Replication Sets and Its Criteria

In this video we are discussing a very important concept that is a replica sets and

its criteria. Now what is a replica set? A replica set is actually collection of multiple

MongoD applications who are working on the same dataset. So, replica set is nothing

but collection of multiple MongoD applications who are running on the same data set. Out of

these multiple MongoD applications, one application will be treated as the primary and rest of

them will be treated as the secondary. Read and write operations will be at first will be landed

at the primary MongoD application and then replications will be done on the secondary

but sometimes some read operations can also directly access the secondary MongoD. So, let us go

for some further discussions with diagrams regarding the topic that is a replica sets and it

criteria. So, here we are having one diagram. The replica set is basically a group of MongoD

applications that work on same dataset In a replica set there is only one primary MongoD instance and others are secondary So only one primary and multiple secondary nodes will be there

The primary instance takes the read-write requests from the client and it sends the

modification details to the respective secondary instances. So just consider this diagram

Here we are having the client application running. Through this driver, the read-write requests or the option

will be done with the primary MongoD instance and this primary MongoD instance will

put the replication details to the secondary MongoD instances. And what about the modifications, updations or deletions, whatever is taking place due to the

purpose of this read-write operations, then though that modifications are to be reflected or

to be conveyed to these secondary MongoD instances. So only one primary instance and more than one

at least one or more than one secondary instances will be there and that is known as the replica set

what are the features of the replica set a replica set is a collection of n different nodes and the number of n can be anything from two three and so on so at least 2 nodes must be there And we can have the 3 nodes 4 nodes and so on So replica set is nothing but a collection of n number of different nodes

Among these n nodes, only one node will be the primary instance, and rest n-manus 1 nodes

will be for the second instance. For the instances of the replica set, read and write operations goes to the primary node

you can easily find that these read-right operations are going to the primary node and then

sometimes the read operations only read not right only read operations may perform on the

secondary nodes also but right operations must be performed on the primary node only so here

we're having the restrictions that read and write will be done on the primary sometimes only read

operations can directly access the secondary instances just consider this respective we can find that we're having multiple MongoD applications are running here and you can

find that only one is primary and rest of them are secondary. So a cluster of N nodes

this is a cluster of N nodes are there Any one node can be primary So that is no priority Any one of the nodes can be primary All right operations goes to the primary as we have mentioned earlier Then automatically failover

Automatic failover means what? If the primary nodes goes down, then any one of the rest secondary nodes will be treated

as a primary node. So, election will take place so that the respective secondary node will be treated as a primary

node after failure of the current primary node. So automatic failover. next one is automatic recovery and next one is consensus election of primary so in

this way the things will take place if the primary node goes down the process

selects another node as a primary to overcome the problem so out of the rest

secondary nodes one of the nodes will be selected as a primary this process are

done automatically so MongoDB has automatic fail over and recovery mechanism. The MongoD will choose the primary node from the secondary nodes by an

election mechanism. So, the as in the way in which we select our ministers, so in the same

way out of the secondary nodes, one of the nodes will be treated as a, will be selected

as a primary through this election mechanism. When the primary node goes down, the