Introduction to Cassandra Consistency Levels (ONE, QUORUM, ALL, etc.)

Consistency levels refer to the strategies implemented in distributed systems to achieve consistency. They define whether commands should continue to be executed even when some nodes fail or are unavailable, until all available nodes have received the command.

WHAT are Consistency Levels?

Consistency levels are strategies used to achieve consistency in distributed systems. They define whether commands should continue to be executed even when some nodes fail or are unavailable.

Types of Consistency Levels

ONE

ONE is a strong consistency strategy, meaning a command is considered successfully executed only when all nodes have received it. This strategy is very strict regarding data consistency, but it also means that if any single node fails, all commands will be discarded.

QUORUM

QUORUM maximizes consistency, requiring that more than half of the nodes have received the command before it is considered successfully executed. This strategy represents a compromise, balancing the requirements of strong consistency and availability.

ALL

ALL is a strong consistency strategy, but unlike ONE, it does not require all nodes to receive the command. If more than half of the nodes have received the command, it is considered successfully executed. This strategy offers better availability than ONE in certain scenarios.

LOCAL_QUORUM (Local QUORUM)

LOCAL_QUORUM is a strong consistency strategy, but it only applies to specific nodes or data tables. This means that if these nodes are all down, it will not affect the integrity of the entire system.

Pros and Cons

Each consistency level has its own pros and cons. For example:

  • ONE
  • Pros: Strong consistency
  • Cons: If any single node fails, all commands will be discarded.
  • QUORUM
  • Pros: Achieves a good balance between availability and strong consistency
  • Cons: If more than half of the nodes have not received the command, some commands will be lost.

Conclusion

Choosing the correct consistency level is key to achieving the reliability and consistency of a distributed system. Each strategy has its own pros and cons, and the choice depends on the specific application scenario.