a mechanism for handling the result of an asynchronous operation. In programming, it refers to a function passed as an argument for later invocation. In web architectures, it refers to a URL that receives an HTTP request upon event completion.

Callbacks in Programming

// Function callback
fetchData(url, function(error, result) {
  if (error) handleError(error);
  else processResult(result);
});

// Modern async/await (still callbacks under the hood)
try {
  const result = await fetchData(url);
  processResult(result);
} catch (error) {
  handleError(error);
}

Callback URLs in Web Services

In task orchestration and webhook systems, a callback URL is an HTTP endpoint that receives a request when an operation completes:

  1. Your application creates a task with a callback URL
  2. The task orchestrator executes the work
  3. When complete, it sends an HTTP POST to your callback URL with the result

This pattern is essential for long-running operations that cannot hold a connection open.

Callbacks in AsyncQueue

AsyncQueue’s core feature is callback-based task orchestration:

await asyncqueue.tasks.create({
  callbackUrl: "https://api.example.com/on-complete",
  payload: { orderId: "12345" },
  retries: 3
});
// Your app continues immediately
// AsyncQueue calls your URL when the task completes

The callback URL receives the task result, status, and metadata via HTTP POST. AsyncQueue retries automatically if delivery fails.