Albert Barnes Commentary 2 Samuel 17:27

"And it came to pass, when David was come to Mahanaim, that Shobi the son of Nahash of Rabbah of the children of Ammon, and Machir the son of Ammiel of Lodebar, and Barzillai the Gileadite of Rogelim," — 2 Samuel 17:27 (ASV)

Shobi’s father may have been the king of the Ammonites, and Shobi was appointed by David as a tributary king or governor of Ammon after he took Rabbah (2 Samuel 12:29). On the other hand, Nahash may have been a common name among the Ammonites, and the Nahash mentioned in 2 Samuel 17:25 may have been from that nation.

Regarding Machir, see the marginal reference.

Barzillai was the ancestor, through a daughter, of a family of priests who were called “sons of Barzillai” after him. They returned from captivity with Zerubbabel but were not allowed to officiate as priests or eat of the holy things, because they lacked a proper register (Ezra 2:61–63). It is likely that because they were wealthy, they had neglected their priestly privileges as a means of maintenance before the captivity.

Rogelim was located in the highlands of Gilead, but its exact location is not known. The name means “the fullers,” as it is the plural of the word “Rogel,” found in “En-Rogel” (2 Samuel 17:17).