CAP Theorem, PACELC & Consistency Models

# Python Tunable Quorum Read Coordinator
import asyncio

async def quorum_read(key: str, nodes: list, R: int, N: int) -> dict:
    # Dispatch parallel reads to all N nodes
    tasks = [asyncio.create_task(node.get(key)) for node in nodes]
    responses = []
    
    for completed in asyncio.as_completed(tasks):
        resp = await completed
        if resp.is_success:
            responses.append(resp)
        if len(responses) >= R:
            break  # Quorum threshold met!

    if len(responses) < R:
        raise Exception("HTTP 503: Read Quorum Not Met")

    # Pick highest timestamp version (Last-Write-Wins / Vector Clock)
    latest = max(responses, key=lambda r: r.version)
    return latest.value

// C++20 Quorum Write Coordinator
#include <vector>
#include <future>

bool quorumWrite(const std::string& key, const std::string& val, int W, const std::vector<Node*>& nodes) {
    std::atomic<int> acks{0};
    uint64_t version = getCurrentTimestampMicros();
    
    for (auto* node : nodes) {
        if (node->write(key, val, version)) {
            acks++;
        }
    }
    return acks.load() >= W; // Strong consistency if R + W > N
}

// Java Cassandra-Style Quorum Evaluator
public class QuorumCoordinator {
    public Value read(String key, ConsistencyLevel level, List<Node> replicas) {
        int required = (level == ConsistencyLevel.QUORUM) ? (replicas.size() / 2) + 1 : 1;
        List<Record> responses = replicas.parallelStream()
            .map(n -> n.get(key))
            .filter(Objects::nonNull)
            .limit(required)
            .collect(Collectors.toList());

        if (responses.size() < required) {
            throw new ReadTimeoutException("Quorum failed");
        }
        return responses.stream().max(Comparator.comparing(Record::getVersion)).get().getValue();
    }
}

// Go Quorum Coordinator with Channels
func QuorumWrite(ctx context.Context, key, value string, nodes []Node, W int) error {
    resChan := make(chan bool, len(nodes))
    for _, n := range nodes {
        go func(node Node) {
            resChan <- node.Put(key, value)
        }(n)
    }

    acks := 0
    for i := 0; i < len(nodes); i++ {
        if <-resChan { acks++ }
        if acks >= W { return nil } // Quorum met!
    }
    return errors.New("write quorum failed")
}

// TypeScript Quorum Coordinator
export async function quorumRead<T>(key: string, nodes: Node[], R: number): Promise<T> {
  const results = await Promise.allSettled(nodes.map(n => n.get(key)));
  const successful = results
    .filter((r): r is PromiseFulfilledResult<Versioned<T>> => r.status === 'fulfilled')
    .map(r => r.value);

  if (successful.length < R) {
    throw new Error('HTTP 503: Quorum not met');
  }
  successful.sort((a, b) => b.version - a.version);
  return successful[0].data;
}