What is a cluster in MongoDB?

I am someone new to mongoDB and has absolutely no knowledge regarding databases so would like to know what is a cluster in MongoDB and what is the point of connecting to one in MongoDB? Is it a must to connect to one or can we just connect to the localhost?

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A mongodb cluster is the word usually used for sharded cluster in mongodb. The main purposes of a sharded mongodb are:

  • Scale reads and writes along several nodes
  • Each node does not handle the whole data so you can separate data along all the nodes of the shard. Each node is a member of a shard (which is a replicaset, see below for the explanation) and the data are separated on all shards.

This is the representation of a mongodb sharded cluster from the official doc.

mongodb sharded cluster

If you are starting with mongodb, I do not recommend you to shard your data. Shards are way more complicated to maintain and handle than replicasets are. You should have a look at a basic replicaset. It is fault tolerant and sufficient for simple needs.

The ideas of a replicaset are :

  • Every data are repartited on each node
  • Only one node accept writes

A replicaset representation from the official doc

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For simple apps there is no problem to have your mongodb cluster on the same host than your application. You can even have them on a single member replicaset but you won't be fault tolerant anymore.

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It will be more clear, if you create a cluster in the cloud website of mongo. Try free version. You can see live monitoring of servers.enter image description here

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Atlas-managed MongoDB deployments, or “clusters,” can be either a replica set or a sharded cluster.

replica set: a cluster of MongoDB servers that implements replication and automated failover. (Failover is a method of protecting computer systems from failure, in which standby equipment automatically takes over when the main system fails.)

– replication: a feature allowing multiple database servers to share the same data, thereby ensuring redundancy and facilitating load balancing. (Redundancy is a system design in which a component is duplicated so if it fails there will be a backup.)

sharded cluster: the set of nodes comprising a sharded MongoDB deployment. A sharded cluster consists of config servers, shards, and one or more mongos routing processes.

– sharding: a method for distributing data across multiple machines.

A sharded cluster consists of ten servers: one server for the mongos [interface between the client applications and the sharded cluster] and three servers each for the first replica set, the second replica set, and the config server replica set

– shard: a single mongod instance or replica set that stores some portion of a sharded cluster’s total data set. In production, all shards should be replica sets.

Sources: MongoDB docs: Glossary, Create a Cluster, Convert a Replica Set to a Sharded Cluster


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