Byzantine Fault Tolerance (BFT) Consensus Interview Questions

calendarDec 14, 2025 · 2 min read · consensus blockchain interview bft byzantine-fault-tolerance  ·
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Byzantine Fault Tolerance (BFT) consensus algorithm interview questions covering distributed systems that handle arbitrary failures.

Q1: How does Byzantine Fault Tolerance (BFT) work?

Answer:

Byzantine Fault Tolerance handles arbitrary failures, including malicious behavior.

Sequence Diagram:

Overall Flow Diagram:

Individual Node Decision Diagram:

BFT Requirements:

  • Total Nodes: n = 3f + 1 (where f is max Byzantine nodes)
  • Honest Nodes: 2f + 1 (majority)
  • Fault Tolerance: Up to f Byzantine nodes

BFT Phases:

1. Request Phase:

  • Client sends request to primary
  • Primary broadcasts to all replicas

2. Pre-Prepare Phase:

  • Primary assigns sequence number
  • Broadcasts pre-prepare message

3. Prepare Phase:

  • Replicas broadcast prepare messages
  • Wait for 2f matching prepares

4. Commit Phase:

  • Replicas broadcast commit messages
  • Wait for 2f + 1 commits (including self)

5. Reply Phase:

  • Execute request
  • Send reply to client
  • Client waits for f + 1 matching replies

Example:

 1class BFTNode:
 2    def __init__(self, node_id, total_nodes):
 3        self.node_id = node_id
 4        self.total_nodes = total_nodes
 5        self.f = (total_nodes - 1) // 3
 6        self.quorum = 2 * self.f + 1
 7        self.log = {}
 8    
 9    def pre_prepare(self, sequence, request):
10        # Primary assigns sequence number
11        self.log[sequence] = {
12            'request': request,
13            'prepares': set([self.node_id]),
14            'commits': set()
15        }
16        return {'pre_prepare': (sequence, request)}
17    
18    def prepare(self, sequence, request):
19        if sequence in self.log:
20            self.log[sequence]['prepares'].add(self.node_id)
21            if len(self.log[sequence]['prepares']) >= self.quorum:
22                return {'prepared': True}
23        return {'prepared': False}
24    
25    def commit(self, sequence):
26        if sequence in self.log:
27            self.log[sequence]['commits'].add(self.node_id)
28            if len(self.log[sequence]['commits']) >= self.quorum:
29                # Execute request
30                return {'committed': True, 'result': self.execute(sequence)}
31        return {'committed': False}

Use Cases:

  • Hyperledger Fabric
  • Stellar
  • Ripple

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