Consensus Algorithms
Understanding how distributed systems agree on a single data value or state.
What is Consensus?
Consensus is the process of achieving agreement among distributed system participants, even when some components may fail or act maliciously. It's crucial for systems where multiple computers must maintain consistency despite network partitions, crashes, or delays.
Key Concepts
- Agreement: All honest nodes agree on the same value
- Validity: The agreed value was suggested by some node
- Totality: All nodes are aware of the agreement
1. The Two Generals' Problem
Consensus is impossible in asynchronous systems when communication failures can occur. This famous problem demonstrates why achieving consensus requires special algorithms in unreliable environments.
// Simplified simulation
const messages = [];
const send = (message) => {
messages.push(message);
return Math.random() < 0.3 ? 'lost' : 'delivered';
};
```
2. Paxos Algorithm
Paxos is a consensus protocol designed by Leslie Lamport. It ensures agreement through a series of coordinated propose-and-accept cycles, even in partitioned networks.
class Paxos {
// Simplified pseudocode
propose(value) {
// 1. Prepare phase
let promises = this.sendPrepareRequests();
if (this.hasQuorum(promises)) {
// 2. Accept phase
this.sendAcceptRequests(value);
}
}
};
3. Raft Algorithm
Raft divides the consensus problem into leader election, log replication, and safety mechanisms. It's designed for practical use in real-world distributed systems.
const Raft = {
states: ['follower', 'candidate', 'leader'],
currentTerm: 0,
vote: {
// Heartbeat message
sendHeartbeat: () => {
console.log('Leader heartbeat sent');
}
}
};
Why Consensus Matters
- Guarantees data integrity across clusters
- Prevents conflicting updates to shared resources
- Enables fault-tolerant distributed databases
- Supports blockchain and distributed ledgers
Practical Applications
- Replication across database clusters
- Blockchain consensus mechanisms
- Distributed configuration services
- High-availability systems
Ready to Explore?
Understanding consensus algorithms opens the door to building resilient distributed systems. Start experimenting with real-world code samples and projects.
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