cassandra – write consistency | Start-Up Ideas, Tech Code, Use Cases, Thoughts

The consistency level determines the number of replicas on which the write must succeed before returning an acknowledgment to the client application.

Write Consistency Levels
Level	Description	Usage
`ALL`	A write must be written to the commit log and memtable on all replica nodes in the cluster for that partition key.	Provides the highest consistency and the lowest availability of any other level.
`EACH_QUORUM`	Strong consistency. A write must be written to the commit log and memtable on a quorum of replica nodes in all data center.	Used in multiple data center clusters to strictly maintain consistency at the same level in each data center. For example, choose this level if you want a read to fail when a data center is down and the `QUORUM` cannot be reached on that data center.
`QUORUM`	A write must be written to the commit log and memtable on a quorum of replica nodes.	Provides strong consistency if you can tolerate some level of failure.
`LOCAL_QUORUM`	Strong consistency. A write must be written to the commit log and memtable on a quorum of replica nodes in the same data center as thecoordinator node. Avoids latency of inter-data center communication.	Used in multiple data center clusters with a rack-aware replica placement strategy, such as NetworkTopologyStrategy, and a properly configured snitch. Use to maintain consistency locally (within the single data center). Can be used with SimpleStrategy.
`ONE`	A write must be written to the commit log and memtable of at least one replica node.	Satisfies the needs of most users because consistency requirements are not stringent.
`TWO`	A write must be written to the commit log and memtable of at least two replica nodes.	Similar to `ONE`.
`THREE`	A write must be written to the commit log and memtable of at least three replica nodes.	Similar to `TWO`.
`LOCAL_ONE`	A write must be sent to, and successfully acknowledged by, at least one replica node in the local data center.	In a multiple data center clusters, a consistency level of `ONE` is often desirable, but cross-DC traffic is not. `LOCAL_ONE` accomplishes this. For security and quality reasons, you can use this consistency level in an offline datacenter to prevent automatic connection to online nodes in other data centers if an offline node goes down.
`ANY`	A write must be written to at least one node. If all replica nodes for the given partition key are down, the write can still succeed after a hinted handoff has been written. If all replica nodes are down at write time, an `ANY` write is not readable until the replica nodes for that partition have recovered.	Provides low latency and a guarantee that a write never fails. Delivers the lowest consistency and highest availability.
`SERIAL`	Achieves linearizable consistency for lightweight transactions by preventing unconditional updates.	You cannot configure this level as a normal consistency level, configured at the driver level using the consistency level field. You configure this level using the serial consistency field as part of the native protocol operation. See failure scenarios.
`LOCAL_SERIAL`	Same as SERIAL but confined to the data center. A write must be written conditionally to the commit log and memtable on a quorum of replica nodes in the same data center.	Same as SERIAL. Used for disaster recovery. See failure scenarios.

Even at low consistency levels, the write is still sent to all replicas for the written key, even replicas in other data centers. The consistency level just determines how many replicas are required to respond that they received the write.