Micro-services is building style for building distributed systems. Each service is a small independent process. Using the messaging service the microservices can be decoupled from each other. This is so decoupled that it is not even necessary to use the same programming language for each service. That's why the API has to be language agnostic. A message broker facilitates this language agnostic requirement. The services are called microservices because they are typically small. They focus on doing one isolated task in a distributed system. Each service should be autonomous. Microservices should keep the code sharing to minimum and they should have there own databases which doesn't need to be of the same type. One can use the Relational database while for the other service a nosql database might be more suitable. And because the each service is autonomous the building of the distributed system is very modular. Each microservice can be build by a separate team and can be separately deployed.
There is an important question over here that if each microservice has its own database what about the data integrity. In microservice architecture each microservice maintains its own version of the entity with only the relevant data which is received with the message. For eg the registration service gets a new order from the user interface because it must be capable of producing the list of received orders later it stores the order in the database but it only stores the data necessary to produce the list. It than sends the message to the dispatcher and the finance service using a message broker. In the message it only keeps the data which the receiving service needs that service than processes the order and keeps the data of order it needs later. In this way the definition of order is different for each service. There is no single database which keeps all data that is to know about the order. The order data is distributed across the different microservices.
There is an important question over here that if each microservice has its own database what about the data integrity. In microservice architecture each microservice maintains its own version of the entity with only the relevant data which is received with the message. For eg the registration service gets a new order from the user interface because it must be capable of producing the list of received orders later it stores the order in the database but it only stores the data necessary to produce the list. It than sends the message to the dispatcher and the finance service using a message broker. In the message it only keeps the data which the receiving service needs that service than processes the order and keeps the data of order it needs later. In this way the definition of order is different for each service. There is no single database which keeps all data that is to know about the order. The order data is distributed across the different microservices.
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