Cancellation & Timeouts

There may be cases where you want to cancel a call, because you are no longer interested in the response. For example, you want to show search results as soon as the user starts typing, but when the user enters another term, you want to cancel the previous call. Let's take a look at cancellation, and the complementary topic of timeouts.

Cancellation

To cancel a call, you can pass an AbortSignal to every client method. If the signal triggers, the network call is cancelled, and the response promise for your call is rejected:

import { createClient, ConnectError, Code } from "@connectrpc/connect";

const client = createClient(ElizaService, ...);

// For the sake of this example, let's cancel the request right away
const abort = new AbortController();
abort.abort();

try {
  await client.say({sentence: "I feel..."}, { signal: abort.signal });
} catch (err) {
  if (err instanceof ConnectError && err.code != Code.Canceled) {
    // handle the genuine error, ignoring cancelled requests
  }
}

Note that our client raises a ConnectError with code Canceled, instead of a DOMException that fetch() would raise.

Timeouts

Similar to the signal option for cancellation, there is also the timeoutMs option, which allows you to specify a timeout, in milliseconds, for an individual call. Here is an example:

try {
  const req = { name: "Joe" };
  // If this call takes more than 200 milliseconds, it is canceled
  for await (const res of client.introduce(req, { timeoutMs: 200 })) {
    console.log(res.sentence);
  }
} catch (err) {
  // Note that our client raises a ConnectError with Code.DeadlineExceeded
  if (err instanceof ConnectError && err.code === Code.DeadlineExceeded) {
    // handle the timeout error
  }
}

When a timeout is reached before the request finishes, a ConnectError with code DeadlineExceeded is raised.

When you specify a timeout, the value is sent to the server in a request header that is understood by Connect, gRPC, and gRPC-web servers. Servers and clients honor the timeout, which can be immensely helpful for streaming calls in a fragile network environment. Timeouts can also be propagated to upstream services. In gRPC, the concept is also known as deadlines.