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雄关漫道真如铁,而今迈步从头越。
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Consistent Hashing Algorithm

Consistent Hashing is a hashing algorithm used to solve data distribution and load balancing issues in distributed systems. It is particularly suitable for scenarios where dynamic nodes (such as servers) join or leave, minimizing data redistribution and thereby enhancing system stability and scalability.

Basic Principles of Consistent Hashing:#

  1. Virtual Nodes:
    Consistent hashing typically introduces the concept of virtual nodes. Each physical node is mapped to multiple virtual nodes, which helps to distribute the load more evenly. Virtual nodes are mapping points at multiple "positions" on the hash ring.
  2. Hash Ring:
    The hash ring is a logical circular structure that can be viewed as a hash value space ranging from 0 to 2^32-1 (or other ranges). Each node (physical or virtual) is mapped to a certain position on this hash ring through a hashing algorithm.
  3. Data Allocation:
    Data is mapped to a certain position on the hash ring using a hashing algorithm (such as MD5, SHA-1, etc.). Then, the data will be stored on the first node encountered in a clockwise direction (either a physical or virtual node).

Workflow:#

  • Node Joining:
    When a new node joins, it only affects a small amount of data adjacent to it. That is, data is located through the hash ring, and only a portion of the data is migrated to the new node, avoiding large-scale data migration.
  • Node Leaving:
    When a node leaves, the data on the hash ring is transferred to the next node in a clockwise direction. Due to the design of consistent hashing, the leaving node only affects the data it is responsible for, rather than causing global data migration.

Advantages:#

  • Minimizing Data Migration: The joining or leaving of nodes does not lead to large-scale migration of the entire dataset. Usually, only a small portion of the data is affected, which is very useful in dynamic environments.
  • Load Balancing: By using virtual nodes, the load can be distributed more evenly, preventing some nodes from becoming overloaded while others remain idle.
  • Strong Scalability: When the system scales, it does not have a drastic impact on the entire system, and the expansion of nodes is relatively simple.

Disadvantages:#

  • Uneven Data Distribution: If the number of virtual nodes is too small, data may become concentrated on a few physical nodes, leading to load imbalance.
  • Node Failure: If a node fails, due to the clockwise nature of the hash ring, data transfer may encounter situations of high load.

Application Scenarios:#

The consistent hashing algorithm is widely used in many distributed systems, especially when the system needs to support dynamic increases or decreases in nodes, such as:

  • Distributed Caches (e.g., Memcached, Redis)
  • Distributed Databases (e.g., Cassandra)
  • CDNs (Content Delivery Networks)

In summary, consistent hashing greatly improves the flexibility and scalability of distributed systems by allocating node positions in the hash ring and combining virtual node technology.

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