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The usage of an API gateway enforces a physical implementation of what so far was only a logical boundary, based on the social and business notion of the application itself: it mandates that communication across applications goes through a formal layer, which is the sole gateway to what’s wrapped within the ownershi p boundary. As a direct consequence, other microservices that are owned by a different group — and therefore outside of the boundary — should be able to access application components only via this gateway. 

What about the advantages that are provided by a service mesh when it comes to service-to-service communication? 

They can all be leveraged — if the mesh is used for communication within the application boundary — for what we have called intra-application communication. See Figure 4-19 on page 125. 

