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Definition of Byzantine fault tolerance system

What is the Byzantine fault tolerance system?

The Byzantine fault tolerance (BFT) system is a set of algorithms and protocols that allows a distributed system to function correctly even when some of the components malfunction or act maliciously. This maliciousness can range from simple failures like crashes to deliberate attempts to mislead other nodes.

These nodes can be unreliable due to network issues, software bugs, or even deliberate attacks. BFT systems guarantee that despite these challenges, all nodes agree on the same state and perform the same action, preventing inconsistent or erroneous behavior.

Imagine a group of generals surrounding a city, each with their own army. Yet, some generals may be traitors, spreading misinformation. How can they guarantee coordinated action and avoid catastrophic defeat? This is the crux of the Byzantine Generals' Problem and the foundation of BFT systems.

Here are some scenarios where BFT is applied:

  • Blockchain technology, to ensure the consistency and security of transactions.
  • Financial systems, to guarantee the integrity of financial transactions and prevent unauthorized access.
  • Air traffic control, to provide reliable communication and safe operation in critical systems.What is the main concept behind practical Byzantine fault tolerance?

What is the main concept behind practical Byzantine fault tolerance?

BFT relies on a core principle of majority consensus. All participants in the system must agree on the same outcome, even if some components fail or act maliciously. This concept is often realized through practical Byzantine fault tolerance (PBFT), a specific BFT algorithm.

PBFT works by:

  • Data replication. Each participant receives and stores the same information.
  • Multi-phase communication. In multiple rounds, participants exchange information and vote on proposed decisions.
  • Fault detection and recovery. Mechanisms are in place to identify and handle faulty or malicious components.

This multi-phase process with excessive communication ensures several crucial things:

  • Consistency. All nodes agree on the final decision (attack or retreat), even if some are faulty or malicious.
  • Liveness. If a correct majority exists, the system eventually reaches a decision without getting stuck.
  • Fault tolerance. The system can tolerate up to a certain number of faulty nodes (typically one-third) and still function correctly.

How does the Byzantine fault tolerance work?

This multi-phase approach with majority voting ensures that only proposals supported by most honest nodes are accepted and executed. Any faulty or malicious nodes can only influence a minority of the votes, ultimately failing to disrupt the system's consensus.

It works with the following 4 steps:

  1. Leader election: One node is chosen as the leader for a specific round of decision-making. This leader is responsible for initiating the proposal and coordinating the communication flow.
  2. Pre-vote phase: The leader broadcasts its proposal to all other nodes. Then, nodes send a pre-vote message, indicating whether they agree with the proposal.
  3. Commit phase: If the leader receives a majority of pre-votes in favor of the proposal, it broadcasts a commit message. Nodes then send a commit message if they receive the leader's commit.
  4. Execution phase: If a node receives most commit messages, it executes the agreed-upon action.

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