如果Mongo複製集叢集掛掉的節點數（Arbiter也算作一個普通節點）達到總節點數的一半，則剩餘節點無法選舉新的Primary，並且會在日誌裡記錄“Not starting an election, since we are not electable due to: Not standing for election because I cannot see a majority (mask 0x1)”。即便是剩餘節點裡有Arbiter存在，也無法選舉新的Primary。

驗證試驗如下：

1臺Primary,4臺Secondary,1臺Arbiter

• Primary和Arbiter同時掛了
  • 4臺Secondary會自動選出一個Primary，暫時沒有發現有平局而選不出的情況
• Primary,Arbiter和一個Secondary同時掛，達到總節點數的一半
  • 3臺Secondary無法自動選出一個Primary，日誌中會報“Not starting an election, since we are not electable due to: Not standing for election because I cannot see a majority (mask 0x1)”
• Primary和2個Secondary同時掛，達到總節點數的一半
  • 2臺Secondary無法自動選出一個Primary，日誌中會報“Not starting an election, since we are not electable due to: Not standing for election because I cannot see a majority (mask 0x1)”