Signals adalah implementasi Django terhadap observer pattern — mereka memungkinkan pengirim tertentu memberitahu serangkaian penerima ketika event spesifik terjadi (seperti model disimpan atau dihapus). Mereka memungkinkan kode yang decoupled: satu bagian aplikasi bereaksi terhadap event di bagian lain tanpa coupling langsung.
Signals built-in yang umum
pre_save / post_save → before / after a model instance is saved
pre_delete / post_delete → before / after a model instance is deleted
m2m_changed → a ManyToMany relationship changed
request_started / finished→ request lifecycle
user_logged_in / out → authentication events
