Apache Cassandra Overview

 Apache Cassandra Overview




























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Virtual Nodes in Ring

Virtual Nodes in Cassandra Ring In Apache Cassandra, virtual nodes (vnodes) are a way to divide the token range into smaller pieces and assign multiple token ranges to each node in the cluster.   What are Virtual Nodes (vnodes)? In the traditional Cassandra architecture, each node was responsible for a single, contiguous token range. This made data distribution and rebalancing more difficult when nodes were added or removed from the cluster because large ranges had to be reassigned. With vnodes, instead of assigning a single, large token range to a node, each node is assigned multiple, smaller token ranges. By default, Cassandra assigns 256 virtual nodes per physical node. This allows the total token space to be divided into many smaller chunks that are spread across the cluster. Example:   Instead of Node A owning the token range from zero to 500, Node B owning 500 to 1000, and so on, each node gets assigned multiple, smaller ranges. One node could own small token ranges li...
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Node

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  Apache Cassandra Node But as the load on single node increases node gets to crack We just add more nodes to the ring   Data is distributed evenly across the ring nodes Each node is assigned a token range   How does cassandra retrieve the data from the correct node ? Query using partition key only:   When we query data using partition key   eg: select * from users where user_id=1234;   Cassandra takes the partition key and calculates the hash value using default Murmur3 algorithms   Calculated hash value of the partition key is the Token. Cassandra takes the token and identifies which node can handle this token range. Once cassandra identifies the node it sends the query , if we have replication factor then it sends to multiple nodes depends on the replication factor and consistency level.   Once data is retrieved from the multiple nodes then it checks which replica node has the latest timestamp   Cassandra then sends the data to the client....
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Read Repair Chance

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Read Repair Chance When a read request is made with consistency as ALL   Coordinator Node send the data request to nearest node   Coordinator Node send the checksum request from replica node Once it receives the responses it checks all the checksum values. If they are all same as below value 23 If. Values are different as below from replica nodes Coordinator nodes checks the checksum which has the latest timestamp , it discards the data from replica node which has stale data. Coordinator node sends the correct data on the replica node on which data is deleted  
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