What is the concept of divine destiny (Qadar) in Islam, what is its relationship with means and human actions, and does supplication (Du'a) avert a divine decree (Qada')?

The doctrine of the People of the Sunnah and the Community (Ahl al-Sunnah wa al-Jama'ah) regarding divine decree (Qada') and destiny (Qadar) is the middle ground between the Jabariyyah (determinists) and the Qadariyyah (free-will advocates). The servant is destined for what he was created for, yet also given choice from another perspective, with will and volition bestowed upon him by Allah. Everything that befalls a person, whether good or evil, is by Allah's decree and destiny, and the means themselves are part of Allah's decree. Sheikh Abdur-Rahman ibn Nasir as-Sa'di clarified this with various examples, including that lifespans and provisions are linked to Allah's decree and have religious and material causes. The Prophet, peace and blessings be upon him, commanded diligence in pursuing what is beneficial and seeking help from Allah. Thus, all causes and their effects are from Allah's divine decree and destiny, and averting harm does not contradict belief in destiny. If doubt arises concerning an issue, it should not become a path to suspicion in religion; rather, one must learn and supplicate for steadfastness.