Does Hashmap use Consistent Hashing?

No, HashMap in most programming languages, including Java, do not use Consistent Hashing. HashMap typically uses a hash function to map keys to indices in an array (often called a bucket or slot). The index is calculated based on the hash code of the key and the size of the array.

• When collisions occur (i.e., two keys hash to the same index), HashMap typically use a linked list or a more efficient data structure like a balanced tree (starting from Java 8) to store multiple key-value pairs at the same index.
• HashMap does not dynamically rehash or remap keys when the size of the map changes, which is a key characteristic of consistent hashing.

How Consistent Hashing Works?

Below is how Consistent Hashing Works:

• Hash Ring: Consistent hashing uses a hash ring, which is a circular array of hash values ranging from 0 to 2^32-1 (or another appropriate range depending on the hashing algorithm).
• Node Assignment: Each node in the system is assigned a random value on the hash ring. This value determines the range of keys that the node is responsible for.
• Key Hashing: When a key needs to be stored or looked up, it is hashed to a value within the same range as the nodes on the hash ring.
• Node Selection: To find which node is responsible for a key, the hash ring is traversed in a clockwise direction from the key’s hash value until the first node is encountered. This node is then responsible for the key.
• Adding or Removing Nodes: When a node is added or removed from the system, only a fraction of the keys need to be reassigned. This is because most keys remain within the same range and are still assigned to the same nodes.
• Load Balancing: Consistent hashing helps in distributing keys evenly across nodes, which helps in balancing the load. This prevents hotspots where a few nodes are responsible for a disproportionate number of keys.
• Fault Tolerance: Consistent hashing provides fault tolerance because if a node fails, only the keys assigned to that node need to be reassigned. The rest of the keys remain assigned to their respective nodes.
• Sticky Sessions: In the context of load balancing, consistent hashing can be used to implement sticky sessions, where requests from the same client are consistently routed to the same backend server based on the hashed value of the client’s IP address or session ID.