Implement Election Restriction for Safety - The Consensus

                      TASK
                    

Implementation

Implement the Election Restriction to ensure safety:

A candidate must have an "up-to-date" log to win election:

Voter compares own log to candidate's
If candidate lastLogTerm > voter lastLogTerm: candidate is ahead
If same term, compare lastLogIndex
Only grant vote if candidate is at least as up-to-date

This ensures the elected leader has all committed entries.

Sample Test Cases

Grant vote to higher termTimeout: 5000ms

Input

{"src":"c0","dest":"n2","body":{"type":"init","msg_id":1,"node_id":"n2","node_ids":["n1","n2","n3"]}}
{"src":"c0","dest":"n2","body":{"type":"set_term","msg_id":2,"term":2}}
{"src":"n1","dest":"n2","body":{"type":"request_vote","msg_id":3,"term":3,"candidate_id":"n1","last_log_index":0,"last_log_term":0}}

Expected Output

{"src":"n2","dest":"c0","body":{"type":"init_ok","in_reply_to":1,"msg_id":0}}
{"src":"n2","dest":"c0","body":{"type":"set_term_ok","in_reply_to":2,"msg_id":1}}
{"src":"n2","dest":"n1","body":{"type":"request_vote_ok","in_reply_to":3,"msg_id":2,"term":3,"vote_granted":true}}

Hints

Hint 1▾

Voter checks candidate log is up-to-date

Hint 2▾

Compare last log term first, then index

Hint 3▾

Reject vote if candidate log is behind

Resources

Raft Safety

Raft paper section on safety

                      OVERVIEW
                    

Theoretical Hub

Election Restriction

This is the key safety property of Raft. By only electing candidates with up-to-date logs, we ensure no committed entries are lost. The leader completeness property follows from this.

Comparison Logic

Higher term always wins. If same term, longer log wins. This captures "more up-to-date" precisely. A leader elected under these rules has everything committed before it.

Key Concepts

election restrictionsafetyup-to-date

main.py

python

#!/usr/bin/env python3
import sys
import json
class Voter:
    def __init__(self, log, current_term, voted_for):
        self.log = log
        self.current_term = current_term
        self.voted_for = voted_for
    
    def is_candidate_up_to_date(self, candidate_last_term, 
    candidate_last_index):
        # TODO: Compare candidate log to own
        pass
    
    def handle_request_vote(self, rv):
        # rv = {term, candidate_id, last_log_index, last_log_term}
        
        # TODO: Check term
        # TODO: Check if already voted
        # TODO: Check if candidate is up-to-date
        # TODO: Grant or reject vote
        pass
def compare_logs(term1, idx1, term2, idx2):
    # TODO: Return True if (term1, idx1) >= (term2, idx2)
    pass