Context (ctx)

The Context API provides methods to manage the lifecycle of your Worker or Durable Object.

Context is exposed via the following places:

• As the third parameter in all handlers, including the fetch() handler. (fetch(request, env, ctx))
• As a class property of the WorkerEntrypoint class

waitUntil

ctx.waitUntil() extends the lifetime of your Worker, allowing you to perform work without blocking returning a response, and that may continue after a response is returned. It accepts a Promise, which the Workers runtime will continue executing, even after a response has been returned by the Worker’s handler.

waitUntil is commonly used to:

• Fire off events to external analytics providers. (note that when you use Workers Analytics Engine, you do not need to use waitUntil)
• Put items into cache using the Cache API

Alternatives to waitUntil

If you are using waitUntil() to emit logs or exceptions, we recommend using Tail Workers instead. Even if your Worker throws an uncaught exception, the Tail Worker will execute, ensuring that you can emit logs or exceptions regardless of the Worker’s invocation status.

Cloudflare Queues is purpose-built for performing work out-of-band, without blocking returning a response back to the client Worker.

You can call waitUntil() multiple times. Similar to Promise.allSettled, even if a promise passed to one waitUntil call is rejected, promises passed to other waitUntil() calls will still continue to execute.

For example:

export default {
  async fetch(request, env, ctx) {
    // Forward / proxy original request
    let res = await fetch(request);

    // Add custom header(s)
    res = new Response(res.body, res);
    res.headers.set('x-foo', 'bar');

    // Cache the response
    // NOTE: Does NOT block / wait
    ctx.waitUntil(caches.default.put(request, res.clone()));

    // Done
    return res;
  },
};

passThroughOnException

Reuse of body

The Workers Runtime uses streaming for request and response bodies. It does not buffer the body. Hence, if an exception occurs after the body has been consumed, passThroughOnException() cannot send the body again.

If this causes issues, we recommend cloning the request body and handling exceptions in code. This will protect against uncaught code exceptions. However some exception times such as exceed CPU or memory limits will not be mitigated.

The passThroughOnException method allows a Worker to fail open ↗, and pass a request through to an origin server when a Worker throws an unhandled exception. This can be useful when using Workers as a layer in front of an existing service, allowing the service behind the Worker to handle any unexpected error cases that arise in your Worker.

export default {
  async fetch(request, env, ctx) {
    // Proxy to origin on unhandled/uncaught exceptions
    ctx.passThroughOnException();
    throw new Error('Oops');
  },
};