Important keywords

• Data centers and racks
• Nodes and replicas
• RF: replication factor
• CL: consistency level
• Tokens and the Ring

Racks

Logical grouping of physically related nodes. Motivated by locality of failures.

Data Centers

Logical grouping of physically related racks.

cassandra-rackdc.properties # These properties are used with GossipingPropertyFileSnitch and will # indicate the rack and dc for this node dc=dc1 rack=rack1

The Ring!

• Locating data always requires knowing the full PK
• hash(PK) -> token
• A consistent hashing function is used to calculate the token

CREATE KEYSPACE Excelsior WITH REPLICATION = { 'class' : 'SimpleStrategy', 'replication_factor' : 3 }; CREATE KEYSPACE Excalibur WITH REPLICATION = { 'class' : 'NetworkTopologyStrategy', 'dc1' : 3, 'dc2' : 2};

Replication strategies

SimpleStrategy

SimpleStrategy

• All nodes are peers (no master-replicas)
• RF=2 means data is on 2 nodes
• Client talks to a "Coordinator Node"

Replication strategies

NetworkTopologyStrategy

NetworkTopologyStrategy

• Network topology communicated via gossip protocol
• Node joining the cluster needs "seed nodes": 1+ node per DC
• Seeds are also used to learn the topology of the ring

Vnodes

No Vnodes: 1 token - 1 node

Vnodes: n tokens - 1 node

• Faster node replacements: randomly distributed, smaller token ranges
• New nodes (scaling) require no rebalancing
• Assign less tokens on slower boxes
• Stackoverflow question

Hinted handoff

• Coordinator node is responsible for storage
• system.hints table holds hinted handoffs
• Write replayed when node comes online