SQL vs NoSQL

Priyanka Sarode — Fri, 23 May 2025 20:29:12 +0000

In today’s data world, there are two databases mostly used in the market to perform operations ( add, retrieve, update and delete) on data :- SQL database and NoSQL database

SQL

SQL database stores data in structured way means in the form of rows and columns (tabular format like Excel). And that data doesn’t change frequently such as bank transaction.
It uses SQL (structured query language). e.g. to retrieve products from products table where price is greater than 1000 Rs, SQL query would be

select * from products where price > 1000

It follows a fixed schema⁣⁣ - need to define all fields first. It ensures consistency and predictability.
SQL databases scales vertically meaning you can increase the load on a single server by adding more CPU, RAM, or SSD capacity i.e. you have to increase hardware..
Also SQL queries helps in deriving insights from data.

NoSQL

NoSQL stores data in unstructured way means it is stored in JSON-like documents, key-value pairs, etc.⁣ and data changes frequently such as social media posts.
NoSQL databases don’t have a single standardized query language. Their queries depending upon database type. e.g. to retrieve products from products table where price is greater than 1000 Rs, MongoDB query would be

db.products.find({ price: { $gt: 1000 } })

It follows flexible schema - no need to define everything upfront⁣. You can add fields later on as needed.
NoSQL databases scales horizontally, meaning they use multiple servers to handle increased workloads
In NoSQL, you can’t derive insights effectively as data changes dynamically.

Examples

If you go to Flipkart site and in one tab select men’s t-shirt and in second tab select any brand of mobile, you will get to see different fields in Product Details section for both products even if it comes under product database table. So in this case, Flipkart uses NoSQL database.
Netflix uses hybrid model. For user and billing data, it uses SQL because for every user fields/ schema won’t change like username, password, email. And for recommendation data it uses NoSQL because recommendation movie cards that display on each user page are different as per user’s interest or likings.
Uber also uses NoSQL because if company introduces new feature later on, so need to add that additional fields in database too.

While SQL follows ACID properties, NoSQL follows the CAP theorem

CAP theorem states that you can only guarantee two out of three properties in a distributed system.

Consistency - with every request I am going to fetch latest response for you
Availability - with every request I am going to fetch some result even if it is not recent one.
Partition tolerance: The system keeps working even if network issues occur

e.g.
1) Banking follows CP - User need to enter correct account data. If some data is wrong, cancel that transaction.
2) Uber follows AP - If there is delay in locating vehicle's exact position, show some nearby location. Means it works on availability basis.

Conclusion
SQL is best for banking, inventory systems, CRMs⁣
Examples of SQL database are Oracle, MySQL, PostgreSQL, SQLite, SQL Server⁣
Choose SQL when you need strict consistency.⁣

NoSQL is best for real-time apps, social media, IoT⁣ etc.
Examples: MongoDB, Cassandra, DynamoDB⁣, Redis, HBase, Neo4j
Choose NoSQL when you need flexibility.

Web Storage API (localStorage, sessionStorage) & cookie

Priyanka Sarode — Tue, 22 Apr 2025 07:57:02 +0000

We all know that for client and server communication HTTP protocol is used. But this protocol is stateless means it doesn't track any response by the server or request by the client. To resolve this problem, cookies, localStorage & sessionStorage come into picture to manage and track data between client-server requests.

localStorage and sessionStorage both are part of Web Storage API provided by browser and introduced in HTML5 to store the data in client-side computer.
Cookie are not a part of web storage API as they were introduced way before web storage API. They are used in both client and server side communication.

We will see each one of behaviors, when to use with example.

1) Cookie

cookies are small piece of plain text data to remember/ persist user's information across sessions on the web.
They are sent to server with each request so that server can identify particular user is authenticated user or not and provide personalized content to him/ her.
They can storage 4KB of data per cookie means can store small amount of data.
They are typically used for authentication, session management, storing login status, user preferences and tracking user's behavior on the site.
They can be set to expire at a specific period of time or can be deleted manually.
They can be accessed by other domains that user visits without user consent. This is the main concern by cookie. We can't store sensitive data in the cookie. Yes we can use HttpOnly, Secure, and Expires/Max-Age attributes that cookies provide to prevent/ leakage of the user's information in the web.
Cookies doesn't expire on closing the browser but will expire at a specific date (Expires) or length of time (Max-Age) .
Mainly suitable for client-server communication

Example - When user login for the first time, the server can create a cookie to remember their username, unique session_ID etc. If the user visits the same website again in future, the browser sends the cookie back to the server with every request. The server can then use the information in the cookie to identify the user and personalize their experience on the site. But here whenever user sends request to server, api call is made and it costs a lot. Some user will experience a significant delay if network is slow.

//setting cookie
document.cookie = "username=John; expires=Fri, 31 Dec 2025 12:00:00 UTC;

2) localStorage

stores data in key-value pairs as string in user's browser.
here data persists even after user closes their browser tab/ window or on page reloades/ refreshes/ reopend. Means it doesn't have expiry date. It remains until user explicitly remove the data through code or clearing by double clicking on that data in Application tab of browser developer tool or by clearing browser's cache.
We can access localstorage data in multiple tabs, different windows if it has the same url.
It can store data upto 5MB or 10MB depending on the browser.
Its data is never transferred to the server but you can manually include its value in fetch request.
It is synchronous. If I perform operations on localStorage, It will block the execution of other javascript code until the current operation will get completed.
It follows same origin policy (per-origin) means http://abc.com’s localStorage object is different from https://blog.abc.com’s localStorage object because the two origins use different protocols (http and https), different hosts (abc and blog.abc). So data will be different for these two sites.
localStorage is a property of the global Window object. Therefore, localStorage.setItem() is equivalent to window.localStorage.setItem().
Use localStorage when you need to store simple, persistent data on a user's device for a longer period of time.

Methods of localStorage

// Save data
localStorage.setItem("key", "value");

localStorage.setItem("username", "Alice");

// Retrieve data
localStorage.getItem("key");

localStorage.getItem("username")
console.log(username)

// Remove data
localStorage.removeItem("key");

localStorage.removeItem("username");

// Clear all data from localStorage
localStorage.clear();

If I store same key with different value, then data will be override with the previous ones and get new value as result.

localStorage.setItem("username", "Alice");
localStorage.setItem("username", "John");

localStorage.getItem("username")
//output - John

The removeItem() method will do nothing if its key does not exist in the local storage. You will get output as undefind.

localStorage.removeItem("username");
//output - undefined

And getItem() method will return null if its key doesn't exist and I try to access that key, it will return null.

localStorage.getItem("username");
//output - null

We can store data as string in localStorage, but what if I want to store object in it. If I do below like this, it will return "[object Object]".

localStorage.setItem("user", {"name": "Alice"});

We need to convert object into string before storing it into localStorage by using JSON.stringify()

const user = {"name": "Alice"}
localStorage.setItem("User", JSON.stringify(user));

And to get the object data from localStorage, again we need to convert string to its original form by using JSON.parse()

JSON.parse(localStorage.getItem("User")))

Usage:

You can use it to create and store notes in user's computer
most suitable for user preferences data like theme, layout or language settings that doesn't need to be sent to server
to store form inputs so users don’t lose data when they refresh the page.
to keep user authentication tokens for faster login whenever visit the website next time.

Example:- When login in social media application or e-commerce website, your login credentials like username, password (same for longer period of time) gets stored in localStorage. So every time you come back, application doesn't require to make api call to get account details & retrieve data from it. It helps to save api call.

3) sessionStorage

It does almost the same work, has the same methods like localStorage but with a few minor differences mentioned below: -

sessionStorage stores the data temporarily as string in key-value pairs in client side but for a single/ current session/ when a tab is open.
It's data gets cleared when user closes the tab/ window/ page session ends/ refreshes that page. i.e. it follows per-instance policy plus same origin policy.
Here browser creates unique page session with each tab/ window. Hence it creates new instance/ object of sessionStorage for an app without interfering other instances of the same app.
It can store 5MB of data means it has a higher storage capacity compared to cookies.
It's useful for storing temporary data that is needed when user is on that website like session-based authentication details that shouldn't persist beyond the current session.
To store large amount of data, use database instead.

Methods of sessionStorage

// Save data
sessionStorage.setItem("username", "John");

// Retrieve data
let username = sessionStorage.getItem("username");

// Remove data
sessionStorage.removeItem("username");

// Clear all data
sessionStorage.clear();

Usage:

to store authentication tokens for a single session
to store user's input in multi-step form. In this case user need that input data for that session only and don't want to lose it when switching between pages.

Tricky Question

Q) Is there any way you can persist sessionStorage data for a longer period of time?
Ans - Yes. We can set manual expiration time in it.

let now = new Date().getTime();
let expiry = now + 1000 * 60 * 30; // Expires in 30 minutes
sessionStorage.setItem("expiryTime", expiry);

Differences and similarity between cookie, localStorage and sessionStorage

	cookie	localStorage	sessionStorage
data accessible	any window	any window	same tab
data persistence	depends on expires/ max-age attribute	data persists even after closing the session	data is lost when session is over
storage capacity	4KB of storage	5-10MB per origin depends on browser	5MB per origin
expire	manually set using expires attribute	till explicitly delete by code/ user	when session ends/ explicitly delete by code/ user
data transfer to server	yes	no	no
can read and write data	client and server	client	client
accessible in domain/ origin	different	same	same
supported by all browsers including older ones	yes	yes	yes
web storage api (introduced in html5)	no	yes	yes
synchronous	yes	yes	yes
property of window object	indirectly (window.document.cookie)	yes	yes
usage	for client-server communication	for data that doesn't need to be sent to server	for storing session-relevant data

DEV Community: Priyanka Sarode

SQL vs NoSQL

Web Storage API (localStorage, sessionStorage) & cookie

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