The Slavonic equivalent is ряб (ryab) which means slave in the vernacular. Thus a priest using vernacular no longer uses the term ryab but слуга (sluga) which means an obligated servant, literally "one who obeys [or listens]," because it is derived from the verb to obey служити , and from which the word for service (includig the muilitary service, as well as divine service, the litugy) is derived, служба (sluzhba), and the word слушати (slushati), to listen (as in obey).
I believe the modern meaning of doulos is different from the ancient one but I may be mistaken. The NT dictionary mentions bondsman as one definition which seems to fit the term obligate servant. Other definitions mention a slave as well as a servant.
Saint Augustine sums this up where it makes absolute sense to me when he says....
“You hear the Lord himself declare: ‘It is not you that speak, but the Spirit of your Father that speaketh in you’. Likewise you hear the Apostle declare: ‘God hath sent the Spirit of His Son into your hearts’. Could there then be two spirits, one the spirit of the Father, the other the spirit of the Son? Certainly not. Just as there is only one Father, just as there is only one Lord or one Son, so there is only one Spirit, Who is, consequently, the Spirit of both. . . Why then should you refuse to believe that He proceeds also from the Son, since He is also the Spirit of the Son? If He did not proceed from Him, Jesus, when He appeared to His disciples after His Resurrection, would not have breathed on them, saying: ‘Receive ye the Holy Ghost’. What, indeed, does this breathing signify, but that the Spirit proceeds also from Him?” (De Trin. II, 5, 7-10; IV, 18, 24 )