Who is Barzillai and why does David make him such an offer?

Verse 32 tells us that Barzillai had provided the king sustenance while he lay at Mahanaim. When did this happen? What was happening in David's life at the time of Barzillai's provision? (This is one of those times when you should look at the cross references to find the answer to when Barzillai provided sustenance to David.)

2nd Samuel 17:27 tells us, 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,
2Sa 17:28 Brought beds, and basons, and earthen vessels, and wheat, and barley, and flour, and parched corn, and beans, and lentiles, and parched pulse,
2Sa 17:29 And honey, and butter, and sheep, and cheese of kine, for David, and for the people that were with him, to eat: for they said, The people is hungry, and weary, and thirsty, in the wilderness.

Absalom is attempting to usurp the throne of his father. Many in Israel are turning their back on David and following Absalom; but not Barzillai. When others were turning their back on God and God's man, Barzillai stayed true. He provided sustenance for David.

Barzillai is a perfect example for us of the need to stay true to God when others may be forsaking Him. When it may no longer be popular to be faithful, we see in Barzillai the need to remain faithful. We also see how God rewards those who provided the means for His work to be accomplished. Barzillai gave of his own sustenance. He shared what God had given Him with others. He had concern for the well being of others. Because Barzillai understood, "it is more blessed to give than receive," God through David will bless Him.