Replication is the process of synchronizing knowledge across multiple servers. Replication provides redundancy and increases knowledge availability with multiple copies of knowledge on different database servers. Replication protects a database from the loss of a single server. Replication also allows you to recover from hardware failure and service interruptions. With additional copies of the data, you can dedicate one to disaster recovery, reporting, or backup.

Why Replication?

• To keep your data safe
• High (24*7) availability of knowledge
• Disaster recovery
• No downtime for maintenance (like backups, index rebuilds, compaction)
• Read scaling (extra copies to read from)
• Replica set is transparent to the application

How Replication Works in MongoDB

MongoDB achieves replication by the use of replica set. A replica set is a group of mongod instances that host the same data set. In a replica, one node is primary node that receives all write operations. All other instances, such as secondaries, apply operations from the primary so that they have the same data set. Replica set can have only one primary node.
• Replica set is a group of two or more nodes (generally minimum 3 nodes are required).
• In a replica set, one node is primary node and remaining nodes are secondary.
• All knowledge replicates from primary to secondary node.
• At the time of automatic failover or maintenance, election establishes for primary and a new primary node is elected.
• After the recovery of failed node, it again join the replica set and works as a secondary node.
A typical diagram of MongoDB replication is shown in which client application always interact with the primary node and the primary node then replicates the knowledge to the secondary nodes.

Replica Set Features

• A cluster of N nodes
• Any one node are often primary
• All write operations go to primary
• Automatic failover
• Automatic recovery
• Consensus election of primary

Set Up a Replica Set

In this tutorial, we will convert standalone MongoDB instance to a replica set. To convert to replica set, following are the steps −
• Shutdown already running MongoDB server.
•
• Start the MongoDB server by specifying — replSet option. Following is the basic syntax of –replSet −
mongod –port “PORT” –dbpath “YOUR_DB_DATA_PATH” –replSet “REPLICA_SET_INSTANCE_NAME”
Example
mongod –port 27017 –dbpath “D:\set up\mongodb\data” –replSet rs0
• It will start a mongod instance with the name rs0, on port 27017.
• Now start the command prompt and connect to this mongod instance.
• In Mongo client, issue the command rs.initiate() to initiate a new replica set.
• To check the replica set configuration, issue the command rs.conf(). To check the status of replica set issue the command rs.status().

Add Members to Replica Set

To add members to replica set, start mongod instances on multiple machines. Now start a mongo client and issue a command rs.add().
Syntax
The basic syntax of rs.add() command is as follows −
>rs.add(HOST_NAME:PORT)
Example
Suppose your mongod instance name is mongod1.net and it’s running on port 27017. To add this instance to replica set, issue the command rs.add() in Mongo client.
>rs.add(“mongod1.net:27017”)
>
You can add mongod instance to replica set only when you’re connected to primary node. To check whether you’re connected to primary or not, issue the command db.isMaster() in mongo client.
So, this brings us to the end of blog. This Tecklearn ‘Deep dive into Replication process in MongoDB’ helps you with commonly asked questions if you are looking out for a job in MongoDB and No-SQL Database Domain.
If you wish to learn and build a career in MongoDB or No-SQL Database domain, then check out our interactive, MongoDB Training, that comes with 24*7 support to guide you throughout your learning period. Please find the link for course details:

MongoDB Training

MongoDB Training

About the Course

Tecklearn’s MongoDB Training helps you to master the NoSQL database. The course makes you job-ready by letting you comprehend schema design, data modelling, replication, and query with MongoDB through real-time examples. Along with this, you’ll also gain hands-on expertise in installing, configuring, and maintaining the MongoDB environment, including monitoring and operational strategies from this online MongoDB training. Upon completion of this online training, you will hold a solid understanding and hands-on experience with MongoDB.

Why Should you take MongoDB Training?

• MongoDB – a $36 billion to a $40 billion market growing at 8% to 9% annually – Forbes.com
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What you will Learn in this Course?

Introduction to MongoDB and Importance of NoSQL
• Understanding the basic concepts of RDBMS
• What is NoSQL Database and its significance?
• Challenges of RDBMS and How NoSQL suits Big Data needs
• Types of NoSQL Database and NoSQL vs. SQL Comparison
• CAP Theorem and Implementing NoSQL
• Introduction to MongoDB and its advantages
• Design Goals for MongoDB Server and Database, MongoDB tools
• Collection, Documents and Key Value Pair
• Introduction to JSON and BSON documents
• MongoDB installation
MongoDB Installation
• MongoDB Installation
• Basic MongoDB commands and operations,
• Mongo Chef (MongoGUI) Installation
Schema Design and Data Modelling
• Why Data Modelling?
• Data Modelling Approach
• Data Modelling Concepts
• Difference between MongoDB and RDBMS modelling
• Challenges for Data Modelling
• Model Relationships between Documents
• Data Model Examples and Patterns
• Model Tree Structures
CRUD Operations
• MongoDB Architecture
• CRUD Introduction and MongoDB CRUD Concepts
• MongoDB CRUD Concerns (Read and Write Operations)
• Cursor Query Optimizations and Query Behaviour in MongoDB
• Distributed Read and Write Queries
• MongoDB Datatypes
Indexing and Aggregation Framework
• Concepts of Data aggregation and types and data indexing concepts
• Introduction to Aggregation
• Approach to Aggregation
• Types of Aggregation: Pipeline, MapReduce and Single Purpose
• Performance Tuning
MongoDB Administration
• Administration concepts in MongoDB
• MongoDB Administration activities: Health check, recovery, backup, database sharing and profiling, performance tuning etc.
• Backup and Recovery Methods for MongoDB
• Export and Import of Data from MongoDB
• Run time configuration of MongoDB
MongoDB Security
• Security Introduction
• MongoDB security Concepts and security approach
• MongoDB integration with Java and Robomongo
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