how is the ias posting

Initially is the probation period. It is nothing but the period of between the first posting after you are recommended by UPSC and your first posting as SDM(sub divisional magistrate),ADM(additional district magistrate or sub collector).

Then is the Foundational Training: This training is imparted to probationers of the All India Services / the IAS / the IPS/ the IFS (Indian Forest Service) and the various Central Services (Group-A) of the Union. The training aims at imparting a basic understanding of the constitutional, political, socio-economic and legal framework of the country; and also fostering greater cooperation among the members of the different public services by building spirit-de-corps and cultivating an attitude of cooperation and inter-dependence.
After you complete your probation which is approx of 2 years then you are allotted a cadre as a SDM.
This is how one is being posted.

Hope this helps.