Motivation:
You need to decide whether to use microservice architecture for a new system or for modernizing a legacy system.
Suggestions:
A. Why select microservice architecture for your new system?
Reason 1: You want to have several independent development teams.
Reason 2: You want or your client wants to have a faster and modern UI (i.e. SPA) that is totally decoupled from the other parts of the system.
Reason 3: You want or your client wants that a part of your system (e.g. user store and authentication) can be shared among or reused by multiple different systems.
Reason 4: You want or your client wants your system to be able to scale out easily later.
Reason 5: You want or your client wants that some parts of your system can be revised or extended or replaced easily later.
B. Why migrate your legacy system to microservice architecture?
Reason 1: You want to have several independent development teams.
Reason 2: You want or your client wants to have a faster and modern UI (i.e. SPA) that is totally decoupled from the other parts of the system while the old technologies (like ASP.NET Web Forms or WordPress) cannot provide it.
Reason 3: You do or your client does have performance or scalability issue with the legacy system.
Reason 4: You find that is too difficult to add a new feature to the legacy system because it is too complicated.
Reason 5: You want or your client wants that some parts of your system (e.g. user store and authentication) can be shared among or reused by multiple different systems.
Reason 6: You want or your client wants your system to be able to scale out easily later.
Reason 7: You want or your client wants that a part of your system can be revised or extended or replaced easily later.
C. Here are my suggestions based on estimated development effort and a number of system users.
For a new small project (3-month effort, less than 1,000 users) I always recommend that we use any simple solution that works (e.g. WordPress, Magento, Salesforce LWC, ABP).
For a new medium or large project (2-year effort, more than 20,000 users) I recommend that we use a microservice-based and SPA solution.
For a legacy project I think that we do need a really “good” reason to migrate it to microservice architecture first.
Maybe your client wants to provide a modern SPA for their users.
Maybe the number of users grows to 1,000,000 and you do have a performance or scalability issue.