Why am I getting "Fatal Exception Unreachable Hosts"?

This error will come up if the credentials you are using (the Application ID and the API key) are incorrect.  Therefore, we first recommend checking that the credentials you are using are correct. 

Then, we recommend reviewing whether the application you are trying to connect to exists. If it does exist, you can verify the status of your cluster by going to our status page here.

If the application you are trying to connect to exists and the status page does not indicate issues on your cluster, it is likely to be a DNS or TCP issue. This is the case regardless of the application you are using. 

For backend indexing you can increase the ulimit size or pool the TCP connections. 

When ulimit is not an option, http agent or http pooling (or a combination of ulimit + http agent) is the best next solution.

For nodejs UV_THREADPOOL_SIZE is useful.