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- Each server stores a term number called currentTerm. This indicates the most recent term that has been seen by this server.
- Every log entry contains the term in which the entry was created. When a server starts up, it sets currentTerm to the later of (a) the term from the last entry in its log and (b) the term in its most recent vote; if neither of these values is present, currentTerm is initialized to 0.
- Every message from server to server contains the term of the sender, plus an indication whether the sender is a candidate or leader (passive servers never issue requests). This value (call it senderTerm) is used to update currentTerm and to detect out-of-data date servers:
- senderTerm < currentTerm: reject the message with an indication that the sender is out of date; the rejection also includes currentTerm. When the sender receives the rejection it updates its currentTerm to match the one in the response, then passivates. This makes sense for leaders because it means their term of leadership is over. This also make sense for candidates because it means there is already a new election cycle with other active candidates; there is no need for the sender to participate.
- senderTerm == currentTerm: process the request normally.
- senderTerm > currentTerm: set currentTerm to senderTerm. If the recipient is currently a leader or candidate, then it passivates. Finally, it processes the request.
- When a server switches from passive to candidate, it increments currentTerm to force a new election cycle.
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