add consistent hashing implementations

implementations/python/consistent_hashing/consistent-hashing.py

@@ -0,0 +1,80 @@
+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