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layer
subject			fact

layer	communicates using procedure calls or by inter-process communication where the lower layers can become servers and the higher layers can become clients
	has definition A subsystem that provides a set of services and has layer cohesion
	has example The set of related services which could form a layer might include: Services for computation Services for transmission of messages or data Services for storage of data Services for managing security Services for interacting with users Services provided by an operating system Services provided directly by the hardware
	has well-defined interface that is used by layer immediately above
	is a subtopic of 9.2 - Principles Leading to Good Design
	is often divided into smaller subsystems
	is part of multi-layer system
	is a kind of component
	only communicates with the layer immediately below it
	sees a lower layer as a set of services it can use
	usually provides services through an API
component	is reusable if it can be used in several different systems with little or no modification
	may perform a special-purpose function such as the user interface for a particular system

Kinds of layer :

• kernel layer (3 facts)
• model^2 (7 facts) - The functional layer in the MVC architectural pattern - the underlying classes whose instances are to be viewed and manipulated
• user interface layer (5 facts)