David was not so engaged in his wars abroad as to neglect the administration of the government at home.
I. His care extended itself to all the parts of his dominion: He reigned over all Israel (Sa2 8:15); not only he had a right to reign over all the tribes, but he did so; they were all safe under his protection, and shared in the fruits of his good government.
II. He did justice with an unbiased unshaken hand: He executed judgment unto all his people, neither did wrong nor denied or delayed right to any. This intimates, 1. His industry and close application to business, his easiness of access and readiness to admit all addresses and appeals made to him. All his people, even the meanest, and those too of the meanest tribes, were welcome to his council-board. 2. His impartiality and the equity of his proceedings, in administering justice. He never perverted justice through favour or affection, nor had respect of persons in judgment. Herein he was a type of Christ, who was faithful and true, and who doth in righteousness both judge and make war, Rev 19:11. See Psa 72:1, Psa 72:2.
III. He kept good order and good officers in his court. David being the first king that had an established government (for Saul's reign was short and unsettled) he had the modelling of the administration. In Saul's time we read of no other great officer than Abner, that was captain of the host. But David appointed more officers: Joab that was general of the forces in the field, and Banaiah that was over the Cherethites and Pelethites, who were either the city train-bands (archers and slingers, so the Chaldee), or rather the life-guards, or standing force, that attended the king's person, the pretorian band, the militia. They were ready to do service at home, to assist in the administering of justice, and to preserve the public peace. We find them employed in proclaiming Solomon, Kg1 1:38. 2. Two ecclesiastical officers: Zadok and Ahimelech were priests, that is, they were most employed in the priests' work under Abiathar, the high priest. 3. Two civil officers: one that was recorder, or remembrancer, to put the king in mind of business in its season (he was prime minister of state, yet not entrusted with the custody of the king's conscience, as they say of our lord chancellor, but only of the king's memory; let the king be put in mind of business and he would do it himself); another that was scribe, or secretary of state, that drew up public orders and despatches, and recorded judgments given. 4. David's sons, as they grew up to be fit for business, were made chief rulers; they had places of honour and trust assigned them, in the household, or in the camp, or in the courts of justice, according as their genius led them. They were chief about the king (so it is explained, Ch1 18:17), employed near him, that they might be under his eye. Our Lord Jesus has appointed officers in his kingdom, for his honour and the good of the community; when he ascended on high he gave these gifts (Eph 4:8-11), to every man his work, Mar 13:34. David made his sons chief rulers; but all believers, Christ's spiritual seed, are better preferred, for they are made to our God kings and priests, Rev 1:6.

Seraiah - the scribe - Most likely the king's private secretary. See Ch1 24:3 (note).

And Zadok the son of Ahitub, and Ahimelech the son of Abiathar, were the priests,.... Not high priests, as Josephus (i) suggests, for there was only one high priest at a time; indeed there was a "sagan", or deputy priest, on occasion; and so Abarbinel says that Zadok was the high priest, and Ahimelech his second or deputy; but the truth of the case was this, Abiathar was high priest only, and continued so until the time of Solomon, when he was thrust out of his office, and Zadok put into it; and Ahimelech his son and Zadok were the principal priests under him, the one of the family of Ithamar, the other of Eleazar; so the Targum on Ch1 18:16 calls them "sagans", or deputies of the high priesthood. Zadok is mentioned first, though Ahimelech was the son of the present high priest, because he was in great favour with David, as afterwards with Solomon, in whose days the high priesthood was translated to him; the family of Eli being now upon the decline, and near being removed from the high priesthood, as was foretold by Samuel it should:

and Seraiah was the scribe; or secretary of state; in Ch1 18:16 he is called Shavsha; he seems to have had two names.

(i) Ut supra, (Antiqu. l. 7. c. 5.) sect. 4.

Zadok . . . and Ahimelech . . . were the priests--On the massacre of the priests at Nob, [Sa1 22:19], Saul conferred the priesthood on Zadok, of the family of Eleazar (Ch1 6:50), while David acknowledged Ahimelech, of Ithamar's family, who fled to him. The two high priests exercised their office under the respective princes to whom they were attached. But, on David's obtaining the kingdom over all Israel, they both retained their dignity; Ahimelech officiating at Jerusalem, and Zadok at Gibeon (Ch1 16:39).