A replication factor is the configured number of copies ScyllaDB maintains for each piece of data. Replication improves fault tolerance, while consistency levels determine how many replicas must acknowledge a query. section 1.2.1
Replication factor affects both placement and continued operation:
- ScyllaDB places replicas on different nodes and uses the hash ring to locate and distribute them. section 8.2.1
- With three nodes and replication factor 3, quorum queries continue operating after one node fails. section 2.4
- NetworkTopologyStrategy spreads replicas across racks and applies a specified replication factor separately within each datacenter. section 5.2.2
Creating a keyspace with three replicas
This keyspace assigns replication factor 3 to datacenter1: CREATE KEYSPACE reviews WITH replication = { 'class': 'NetworkTopologyStrategy', 'datacenter1': '3' }; section 5.2.2
Table 1.1 Sample of consistency level options, assuming a cluster with three replicas
|
Consistency level
|
Description
|
Number required to succeed
|
Failures tolerated
|
|---|---|---|---|
ALL
|
Requires all replicas to succeed
|
3
|
0
|
QUORUM
|
Requires a majority of replicas to succeed
|
2
|
1
|
ONE
|
Requires a single replica to succeed
|
1
|
2
|
Compare ALL, QUORUM, and ONE consistency levels for a cluster with three replicas, including required acknowledgments and tolerated failures. section 1.2.1
