What does the "one business function, one service" principle mean?

The "one business function, one service" principle means that each microservice should be responsible only for one specific task within the business and should contain all the logic and data needed for it.

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#The meaning

Instead of one large universal application, the system is built from independent "building blocks", each of which does one thing well. For example:

• the authentication service handles only login and registration,
• the payments service handles only transaction processing,
• the catalog service handles only products and categories.

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#Why this is needed

• It simplifies understanding and maintaining the code.
• It allows scaling only the part that needs it (for example, payments during a sales spike).
• It makes the system more resilient: a failure in one service does not break the rest.
• It lets different teams develop their parts independently.

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Summary:

One service equals one complete business function. It is autonomous, independent, and can live, update, and scale separately from the others.