Checking if an External Server is Running in Node.js

At Rivet, our testing process involves external services such as our PostgreSQL, ElasticSearch and sometimes an API to mock third-party services. Each test specifies which services it depends on, we then start those services and run a health check to see if they’re running before the test begins.

Some services have a nice way to check if they’re running. PostgreSQL has pg_isready with a timeout parameter and check for a zero exit code and ElasticSearch has the /_cluster/health which we can check for a status code of 200. However, sometimes a service doesn’t provide a built-in way to check for the health and/or status of a service. An alternative way is to see if the server itself is ready to accept connections with a socket:

const checkService = async (host: number, port: string, timeout: number) => {
  const promise = new Promise<string | null>((res, rej) => {
    const socket = new net.Socket();

    const onError = (msg: string) => () => {
      socket.destroy();
      rej(msg);
    };

    socket.setTimeout(timeout);
    socket.once('error', err => onError(String(err))());
    socket.once('timeout', onError('Connection timeout'));

    socket.connect(port, host, () => {
      socket.end();
      // In this situation, a successful connection is null
      // "No news is good news" type of thing
      res(null); 
    });
  });

  try {
    return await promise;
  } catch (e: any) {
    return e;
  }
}

A somewhat annoying aspect of JavaScript are event listeners. They offer some convenience, where you can setup a socket and then add listeners to handle different events. In this case, they didn’t allow us to “await” until a connection was established, we had to set up a callback. Additionally, we can’t directly return from a callback. For instance, in the socket.connect callback, we can return a value, but it won’t be picked up by the outer function since it will be out of scope. We could add a variable outside of the callback and update it in the callback, but that introduces side effects. The nice thing with Promises is that they provide callback functions in an “on success” and “on error” situation.

This code is heavily inspired from the is-port-reachable npm package.