1. Why do we need replication with the Cassandra database?

Here are several reasons why we need replication in Cassandra:

  • Data durability - replication helps to ensure that data is not lost if one node fails. With replication, we'll have multiple copies of data stored on different nodes, so there is always a copy available even if one of the nodes goes down.

  • Data availability - with replication we ensure that data is always available to be read or written, even if some of the nodes in the cluster are unavailable.

  • Load balancing - if we store data on multiple nodes, Cassandra will distribute the load evenly across the cluster, improving performance and scalability.

  • Disaster recovery - in case of a disaster that affects an entire data center, having replicas in a different data center can help ensure that data is still available.

In conclusion, configuring replication in Cassandra helps us ensure the reliability, availability, and performance of our database.

2. Which replication strategies are the most commonly used?

In Apache Cassandra, replication refers to storing multiple copies of data on different nodes in a cluster to ensure data availability and durability. There are several strategies that determine how data is replicated across the nodes in a cluster. Some of the most commonly used replication strategies are:

  • SimpleStrategy: It is used when there is a single data center and all the nodes are in the same datacenter. In this strategy, the number of replicas is specified and the data is replicated on that many nodes in the cluster. Also, it is the default replication strategy for Cassandra.

  • NetworkTopologyStrategy: This strategy is used when there are multiple data centers in a cluster. This strategy allows you to specify the number of replicas for each data center.

  • EveryOtherNodeStrategy: This strategy is used when we want to store data on every other node in the cluster. It is not used so widely.
  • LocalStrategy: This strategy is used when you have a single node in a data center, so this strategy stores all the replicas on that single node.

  • CustomReplicationStrategy: This strategy allows you to create your own replication strategy by implementing the ReplicationStrategy interface in Cassandra. Since existing strategies cover most of the use cases we can have, this one is not so widely used.
  • OldNetworkTopologyStrategy: This strategy is similar to NetworkTopologyStrategy, but it is used in older versions of Cassandra. Since it's obsolete, it should be avoided whenever possible.

  • SkewedStrategy: This strategy is used when you have a small number of very large partitions. It allows us to specify the number of replicas for each partition.

3. Configure replication with Cassandra

To configure the replication in Apache Cassandra, we need to configure the replication property in the cassandra.yaml configuration file. This property specifies the replication strategy and the number of replicas for each data center.

Here is an example of how to configure the replication property for a cluster with two data centers, datacenter1 and datacenter2, using the NetworkTopologyStrategy:

  class: NetworkTopologyStrategy
  datacenter1: 4
  datacenter2: 3

This configuration states that there should be 4 replicas of each piece of data in datacenter1 and 3 replicas in datacenter2.

You can also specify the replication factor (i.e., the number of replicas) using the replication_factor property in the SimpleStrategy:

  class: SimpleStrategy
  replication_factor: 2

This configuration says that there should be 2 replicas of each piece of data in the cluster.

Note that we need to specify the replication strategy and the number of replicas for each data center (for example for NetworkTopologyStrategy)  or the replication factor (for SimpleStrategy) based on our requirements and the architecture of our Cassandra cluster.

You can read more on replication_factor parameter here.

4. Conclusion

In this short tutorial we discussed replication with Cassandra - why we need it, and where replication can be useful for different purposes. We also show how to configure the replication in Cassandra, whether you have a cluster or just one node.