concept hash ring in category nosql
This is an excerpt from Manning's book Making Sense of NoSQL.
One of the most challenging problems with distributed databases is figuring out a consistent way of assigning a document to a processing node. Using a hash ring technique to evenly distribute big data loads over many servers with a randomly generated 40-character key is a good way to evenly distribute a network load.
Hash rings are common in big data solutions because they consistently determine how to assign a piece of data to a specific processor. Hash rings take the leading bits of a document’s hash value and use this to determine which node the document should be assigned. This allows any node in a cluster to know what node the data lives on and how to adapt to new assignment methods as your data grows. Partitioning keys into ranges and assigning different key ranges to specific nodes is known as keyspace management. Most NoSQL systems, including MapReduce, use keyspace concepts to manage distributed computing problems.
In chapters 3 and 4 we reviewed the concept of hashing, consistent hashing, and key-value stores. A hash ring uses these same concepts to assign an item of data to a specific node in a NoSQL database cluster. Figure 6.11 is a diagram of a sample hash ring with four nodes.
Figure 6.11. Using a hash ring to assign a node to a key that uses a 40-character hex number. This number can be expressed in 2160 bits. The first bits in the hash can be used to map a document directly to a node. This allows documents to be randomly assigned to nodes and new assignment rules to be updated as you add nodes to your cluster.
