Microservices intra-communication: using REST or Messaging ?

[Claude Moore]

In my humble experience, I've found that microservices intra-communication, in practice, is usually implemented in two different flavours: a) using REST  and b) using asynchronous messaging.
I wonder which is the best approach, but I cannot find an ultimate answer to this dilemma. Reading pros and cons, I'm  starting to think that the best approach is using messaging, mainly because it decouples at the most level microservices. You send a message, some other service will respond. On the other hand, looks like that Spring Boot first choice is to use a REST based approach, just because a number of example I've seen on the web are all about using Eureka (and service discovery),
Feign and so on; nevertheless, I've the feeling that using REST at the very end force you to work synchronously and induces a deeper coupling between services.
So, at the very end, I'm really confused.
What is your opinion about ?

[Claude Moore]

After furher investigation, I came to the conclusion that the most likely answer is " use both ", I mean both REST and messaging. I ended with this (mental) schema:
- use REST for synchronous communications: mainly for microservices exposing APIs towards front-ends (aka 'backend for frontend' ms), when they need to exchange information with most inner microservices;
- use messaging (with orchestration / choreography) for all asynchronous interactions.

[Tim Moores]

That sounds about right. Microservices should indeed be "micro" - come quickly to a result, and return it synchronously. If a quick result can't be guaranteed, well - you can still implement it as a microservice (so the client doesn't need to bother with other access methods), but its "result" would be more like "yep, I've queued it, check back later for a result".

[Claude Moore]

Tim Moores wrote:That sounds about right. Microservices should indeed be "micro" - come quickly to a result, and return it synchronously. If a quick result can't be guaranteed, well - you can still implement it as a microservice (so the client doesn't need to bother with other access methods), but its "result" would be more like "yep, I've queued it, check back later for a result".

Thanks for your feedback !
The more I study MSA (microservices architecture), the more I believe that the adjective "micro" is greatly misleading. At the very end a Microservices is an independent  business logic unit which must be able to get engaged in really complex scenarios.
Despite the fact a single MS  must be as small as necessary to just "do a single thing well",  interactions between microservices, data exchange and so on is really a though matter.