Yarsanism aka Kaka’i or Ahl-e Haqq, is a Kurdish religious system founded in the 14th century between the border of the KRG (near Halabja) and Iranian-occupied Kurdistan (mostly around Kirmanshah), and more recently also in the Diaspora in Western countries. All adherents of Yarsanism are Kurds. The term Ahl-e Haqq is also used by Islamic sects such as the Hurufites.
In addition to the Shiia facets, the religion of the Yarsanism reveals, above all, clear elements of Yezidism and Alevism. The number of members is estimated to one million. The main sanctuaries of Ahl-e Haqq are the tomb of Baba Yadgar in Dohab and the tomb of Sultan Sahak in Perdiwar.
The religious literature of the Ahl-e Haqq with the principal Kalam-e Saranjam is mainly written on Gorani (Hawrami and Leki), most of which are Ahl-e Haqq. The Ahl-e Haqq regard themselves mostly as Gûran (Goran), regardless of the dialect they speak. 
Yarsanis believe in seven consecutive incarnations of the divinity as well as in succession five epiphanies of divine hypostases or angels. These were developed from the divine, and formed their own essence.  From Sufism (Islamic mysticism), they took over the practice of Dhikr at Cem ceremonies as well as sharing communal meals and fraternal cohabitation. Cem is understood as a mystical union, while a group of singers (kelamxwen), who performs religious verses, is sitting opposite an orchestra of several players of the tembûr.  Tembûr (or temîre) is one of the tanbur-like Kurdish Long-necked lute, which has a sacred meaning in the Ahl-e Haqq.  Of the seven steps, four were already traversed, and the divine essence was thus shown as a form of the Creator God in Kavandagar, Mortaza Ali, Shah Hoshin, and Sultan Sahak, with Sultan Sahak revealing absolute truth.
The salvation through God is reserved only to certain people, depending on which clay they were created from. Thus, people like the Ahl-e Haqq of yellow clay (Zarda-gel) find salvation, while humans of black clay (Kāk-e siyah) are forever condemned.
Among the first scientific descriptions of the Ahl-e Haqq is the work of the Russian Orientalist and diplomat Vladimir Minorsky from 1920. 
Geography and Population
Yarsani, Kakais or Ahl-e Haqq are settled in different pockets in Kurdistan.
Most of the Kakais live in Iranian-occupied Kurdistan, specifically in Kirmanshah and Ilam Provinces. As a result of the opression and assimilation policies of the Islamic regime, Yarsani community is declining. Many Yarsanis leave eastern Kurdistan for Europe or KRG.
There is a significant Kakai community in Southern Kurdistan. Daquq district of Kirkuk is the central settlement of Kakais. In this are, many Kakai-Kurds were displaced to make room for Arab settlers. Plus, dozens of villages were ruined. Since 2003, Kakai community has been working to reestablish themselves in their areas.
There are several Kakai Kurdish villages in south-eastern Ninawa Plain. In addition, Shabaks who consist the majority of Ninawa Plain are sometimes identified as Yarsanis, Kakais.
Yarsanis, Kakais and Ahl-e Haqq are same religious group. Shabak Kurds are related to Kakais if not identical to them.
There are different theories about Shabaks. Although they are mostly Shia and Sunni today, their traditional religious rituels are similar to Yazidis and Kakais. For instance, they visit Yazidi shrines in Bashiqa.
Some elders of Shabaks speak Gorani dialect while the new generation mostly speak Kurmanji and Arabic.
1. Encyclopedia of the Modern Middle East and North Africa. Thompson Gale, Detroit 2004 P.82
2. Bruinessen 2003, Section: Gûran
3. Ende & Steinbach, 2005, P.723
4. Mustafa Dehqan: An Ahl-i Haqq Kurdish Folio on the Music
5. Navid Fozi, 2007, P. 173-205
6. Minorsky, 1920, Pg. 20-97