I've build an iOS and Android app with a nodejs backend server that is now in production, but as I'm new to this I have no idea on how to handle update of server and apps.
First of all, how am I supposed to update the nodejs server without downtimes?
Second, let's suppose I have a chat on my app and for some reasons I have to change it but the change is not compatible with the previous versions, how am I supposed to act?
I think the question is not entirely clear, but I have no idea on what to search on google to point me in the right direction, anything would be helpfull
updating your backend server is a pain indeed. you can't really do that without downtime at all. what you can do though, assuming your clients access your server with a domain rather than with a plain IP address, is prepare another server with an as-up-to-date data as possible and do a DNS record update to redirect the data to it. keep in mind that DNS has a long update time in which some clients get to the old server and some to the new one (which means a big headache if data consistency is important to you)
changing the API is another pain. often times you need to support older versions of your application in parallel to the newer ones. most app stores will let you know the statistics of your app versions and when it's safe to drop support for an old version.
a common practice though is you have the API endpoints versioned so that version 1 of you app accesses URL/API/v1/... and version to accesses URL/API/v2/... which enables you sending different replies based on your client version. you increase the version every since you make a breaking change to the protocol. this makes a "future compatible" protocol
at some cases you initially add a mechanism that lets the server send a message to an old version of the client saying that their version is obsolete and they need to update...
most big apps already has such mechanism while most small apps just take the risk of some downtime and drop support for a few non-updated clients...