Magistrate Judge Leo Sorokin began serving as a Magistrated in the District of Massachusetts on April 11, 2005 and became Chief Magistrate on September 1, 2011, succeeding Magistrate Judge Dein.
His official court bio may be found here.
A somewhat more detailed bio can be found here.
Judge Sorokin presides over the Court Assisted Recovery Effort for Massachusetts to promote the development and maintenance of sover, working felons under the supervision of the court.