hydra-router | service aware router for Hydra Services | Microservice library

It's impossible to give a definite answer to 2 since that is completely up to what the application does, there's no silver bullet configuration that you can apply.

This leaves the first point, which is around something called zero downtime.

So, in the context of having multiple servers returning content to users, e.g. http servers I guess it's fair to say that most production environments have something at the front that's not related to the business logic. This could be a load balancer (comes in many shapes and forms) or a reverse proxy. This is usually the spot where you point your DNS A record. This server should basically never be down.

Now, lets assume you have changed some business logic and want to deploy a new backend. What you normally do is to swap out the already running processes behind the load balancer (or reverse proxy), one by one. So if you have five node processes, you stop one, start up a new with the updated code, and repeat until all running have been swapped out.

You can also utilize this to swap out just one, run tests on that one, then proceed to swap out the rest.

To really make sure you don't disrupt any users, you should stop accepting new http requests on the old processes, so new http requests are routed to the updated processes. This will allow for http requests that are taking place to finish up. Then you stop the old processes.

Hopes this helps.