Use of Callbacks in Layered Architecture

A layered architecture organizes software systems into distinct layers, where each layer handles specific functionality and communicates with adjacent layers through well-defined interfaces. This design pattern is fundamental in networking protocols, operating systems, and enterprise applications.

Most enterprise applications follow a three-layer architecture consisting of the Presentation layer (user interface), Business layer (application logic), and Persistence layer (data storage). Each layer depends only on the layer below it, creating a clean separation of concerns.

What is a Callback?

A callback is executable code passed as an argument to other code, with the expectation that it will be invoked at a specific time. Callbacks can be synchronous (executed immediately) or asynchronous (executed later when certain conditions are met).

Programming languages implement callbacks through various mechanisms including function pointers, lambda expressions, and subroutines. Callbacks are essential for event-driven programming and maintaining loose coupling between system components.

Use of Callbacks in Layered Architecture

In layered architectures, upper layers provide simplified interfaces and APIs, while lower layers handle system-level operations like network communication or hardware interaction. Direct function calls work from upper to lower layers, but the reverse creates circular dependencies.

Callbacks solve this problem by allowing lower layers to communicate back to upper layers without creating tight coupling. Here's how the callback mechanism works:

Callback Registration Process

// Upper layer registers a callback function
register_callback(notify_observer);

// Lower layer stores the function pointer
void register_callback(callback_func_t cb) {
    g_notify_ob = cb;  // Store in global function pointer
}

Callback Invocation

// Lower layer invokes the callback when needed
if (g_notify_ob != NULL) {
    g_notify_ob(data);  // Call the upper layer function
}

Key Benefits

Conclusion

Callbacks in layered architecture provide an elegant solution for upward communication while maintaining proper separation of concerns. This pattern enables flexible, maintainable systems where lower layers can notify upper layers without creating circular dependencies.