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cdrs-session.md

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CDRS Session

Session is a structure that holds as set pools of connections authorised by a Cluster. As well, it provides data decompressing and load balancing mechanisms used for Cassandra frame exchange, or querying in other words.

In order to create new session a cluster config and a load balancing strategy must be provided. Load balancing strategy is used when some query should be performed by driver. At that moment load balancer returns a connection for a node that was picked up in accordance to a strategy. Such logic guarantees that nodes' loads are balanced and there is no need to establish new connection if there is a one that is released after previous query.

Load balancing

Any structure that implements LoadBalancingStrategy trait can be used in Session as a load balancer.

CDRS provides few strategies out of the box so no additional development may not be needed:

  • RandomLoadBalancingStrategy randomly picks up a node from a cluster.

  • RoundRobinLoadBalancingStrategy thread safe round-robin balancing strategy.

  • TopologyAwareLoadBalancingStrategy policy taking dynamic cluster topology into account.

Along with that any custom load balancing strategy may be implemented and used with CDRS. The only requirement is the structure must implement LoadBalancingStrategy trait.

Data compression

CQL binary protocol allows using LZ4 and Snappy (for protocol version < 5) data compression in order to reduce traffic between Node and Client.

CDRS provides methods for creating Session with different compression contexts: LZ4 and Snappy.

Reference

  1. LZ4 compression algorithm https://en.wikipedia.org/wiki/LZ4_(compression_algorithm).

  2. Snappy compression algorithm https://en.wikipedia.org/wiki/Snappy_(compression).