Not sure if my question is explainable enough but I will try to explain it here as much as I can. I am currently exploring and playing with a microservices architecture, to see how it works and learn more. Mostly I understand how things work, what is the role of API Gateway in this architecture, etc...
So I have one more theoretical question. For example, imagine there are two services: an event (which manages possible events) and a service ticket which manages tickets related to a specific event (there could be many tickets). So these 2 services really depend on each other but they have a separated database, are completely isolated and loosely coupled, just as it should be in "ideal" microservices environment.
Now imagine I want to fetch event and all tickets related to that event and display it in a mobile application or web spa application or whatever. Is calling multiple services / URLs to fetch data and output to UI completely okay in this scenario? Or is there a better way to fetch and aggregate this data?
From my reading of different sources calling one service from another service adds latency, making services depend on each other, future changes in one service will break another one, etc. so it's not a great idea at all.
I'm sorry if I am repeating a question and it was already asked somewhere (although I could not find it), but I need an opinion from someone who met this question earlier and can explain the flow here in a better way.
Is calling multiple services / URLs to fetch data and output to UI completely okay in this scenario? Or is there a better way to fetch and aggregate this data.
Yes it is ok to call multiple services from your UI and aggregate the data in your Fronted code for your needs. Actually in this case you would call 2 Rest API's to get the data from ticket micro-service and event micro-service.
Another option is that you have some Views/Read optimized micro-service which would aggregate data from both micro-services and serve as a Read-only micro-service. This of course involves some latency considerations and other things. For example this approach can be used if you have requirement like a View which consists of multiple of models(something like a Denormalized view) which will be accessed a lot and/or have some complex filter options as well. In this approach you would have a Third micro-service which would be aggregated from the data of your 2 micro-services(tickets and events). This micro-services would be optimized for reading and could also if needed use a different storage type(Document db or similar). For your case if you would decide to do this you could have only one API call to this micro-service which will provide you all your data.
Calling One micro-service from another. In some cases you can not really avoid this. Even though there are some sources online which would tell you not to do it sometimes it is inevitable. For your example I would not recommend this approach as it would produce coupling and unnecessary latency which can be avoided with other approaches.
Background info:
You can read this answer where the topic is about if it is ok to call one micro-service from another micro-service. For your particular case it is not a good option but for some cases it might be. So read it for some clarification.
Summary:
I have worked with system where we where doing all those 3 things. It really depends on your business scenario and needs of your application. Which approach to pick will depend on a couple of criteria like: usability from UI, scaling(if you have high demand on the micro-services you could consider adding a Third micro-service which could aggregate data from tickets and events micro-service), domain coupling. For your case I would suggest option 1 or option 2 (if you have a high demanding UI) from user prospective. For some cases option 1 is enough and having a third micro-service would be an overkill, but sometimes this is an option as well.