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Consensus and Related Problems in Distributed Systems
1. What is Consensus in Distributed Systems?
Consensus in distributed systems refers to the process by which multiple nodes (computers) agree on a single, consistent state or value despite failures and network issues. It is crucial for ensuring data consistency, fault tolerance, and synchronization in distributed applications like blockchain, cloud computing, and databases.
2. Challenges in Achieving Consensus
- Network Failures: Delays, packet loss, and partitions make communication unreliable.
- Fault Tolerance: Some nodes may crash or behave maliciously (Byzantine faults).
- Synchronization Issues: Nodes may process information at different speeds.
- Scalability: As the system grows, maintaining consensus becomes harder.
- Concurrency Conflicts: Multiple nodes might propose different values at the same time.
3. Popular Consensus Algorithms
- Paxos: A classic fault-tolerant consensus algorithm used in distributed databases.
- Raft: A more understandable alternative to Paxos, used in Kubernetes and etcd.
- Byzantine Fault Tolerance (BFT): Designed for systems where some nodes may act maliciously (e.g., Practical BFT in blockchain).
- Proof-of-Work (PoW) & Proof-of-Stake (PoS): Used in blockchain networks like Bitcoin and Ethereum.
4. Related Problems in Consensus
- Leader Election: Selecting a single leader node in a distributed network.
- Atomic Broadcast: Ensuring that all nodes receive and process the same messages in the same order.
- Split Brain Problem: When a network partition causes two sets of nodes to operate independently, leading to data inconsistency.
- CAP Theorem: A distributed system can only provide two out of three: Consistency, Availability, and Partition Tolerance.
Would you like a deeper explanation of any of these problems, or help with an assignment on this topic?
Consensus is a fundamental problem in distributed systems. It involves getting a group of independent processes or nodes to agree on a single data value or decision, even in the presence of failures.
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Consensus ensures that all non-faulty processes in a distributed system agree on the same value and maintain a consistent state across the system.
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Leader election
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State machine replication
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Blockchain (e.g., Bitcoin, Ethereum)
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Atomic broadcast
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Fault-tolerant distributed databases
To solve the consensus problem correctly, a protocol must satisfy:
| Property | Description |
|---|---|
| Agreement | All correct (non-faulty) processes must agree on the same value. |
| Validity | If all correct processes propose the same value, that value must be chosen. |
| Termination | Every correct process must eventually decide on a value. |
| Integrity | A process must decide at most once, and only on a proposed value. |
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Asynchrony: No guarantees on message delays or process speeds.
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Failures: Nodes may crash or act maliciously.
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Network Partitioning: Communication may be disrupted.
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FLP Impossibility: In a purely asynchronous system, it is impossible to guarantee consensus with even one faulty process (Fischer, Lynch, Paterson – 1985).
1. Crash Fault Tolerance (CFT) Consensus
Assumes nodes may fail by crashing, but do not act maliciously.
| Protocol | Key Feature |
|---|---|
| Paxos (by Leslie Lamport) | Most well-known, safe under asynchrony |
| Raft | Easier to understand alternative to Paxos |
| Viewstamped Replication | Like Paxos but with primary-backup model |
2. Byzantine Fault Tolerance (BFT) Consensus
Handles malicious or arbitrary faults (Byzantine behavior).
| Protocol | Faults Tolerated | Notes |
|---|---|---|
| PBFT (Practical Byzantine Fault Tolerance) | Tolerates up to ⌊(n−1)/3⌋ Byzantine faults | Used in Hyperledger Fabric |
| Tendermint | BFT-based; used in Cosmos blockchain | |
| HotStuff | BFT-based; scalable; used in Facebook’s Libra | |
| Algorand | Uses cryptographic sortition for scalable BFT |
| Concept | Description |
|---|---|
| Leader Election | Nodes agree on who the coordinator (leader) should be |
| Atomic Broadcast | All nodes receive messages in the same order |
| Total Order Broadcast | Stronger than consensus; implies consensus |
| Replicated State Machine | Consensus used to maintain state across replicas |
| System | Consensus Used |
|---|---|
| Google Spanner | Paxos |
| etcd / Consul | Raft |
| Bitcoin | Nakamoto Consensus (Proof of Work) |
| Ethereum 2.0 | Proof of Stake-based BFT |
| Hyperledger | PBFT |
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Consensus is central to ensuring consistency and coordination in distributed systems.
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It becomes complex due to asynchrony, faults, and network issues.
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There are multiple consensus protocols, each designed to balance fault tolerance, performance, and simplicity.
Would you like:
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A comparison chart of Paxos vs Raft?
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A flow diagram of how consensus protocols work?
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Help coding a simple Raft or Paxos simulation?
Let me know!
CONSENSUS AND RELATED PROBLEMS IN DISTRIBUTED SYSTEM
CS 425 / ECE 428 Distributed Systems Fall 2025
Distributed Systems, Failures, and Consensus
Chapter 14: Consensus and Agreement
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