Replication vs Redundancy

I am currently reading about Distributed Systems and I am facing two different terms which are described in a similar manner: Replication and Redundancy.

Can anyone explain each term in part?

The two terms are at the first look pretty similar, but there is a significant difference between them.

The common part of the two terms is the fact that each of has something to do with more nodes/components/processes in a system.

1. Redundancy - describes the fact that you have more than one node/component/process in a system and it's pretty useful for handling failovers. In the case that one of your nodes fail, another node in the system can take over and carry on. Redundancy can be:

• active - where all the traffic goes to all nodes at the same time
• passive - where one node receive traffic and in the case of failure, a switch will be made to another node.

"Redundancy is the duplication of nodes, in case of some of them are failing"

2. Replication - includes redundancy, but involves the copying of data from one node to another or the synchronization of state between nodes. An example of where replication is done is at the databases or MQs level that forms a cluster. Replication can be:

• active: each node receives each message in order to keep in sync with the rest of the nodes
• passive: this is the leader-follower model, where the leader receives all the requests and then forwards them to the followers.

"Replication is the synchronization of state between redundant nodes."