请帮忙讲解一下这个英语句子的结构,以及前半部分的两个by的作用~

题目

请帮忙讲解一下这个英语句子的结构,以及前半部分的两个by的作用~
Frustrated by the old-boy network by which Minnesota judges were always appointed ,Page challenged the system in court and was eventually elected judge in the Supreme Court

答案

　　An old boy network,or society (also old boys' club),can refer to social and business connections among former pupils of male-only private schools.校友关系网　　明尼苏达州的法官本来总是由联谊会任命...