What is an OTP application (.app file) and how does it differ from a single module?

An OTP application is a packaged, independently startable unit of functionality — typically a whole subsystem (a web server, a connection pool, a whole product component) — described by a .app file that declares its name, version, modules, registered processes, and dependencies on other applications.

{application, my_app,
 [{description, "My application"},
  {vsn, "1.0.0"},
  {modules, [my_app_sup, my_worker]},
  {registered, [my_app_sup]},
  {applications, [kernel, stdlib, sasl]},
  {mod, {my_app, []}}]}.

Where a module is just a unit of compiled code, an application is a lifecycle-managed unit: it has a defined start function (the mod entry, typically starting a top-level supervisor), can declare dependencies that must start first, and can be started, stopped, and monitored as a whole via application:start/1 and application:stop/1, rather than by manually spawning individual processes yourself.

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