CODEWITHSUNDEEP
hadoopcassandra-2.0
drnushooz
drnushooz

Reputation: 21

Replica placement logic in Cassandra with multiple datacenters

When write is being performed with consistency EACH_QUORUM and replication 4 with 2 data centers DC1 and DC2 with replica placement 3 in DC1 and 1 in DC2, which class picks the node where the secondary and tertiary copy should reside? The snitch is GossipingPropertyFileSnitch and NetworkTopologyStrategy. The client creates a new file using FileSystem.create and perform a write to it. First copy will go to node based on the token and row key hash. Where does the second and third copy go in DC1 and in DC2?

Upvotes: 2

Views: 93

Answers (1)

Roman Tumaykin
Roman Tumaykin

Reputation: 1931

The consistency level does not have anything to do with the placement strategy. It is simply, how many nodes should report back to the coordinator before the success or failure is reported back to the client.

Each DC places copies according to their replication factor independently. So in the DC2, the only copy will be stored according to the partitioning function. In the DC1, the replica placement is done according to this document: http://www.datastax.com/docs/1.0/cluster_architecture/replication#networktopologystrategy

The NetworkTopologyStrategy determines replica placement independently within each data center as follows:

The first replica is placed according to the partitioner (same as with SimpleStrategy). Additional replicas are placed by walking the ring clockwise until a node in a different rack is found. If no such node exists, additional replicas are placed in different nodes in the same rack. NetworkTopologyStrategy attempts to place replicas on distinct racks because nodes in the same rack (or similar physical grouping) can fail at the same time due to power, cooling, or network issues.

Upvotes: 1

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