Consistent Hashing Implementation

package implementations.java.consistent_hashing;

import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.*;

public class ConsistentHashing {
    private final int numReplicas; // Number of virtual nodes per server
    private final TreeMap<Long, String> ring; // Hash ring storing virtual nodes
    private final Set<String> servers; // Set of physical servers

    public ConsistentHashing(List<String> servers, int numReplicas) {
        this.numReplicas = numReplicas;
        this.ring = new TreeMap<>();
        this.servers = new HashSet<>();

        // Add each server to the hash ring
        for (String server : servers) {
            addServer(server);
        }
    }

    private long hash(String key) {
        try {
            MessageDigest md = MessageDigest.getInstance("MD5");
            md.update(key.getBytes());
            byte[] digest = md.digest();
            return ((long) (digest[0] & 0xFF) << 24) |
                   ((long) (digest[1] & 0xFF) << 16) |
                   ((long) (digest[2] & 0xFF) << 8) |
                   ((long) (digest[3] & 0xFF));
        } catch (NoSuchAlgorithmException e) {
            throw new RuntimeException("MD5 algorithm not found", e);
        }
    }

    public void addServer(String server) {
        servers.add(server);
        for (int i = 0; i < numReplicas; i++) {
            long hash = hash(server + "-" + i); // Unique hash for each virtual node
            ring.put(hash, server);
        }
    }

    public void removeServer(String server) {
        if (servers.remove(server)) {
            for (int i = 0; i < numReplicas; i++) {
                long hash = hash(server + "-" + i);
                ring.remove(hash);
            }
        }
    }

    public String getServer(String key) {
        if (ring.isEmpty()) {
            return null; // No servers available
        }

        long hash = hash(key);
        // Find the closest server in a clockwise direction
        Map.Entry<Long, String> entry = ring.ceilingEntry(hash);
        if (entry == null) {
            // If we exceed the highest node, wrap around to the first node
            entry = ring.firstEntry();
        }
        return entry.getValue();
    }

    public static void main(String[] args) {
        List<String> servers = Arrays.asList("S0", "S1", "S2", "S3", "S4", "S5");
        ConsistentHashing ch = new ConsistentHashing(servers, 3);

        // Step 2: Assign requests (keys) to servers
        System.out.println("UserA is assigned to: " + ch.getServer("UserA"));
        System.out.println("UserB is assigned to: " + ch.getServer("UserB"));

        // Step 3: Add a new server dynamically
        ch.addServer("S6");
        System.out.println("UserA is now assigned to: " + ch.getServer("UserA"));

        // Step 4: Remove a server dynamically
        ch.removeServer("S2");
        System.out.println("UserB is now assigned to: " + ch.getServer("UserB"));
    }
}

import hashlib
import bisect

class ConsistentHashing:
    def __init__(self, servers, num_replicas=3):
        """
        Initializes the consistent hashing ring.

        - servers: List of initial server names (e.g., ["S0", "S1", "S2"])
        - num_replicas: Number of virtual nodes per server for better load balancing
        """
        self.num_replicas = num_replicas  # Number of virtual nodes per server
        self.ring = {}  # Hash ring storing virtual node mappings
        self.sorted_keys = []  # Sorted list of hash values (positions) on the ring
        self.servers = set()  # Set of physical servers (used for tracking)

        # Add each server to the hash ring
        for server in servers:
            self.add_server(server)

    def _hash(self, key):
        """Computes a hash value for a given key using MD5."""
        return int(hashlib.md5(key.encode()).hexdigest(), 16)

    def add_server(self, server):
        """
        Adds a server to the hash ring along with its virtual nodes.

        - Each virtual node is a different hash of the server ID to distribute load.
        - The server is hashed multiple times and placed at different positions.
        """
        self.servers.add(server)
        for i in range(self.num_replicas):  # Creating multiple virtual nodes
            hash_val = self._hash(f"{server}-{i}")  # Unique hash for each virtual node
            self.ring[hash_val] = server  # Map hash to the server
            bisect.insort(self.sorted_keys, hash_val)  # Maintain a sorted list for efficient lookup

    def remove_server(self, server):
        """
        Removes a server and all its virtual nodes from the hash ring.
        """
        if server in self.servers:
            self.servers.remove(server)
            for i in range(self.num_replicas):
                hash_val = self._hash(f"{server}-{i}")  # Remove each virtual node's hash
                self.ring.pop(hash_val, None)  # Delete from hash ring
                self.sorted_keys.remove(hash_val)  # Remove from sorted key list

    def get_server(self, key):
        """
        Finds the closest server for a given key.

        - Hash the key to get its position on the ring.
        - Move clockwise to find the nearest server.
        - If it exceeds the last node, wrap around to the first node.
        """
        if not self.ring:
            return None  # No servers available

        hash_val = self._hash(key)  # Hash the key
        index = bisect.bisect(self.sorted_keys, hash_val) % len(self.sorted_keys)  # Locate nearest server
        return self.ring[self.sorted_keys[index]]  # Return the assigned server

# ----------------- Usage Example -------------------

# Step 1: Initialize Consistent Hashing with servers
servers = ["S0", "S1", "S2", "S3", "S4", "S5"]
ch = ConsistentHashing(servers)

# Step 2: Assign requests (keys) to servers
print(ch.get_server("UserA"))  # Maps UserA to a server
print(ch.get_server("UserB"))  # Maps UserB to a server

# Step 3: Add a new server dynamically
ch.add_server("S6")
print(ch.get_server("UserA"))  # Might be reassigned if affected

# Step 4: Remove a server dynamically
ch.remove_server("S2")
print(ch.get_server("UserB"))  # Might be reassigned if affected