CODEWITHSUNDEEP
cassandra
Maciej Miklas
Maciej Miklas

Reputation: 3331

Cassandra - reading with consistency level ONE

How is reading with CL ONE implemented by Cassandra?

Does coordinator query all replicas and waits for the first to answer?

According to documentation, coordinator should query one single closest replica. What happens if timeout occurs during this query - does it try another replica, or it returns error to client?

Upvotes: 2

Views: 662

Answers (1)

Aaron
Aaron

Reputation: 57748

Does coordinator query all replicas and waits for the first to answer?

As you mentioned, it queries the closest node, as determined by the snitch.

What happens if timeout occurs during this query

There is additional documentation on the Dynamic Snitch, which states that:

By default, all snitches also use a dynamic snitch layer that monitors read latency and, when possible, routes requests away from poorly-performing nodes.

By that definition, if the node chosen by the snitch should fail, the snitch should route the transaction to the [next] closest node.

Note that as of 2.0.2, Cassandra has a feature called Rapid Read Protection, which:

[A]llows Cassandra to tolerate node failure without dropping a single request

Upvotes: 3

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