MongoDB

Created by Cherry Lee / @Happy Tech Group

Contents

• Introduction
• Replication
• Sharding

Introduction

MongoDB is an open-source document database that provides high perfotmance, high availability, and automatic scaling.

Key Features

• High Performance
• High Availability
• Automatic Scaling

BSON

A record in MongoDB is a document, which is a data structure composed of field and value pairs. MongoDB documents are similar to JSON objects.

SQL to MongoDB

SQL to MongoDB Aggregation

Executable

Create and Drop

Insert

Select

Select

Select

Select

Start

Start MongoDB with a configuration file.

mongod --config /etc/mongodb.conf

Replication

A replica set is a group of mongod instances that host the same data set.

Replication set Members

• Primary
• Secondary
• Arbiter

Primary

• The only member receives write operations
• Only one

Secondary

• Maintain a copy
• Can become a primary
• One or more secondaries
• Specific: priority 0; hidden; delayed

Priority 0 Secondary

• Can not become primary

Hidden Secondary

• Must be priority 0
• Cannot read
• For backup

Delayed Secondary

• Must be hidden
• Keep an earlier or delayed state of the set
• For rollback

Replication Set Election

Election Triggering Events

An election happens when:

• The initiation of a new replica set
• A secondary loses contact with a primary
• A primary step down

Election Factors and Conditions

• Heartbeats
• Priority
• Optime
• Connections
• Network Partitions

Sharding

Sharding is a method fot storing data across multiple machines.

Components

Shards

• Hold a subset of a collection's data
• Each shard is a mongod instance or a replication set

Query Routers

• Process and target operations to shards
• Return results to clients
• Each query router is a mongos instance

Config Servers

• Store the cluster's metadata which maps chunks to shards
• The query router uses metadata to target opeartions to specific shards
• Each config server is a mongod instance

Shard Keys

The shard key determines the distribution of the collection’s documents among the cluster’s shards.

Shard Collection Balancing

Balancing is the process MongoDB uses to distribute data of a collection evenly across a cluster.

Chunk Migration

Chunk migration moves the chunks of a shard collection from one shard to another and is part of the balancer process.

Chunk Splits

As chunks grow beyond the specified chunk size a mongos instance will attempt to split the chunk in half. 

More

• MongoDB Org
• MongoDB In Action
• MongoDB:The.Definitive.Guide

The End

Created by Cherry Lee / @Happy Tech Group