One of the core requirement for Distributed DBs is to be as High Availability as possible. What this literally means is that, even if node/nodes go down the DB should function (on its own or with minimum intervention). This is possible only if there are backup copies of the data.
Replication factor controls number of replicas or backup of an item/data/document stored in a DB. The general rule is to have replica for each node which can fail in the cluster.
Let's check how one of the famous NoSQL distributed Db handles Replication Factor.
Couchbase
Default replication factor is 1 in Couchbase (if it's enabled). Drop down field (as shown below) has only 3 values i.e. 1, 2 and 3. Practically, it doesn't make sense to have replication factor more than 3 no matter how large your cluster is.
So even if you have only one node and enable replicas then in the same node there will be two copies of the same data (one original and one backup). Once you add more nodes to the cluster original and replicas will get re-distributed automatically.
Recommendation:
Number of Nodes <= 5 - RF = 1
5 <= Number Of Nodes <= 10 - RF =2
Number of Nodes > 10 - RF = 3
Number of nodes mentioned above is only for data nodes if you are using Multi Dimensional Scaling. If you are not using MDS then also above rule should hold good.
In the event of failure we can fail over (manually or automatically) to replicas.
- In a 5 node cluster with 1 replica. If one node goes down cluster can fail it over. Now before the the failed node is up, what if another node goes down ? You are out of luck. You will have to add another node to the cluster.
- After a node goes down and it's failed over try to replace that node ASAP and perform rebalance. Rebalance creates the replica copies if there are enough nodes available.
References
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