Big Data - 4

Big Data - 4

Introduction to NoSQL Databases

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Introduction to NoSQL

What is NoSQL

NoSQL stands for "Not only SQL" or "Non-relational".

  • NoSQL databases provide an alternative to traditional relational databases.

  • They are designed to overcome some limitations of relational databases:

    • Scaling

    • Agility

    • Performance

    • Flexibility

  • Examples:

    • Key-Value stores: Redis, Voldemort

    • Document stores: MongoDB

    • Column stores: Cassandra, HBase

    • Graph databases: Neo4j, Titan

Differences from SQL databases

  • Schema

    • NoSQL databases have dynamic schemas

    • SQL databases have rigid schemas

  • Scaling

    • NoSQL scales horizontally

    • SQL scales vertically

  • Distribution

    • NoSQL is designed to be distributed

    • SQL is designed for single-server

  • Query language

    • NoSQL uses limited query languages

    • SQL uses powerful SQL

  • ACID compliance

    • NoSQL sacrifices ACID for performance

    • SQL provides ACID transactions

  • Data model

    • NoSQL uses non-relational models

    • SQL uses a relational model

  • Joins

    • NoSQL does not support joins like SQL

NoSQL data models

Types of NoSQL Databases - GeeksforGeeks

  • Key-value store

    • Data stored as keys associated with values
  • Document store

    • Data stored as documents (JSON/BSON) with dynamic schema
  • Column store

    • Data stored in columns rather than rows
  • Graph database

    • Data stored as nodes and edges
  • Wide column store

    • A variant of column store with column families
  • The choice of data model depends on requirements.

  • Each model has pros and cons in performance, scalability, flexibility, etc.

Business Drivers for NoSQL

Scalability

  • NoSQL databases are designed to scale horizontally by adding more servers.

  • They can scale to massive amounts of data and handle high volumes of read/write operations.

  • This makes them suitable for applications with very large and fast-growing datasets.

Flexible schema

  • NoSQL databases have dynamic or flexible schemas as opposed to the rigid schemas of SQL databases.

  • Schema changes do not require downtime and can be done on the fly.

  • This agility makes NoSQL databases suitable for rapidly evolving datasets and models.

High availability

  • Most NoSQL databases are designed with high availability and fault tolerance in mind.

  • They employ replication and data distribution techniques to ensure data is always accessible.

  • This makes them suitable for applications that require constant uptime and access to data.

Low latency

  • Since NoSQL databases do not require complex joins and transactions, they can provide lower latency.

  • This is especially true for read operations that can be served from memory.

  • Low and predictable latency makes NoSQL suitable for real-time applications that have strict response time requirements.

NoSQL Data Architectural Patterns

Key-value store

What is a Key Value Store? Definition & FAQs | ScyllaDB

  • Data is stored as a collection of key-value pairs.

  • Simple and fast data model.

  • Suitable for:

    • Caching data

    • Storing session data

    • Storing user profiles

  • Example databases: Redis, Voldemort

Document store

Relational Databases vs. NoSQL Document Databases | Lenni's Technology Blog

  • Data is stored as documents (JSON/BSON) with dynamic schema.

  • Documents have a nested structure.

  • Suitable for:

    • Storing and querying semi-structured data

    • Storing log data

    • Storing product catalogs

  • Example databases: MongoDB, Couchbase

Column store

What is a Columnar Database? Definition and Related FAQs | HEAVY.AI

  • Data is stored in columns rather than rows.

  • Columns are grouped into column families.

  • Suitable for:

    • Handling large datasets with sparse data

    • Analytical workloads with heavy read loads

  • Example databases: Cassandra, HBase

Graph database

What is a NoSQL Graph Database? | Ontotext Fundamentals

  • Data is stored as nodes and edges (relationships).

  • Suited for data with complex relationships.

  • Suitable for:

    • Social networking queries

    • Recommendation engines

    • Knowledge graphs

  • Example databases: Neo4j, Titan

The choice of data model depends on:

  • The type of data and queries

  • Performance requirements

  • Scalability requirements

  • Data relationships

Using NoSQL to Manage Big Data

Horizontal scaling

  • NoSQL databases are designed to scale horizontally by adding more servers.

  • As data and traffic grow, more servers can be added to the cluster.

  • This allows NoSQL databases to easily scale to massive amounts of big data.

Schema-less design

  • Since NoSQL databases have dynamic schemas, they can accommodate rapidly growing and changing datasets.

  • New attributes can be added to documents on the fly without affecting existing data.

  • This makes NoSQL a good fit for big data projects with evolving data models.

Real-time access

  • Many NoSQL databases are optimized for low latency and high throughput.

  • They can provide real-time access to big data by caching frequently accessed data in memory.

  • This makes NoSQL suitable for applications requiring real-time insights from big data.

Flexible queries

  • While SQL queries are powerful, they become inefficient at the big data scale.

  • NoSQL databases offer more flexible query mechanisms that can scale to massive data volumes.

  • This includes map-reduce functions, secondary indexes, and filtering on specific attributes.

Introduction to MongoDB

What is MongoDB?

  • MongoDB is a popular open-source document database (NoSQL database)

  • It stores data in flexible, JSON-like documents.

  • It is horizontally scalable and high-performance.

  • MongoDB is written in C++.

Data model - documents, collections, schemas

The MongoDB Basics: Databases, Collections & Documents | Studio 3T

  • Data is stored in documents rather than tables.

  • A document is a JSON-like data structure that consists of field-value pairs.

  • Documents have a dynamic schema - different documents in a collection do not have to have the same fields.

  • Documents with similar characteristics are grouped into collections.

  • Collections live within databases.

  • MongoDB has a dynamic schema - you do not define the schema in advance, it is defined by the data itself.

CRUD operations in MongoDB

  • Create: Use the insert() or insertOne()/insertMany() methods to insert documents into collections.

  • Read: The find() and findOne() methods are used to query documents. You can use queries, projections and sorting.

  • Update: The update() and updateOne()/updateMany() methods are used to update existing documents.

  • Delete: The remove() and deleteOne()/deleteMany() methods are used to delete documents from a collection.

MongoDB Architecture

Sharding

MongoDB Sharding | MongoDB

  • Sharding allows MongoDB to split data across multiple servers.

  • In MongoDB, sharding is done on the _id field by default.

  • A shard key is used to determine how data is distributed across shards.

  • MongoDB uses a routing algorithm to determine which shard a document belongs to based on its shard key value.

  • Sharding allows MongoDB to scale horizontally almost linearly by adding more shards.

Replication

Replication — MongoDB Manual

  • Replication in MongoDB involves copying and mirroring data on multiple servers.

  • It provides data redundancy and high availability.

  • MongoDB uses a primary-secondary replication model.

  • There is one primary node that handles writes and secondaries that handle reads.

  • When the primary node fails, a secondary is automatically elected as the new primary.

Indexing

  • Like any database, indexing improves the performance of queries in MongoDB.

  • MongoDB supports several index types:

    • Single field index

    • Compound index

    • Multikey index

    • Hashed index

    • Text index

    • Geospatial 2d and 2dsphere index

  • Indexes are created on one or more fields in a collection.

  • MongoDB automatically creates indexes on the _id field and any indexed fields.

Storing Data in MongoDB

Insert, update, and delete documents

  • Documents are inserted into collections using the insertOne() and insertMany() methods.

      db.collection.insertOne({ name: "John", age: 30 })
      db.collection.insertMany([{ name: "Mary", age: 25 }, { name: "Steve", age: 35 }])
    
  • Documents can be updated using the updateOne() and updateMany() methods.

      db.collection.updateOne({ name: "John" }, { $set: { age: 31 } })
    
  • Documents can be deleted using the deleteOne() and deleteMany() methods.

      db.collection.deleteOne({ name: "John" })  
      db.collection.deleteMany({ age: { $lt: 25 } })
    

Embedded documents

  • MongoDB supports embedding related data as sub-documents within a document.

  • This is useful when the related data has a one-to-one or one-to-few relationship.

  • For example, a blog post may have embedded comments:

      {
        "_id": 1,
        "title": "My First Blog",
        "body": "Lorem ipsum...",
        "comments": [
          {
            "user": "john", 
            "message": "Great blog!" 
          },
          {
            "user": "jane",
            "message": "Nice write up!"  
          }  
        ]
      }
    

Referencing other documents

  • MongoDB also supports referencing related data by storing the ID of the related document.

  • This is useful when the related data has a one-to-many relationship.

  • For example, a user document may reference many post documents:

      { "name": "John", "posts": [ObjectId("5b11ca86aefd4f0474fcc4bb"), ObjectId("5b11d1cdaefd4f0474fcc4bd")] }
    
      { "_id": ObjectId("5b11ca86aefd4f0474fcc4bb"), "title": "My First Blog" }
      { "_id": ObjectId("5b11d1cdaefd4f0474fcc4bd"), "title": "My Second Blog" }
    

Querying MongoDB Data

find() and findOne()

The find() method is used to query documents in a collection. It returns a cursor to the matched documents.

The findOne() method returns only one document and is useful when you want to retrieve a single document that matches the query criteria.

Basic queries in MongoDB use the same selectors as JSON:

  • Equality: {"name": "John"}

  • Comparison: {"age": {$gt: 30}}

  • Logical: {"$or": [{"age": 18}, {"age": 30}]}

  • Regular expression: {"name": /^J/}

For example:

  db.users.find({"age": {$gt: 30}})
  db.users.findOne({"name": "John"})

Projection

Projection allows you to specify which fields to include or exclude in the result documents.

You can project fields using the 1 to include and 0 to exclude:

  db.users.find({}, {"name": 1, "age": 1, "_id": 0})

Sorting, limiting and skipping results

You can sort the results of a query using the sort() method:

  db.users.find().sort({"age": 1})  // Sort by age in ascending order

You can limit the number of results using the limit() method:

  db.users.find().limit(5)  // Limit to 5 results

You can skip the first n results using the skip() method:

  db.users.find().skip(5)  // Skip first 5 results

Aggregation framework

The aggregation framework allows you to perform aggregations like:

  • $group to group by some criteria and apply aggregate functions

  • $match to filter the data

  • $project to transform the data

  • $sort to sort the data

  • And many more stages.

For example, to calculate the average age by gender:

  db.users.aggregate([
    {
      $group: {
        _id: "$gender", 
        avgAge: { $avg: "$age" } 
      }
    }
  ])

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