Stored Procedure (SP) pagination for a data grid

Implementing Stored Procedure (SP) pagination for a data grid involves retrieving only a specific "page" of data from the database, rather than the entire dataset, which significantly improves performance and reduces network traffic. The modern and most efficient method in SQL Server (2012 and later) is using the OFFSET...FETCH NEXT clause. 

Stored Procedure Implementation (SQL Server)

The stored procedure needs input parameters for the page number, page size, and an ORDER BY clause (which is mandatory for OFFSET...FETCH). It may also include an output parameter for the total record count, which is useful for displaying navigation controls on the front end. 

sql

CREATE OR ALTER PROCEDURE [dbo].[uspGetPagedData]
    @PageNumber INT,
    @PageSize INT,
    @TotalRecords INT OUTPUT
AS
BEGIN
    SET NOCOUNT ON;

    -- Get the total number of records
    SELECT @TotalRecords = COUNT(*) FROM [YourTableName];

    -- Retrieve the specific page of data
    SELECT *
    FROM [YourTableName]
    ORDER BY [YourSortColumn] ASC -- ORDER BY is required for OFFSET/FETCH
    OFFSET (@PageNumber - 1) * @PageSize ROWS
    FETCH NEXT @PageSize ROWS ONLY;
END

• @PageNumber, @PageSize: Input parameters to determine which data to retrieve.
• @TotalRecords: An output parameter to return the total count of records in the table, enabling the UI to calculate the total number of pages.
• OFFSET (@PageNumber - 1) * @PageSize ROWS: Skips the appropriate number of rows based on the current page and size.
• FETCH NEXT @PageSize ROWS ONLY: Selects only the number of rows specified by the page size. 

Object Vs var Vs Dynamic Type in C#

 https://www.c-sharpcorner.com/UploadFile/ff2f08/object-vs-var-vs-dynamic-type-in-C-Sharp/

Introduction to object

The object class is the base class for all ohter classes; in other words all derived types are inherited from the object base type. We can assign values of any type to the object type variable (when the value type is converted to an object it is called "boxing" and when the variable type object is converted into any other value type is called "unboxing").

Example

object a = 10;
object b = "This is test string";
object c = a;

//Below is also possible with object type
object k = "jignesh";
k = 10;

C#

Introduction to var keyword

The var keyword was introduced in C# 3.0. It is an implicit type and the type is determined by the C# compiler. There is no performance penalty when using var. The C# compiler assigns types to the variable when it assigns the value.

Example

var a = 10; // it is same as int a = 10;
var b = "This is test string";
var c = b; // gives compilation error.

var words = new string[] { "apple", " banana", "peach", " grape" };

C#

It makes programs shorter and easier to read. In short the var keyword is an implicit way of defining data types. It is an indirect way of defining the variable type. The “var” keyword defines the data type statically (at compile time) and not at runtime.

The var keyword is very useful when.

1. We have a long class name so that our code is not so readable. var makes it short and sweet.
2. We use LINQ and anonymous types that help us reduce the code (reduce effort to create a new class).

Introduction to Dynamic Keyword

C# 4.0 introduced a new type called "Dynamic". Dynamic is a new static type that is not known until runtime. At compile time, an element with dynamic is assumed to support any operation so that we need not be worried about the object, whether it get the value from COM, the DOM, from reflection or anything else in the program. Errors are caught at runtime only. The dynamic type in C# uses the Dynamic Language Runtime (DLR) introduced as a new API in the .Net Framework 4.0. Type dynamic behaves like an object in most cases, but the expression of type dynamic is not checked by the compiler. The type's dynamic status is compiled into a variable type object that is dynamic at compile time, not at run time.

At compilation a dynamic is converted into a System.object and the compiler will emit the code for type safety during runtime. It suffers from boxing and unboxing because it is treated as a System.Object and also the application's performance will suffer siince the compiler emits the code for all the type safety.

Example

dynamic i = 10;
dynamic s = "Jignesh Trivedi";

string p = "This is test";
dynamic k = p;

dynamic other = 10; //assign int value so run time type is integer

other = "Jignesh"; // now runtime type of other varible become string.

C#

We can define a dynamic variable, property, field and parameter, return value, type constraint and so on.

// A dynamic field.
static dynamic field;

// A dynamic property.
dynamic name { get; set; }

// A dynamic return type and a dynamic parameter type.
public dynamic Method(dynamic parameter)
{
    dynamic local = "return string";
    int other = 0;

    if (parameter is string)
    {
        return local;
    }
    else
    {
        return other;
    }
}

C#

	Object	VAR	Dynamic
Introduce in Framework	C# 1.0	C#3.0	C#4.0
Stored value type	It can store any type of value because it is base class i.e. All types in .net framework are derived from object.	"It can store any type of value, but We must initialize ""var"" types at the time of declaration."	It can store any type of value just like old VB language variables.
The compiler has information about the type?	Compiler has very little information about the type.	It is type safe. It means that the compiler has all information about the type so that there is no issue at run time.	It is not type safe. It means that compiler doesn't have any information about the type of variable.
Casting	Need to cast object variable to original type when use it.	No need to cast because the compiler has all information about the type.	Casting is not required, but we need to know the properties and methods related to stored type because intelligence is not available for dynamic type.
Intellisense support?	No	Yes	No
Cause the problem at run time?	Yes. It throws runtime exception when compiler is not able to convert values from stored value type to required type.	No, because type is already known by compiler.	Yes. It throws runtime exception When properties or method name are mismatched from original.
Usefulness	It is very useful when we don't have more information about the type.	It is useful when we don't know the actual type.	It is very useful when we are using dynamic language for coding or use COM component in our code.
Does it pass to a method or create typed properties?	Yes. It can be passed as method parameter and method also can return object type.	"No, we cannot declare a method parameter or return type as ""VAR"""	"Yes, we can declare a method parameter or return type as ""VAR"""

 This is a very important real-time EF Core question.

Let’s break it down clearly, practically, and safely, exactly how it’s done in production systems.

---

How to Migrate in EF Core Without Data Loss

(Adding / Removing Columns Safely)

---

Golden Rule (Interview Answer)

EF Core migrations never delete data unless you explicitly tell them to.
Data loss happens only when:

• You drop a column/table

• You change data type incorrectly

• You don’t provide defaults for non-nullable columns

---

1️⃣ Adding a Column WITHOUT Data Loss (Safe Way)

Scenario

Existing Users table already has data.
Now you want to add a new column.

---

✅ Case 1: Add Nullable Column (Safest)

public string PhoneNumber { get; set; }

Migration generated

migrationBuilder.AddColumn<string>(
    name: "PhoneNumber",
    table: "Users",
    nullable: true);

✔️ No data loss
✔️ Existing rows remain untouched

👉 Recommended approach

---

✅ Case 2: Add Non-Nullable Column (Correct Way)

public bool IsActive { get; set; }

❌ This will fail if table already has rows.

✔️ Correct Solution – Provide Default Value

migrationBuilder.AddColumn<bool>(
    name: "IsActive",
    table: "Users",
    nullable: false,
    defaultValue: true);

✔️ Existing rows get default value
✔️ No data loss

---

🚫 WRONG WAY

public bool IsActive { get; set; } // no default

❌ Migration fails
❌ Causes runtime exception

---

2️⃣ Removing a Column WITHOUT Data Loss (Safe Strategy)

Important Truth

Dropping a column ALWAYS deletes data
There is no automatic safe delete.

So how do real projects do it?

---

✅ Strategy 1: Soft Remove (Recommended)

Instead of deleting column:

public bool IsDeprecated { get; set; }

✔️ Column remains
✔️ App stops using it
✔️ Data preserved

---

✅ Strategy 2: Two-Step Migration (Production Safe)

Step 1: Backup Data

SELECT OldColumn INTO Users_OldColumn_Backup FROM Users;

Step 2: Drop Column

migrationBuilder.DropColumn(
    name: "OldColumn",
    table: "Users");

✔️ No business data lost
✔️ Recoverable if needed

---

✅ Strategy 3: Move Data Before Removing Column

migrationBuilder.Sql(
    "UPDATE Users SET NewColumn = OldColumn");

Then drop old column.

✔️ Data preserved
✔️ Schema cleaned

---

3️⃣ Renaming Column (Common Data Loss Mistake)

❌ Wrong Way

public string MobileNumber { get; set; } // renamed from PhoneNumber

EF Core thinks:

• Drop PhoneNumber

• Add MobileNumber

➡️ ❌ DATA LOSS

---

✅ Correct Way – Rename Column

migrationBuilder.RenameColumn(
    name: "PhoneNumber",
    table: "Users",
    newName: "MobileNumber");

✔️ Data retained
✔️ Safe migration

---

4️⃣ Changing Column Data Type (Safely)

❌ Risky

public int Age { get; set; } // previously string

✅ Safe Migration

migrationBuilder.Sql(
    "UPDATE Users SET Age = TRY_CAST(Age AS INT)");

Then change column type.

✔️ Converts data
✔️ Prevents failure

---

5️⃣ Best Practices to Avoid Data Loss (Real-World)

✔️ Always review migration script

dotnet ef migrations script

✔️ Take DB backup before prod migration

✔️ Never use EnsureCreated() in prod

✔️ Avoid auto-migrate on app startup

✔️ Use feature-based migrations

---

6️⃣ Real Interview Answer (Concise)

To migrate without data loss in EF Core, we add nullable or default-valued columns, rename columns using RenameColumn, migrate data using SQL before dropping columns, and never delete schema elements directly in production without backup.

---

7️⃣ Quick Safety Checklist

Operation	Safe?	How
Add column	✅	Nullable or default
Rename column	✅	RenameColumn
Remove column	❌	Backup or soft delete
Change type	⚠️	Data conversion SQL
Drop table	❌	Archive first

---

If you want next:

• 🔥 Zero-downtime production migration

• 🔥 Blue-green DB migration strategy

• 🔥 EF Core migration rollback

• 🔥 Handling huge tables (millions of rows)

• 🔥 Migration interview questions

Just say the word 👍

Entity Framework core code first migration steps in detailed with example

 Below is a very detailed, step-by-step explanation of EF Core Code-First migrations with a complete real-time example, exactly how it’s expected in enterprise projects and interviews.

---

EF Core Code-First Migration – Detailed Steps with Example

---

What is Code-First in EF Core?

Code-First means:

• You design C# entities first

• EF Core generates the database schema

• Migrations track schema changes over time

Database = output
C# code = source of truth

---

Real-Time Scenario

We are building a User Management API.

Initial Requirements

• Create Users table

• Later add Email, DateOfBirth, IsActive

---

Step 1: Create Web API Project

dotnet new webapi -n UserManagement.API
cd UserManagement.API

---

Step 2: Install Required Packages

dotnet add package Microsoft.EntityFrameworkCore
dotnet add package Microsoft.EntityFrameworkCore.SqlServer
dotnet add package Microsoft.EntityFrameworkCore.Tools

📌 Tools is mandatory for migrations

---

Step 3: Create Entity Classes (Models)

📁 Entities/User.cs

using System.ComponentModel.DataAnnotations;

public class User
{
    [Key]
    public int Id { get; set; }

    [Required]
    [MaxLength(100)]
    public string Name { get; set; }

    [MaxLength(200)]
    public string Email { get; set; }

    public DateTime DateOfBirth { get; set; }

    public bool IsActive { get; set; }
}

---

Step 4: Create DbContext

📁 Data/AppDbContext.cs

using Microsoft.EntityFrameworkCore;

public class AppDbContext : DbContext
{
    public AppDbContext(DbContextOptions<AppDbContext> options)
        : base(options)
    {
    }

    public DbSet<User> Users { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<User>()
            .Property(x => x.IsActive)
            .HasDefaultValue(true);
    }
}

---

Step 5: Configure Connection String

📁 appsettings.json

"ConnectionStrings": {
  "DefaultConnection": "Server=.;Database=UserManagementDb;Trusted_Connection=True;TrustServerCertificate=True"
}

---

Step 6: Register DbContext (Program.cs)

builder.Services.AddDbContext<AppDbContext>(options =>
    options.UseSqlServer(
        builder.Configuration.GetConnectionString("DefaultConnection")));

---

Step 7: Add Initial Migration

dotnet ef migrations add InitialCreate

What EF Core Does

✔️ Scans entity classes
✔️ Compares model vs empty database
✔️ Generates migration files

---

Generated Migration (Simplified)

📁 Migrations/xxxx_InitialCreate.cs

protected override void Up(MigrationBuilder migrationBuilder)
{
    migrationBuilder.CreateTable(
        name: "Users",
        columns: table => new
        {
            Id = table.Column<int>(nullable: false)
                .Annotation("SqlServer:Identity", "1, 1"),
            Name = table.Column<string>(maxLength: 100, nullable: false),
            Email = table.Column<string>(maxLength: 200, nullable: true),
            DateOfBirth = table.Column<DateTime>(nullable: false),
            IsActive = table.Column<bool>(nullable: false, defaultValue: true)
        },
        constraints: table =>
        {
            table.PrimaryKey("PK_Users", x => x.Id);
        });
}

---

Step 8: Apply Migration to Database

dotnet ef database update

✔️ Database created
✔️ Tables created
✔️ __EFMigrationsHistory table created

---

Step 9: Change Entity (New Business Requirement)

Add PhoneNumber field.

📁 User.cs

[MaxLength(15)]
public string PhoneNumber { get; set; }

---

Step 10: Create New Migration

dotnet ef migrations add AddPhoneNumberToUser

Generated Migration

protected override void Up(MigrationBuilder migrationBuilder)
{
    migrationBuilder.AddColumn<string>(
        name: "PhoneNumber",
        table: "Users",
        maxLength: 15,
        nullable: true);
}

protected override void Down(MigrationBuilder migrationBuilder)
{
    migrationBuilder.DropColumn(
        name: "PhoneNumber",
        table: "Users");
}

---

Step 11: Update Database Again

dotnet ef database update

✔️ Schema updated
✔️ Existing data preserved

---

Step 12: Verify Database

Tables:

• Users

• __EFMigrationsHistory

---

Step 13: Rollback a Migration (If Needed)

dotnet ef database update InitialCreate

or

dotnet ef database update 0

---

Step 14: Remove Last Migration (Not Applied)

dotnet ef migrations remove

📌 Only works if migration not applied

---

Step 15: Common EF Core Migration Commands

dotnet ef migrations list
dotnet ef migrations script
dotnet ef database update
dotnet ef migrations remove

---

Step 16: Data Seeding (Optional)

modelBuilder.Entity<User>().HasData(
    new User { Id = 1, Name = "Admin", Email = "admin@test.com", IsActive = true }
);

Then:

dotnet ef migrations add SeedUsers
dotnet ef database update

---

Real-Time Best Practices

✔️ One migration per feature
✔️ Never edit applied migrations
✔️ Avoid auto-migrate in production
✔️ Review generated SQL
✔️ Always keep migrations in Git

---

Common Interview Questions

Q: Difference between EnsureCreated() and migrations?
➡️ EnsureCreated() skips migrations – NOT for production

Q: Where migration history stored?
➡️ __EFMigrationsHistory

---

One-Line Interview Summary

EF Core Code-First migration allows developers to evolve database schema incrementally using C# models while maintaining version control and data safety.

---

If you want, I can next explain:

• 🔥 How EF Core tracks model changes internally

• 🔥 Multiple DbContext migrations

• 🔥 Zero-downtime production migrations

• 🔥 Handling breaking changes

• 🔥 EF Core migration interview Q&A

Just tell me 👍

PLease describe in detailed with example in .net core if one of the microservice failed then how to rollback the tansaction in orchestrator saga pattern

 Below is a step-by-step, real-world explanation of Orchestrator Saga Pattern with rollback using .NET Core, exactly the way it is implemented in production systems and expected in senior interviews.

---

🔷 Problem Statement

You have multiple microservices, each with its own database.

Scenario:
Order → Payment → Inventory → Shipping

👉 If any service fails, you must rollback previously completed steps.

❌ You cannot use DB transactions across services
✅ You must use Saga + Compensating Transactions

---

🔷 What Is Orchestrator Saga Pattern?

A central Saga Orchestrator controls:

• The order of service calls

• Success/failure handling

• Compensation (rollback) logic

The orchestrator does NOT share DBs
It only invokes APIs / publishes commands

---

🔷 High-Level Flow Diagram

Client
  |
  v
Saga Orchestrator
  |
  |---> Order Service (Create Order)
  |---> Payment Service (Charge)
  |---> Inventory Service (Reserve)
  |---> Shipping Service (Create Shipment)

Failure Example

❌ Inventory fails → Orchestrator:

• Refund Payment

• Cancel Order

---

🔷 Step-by-Step Implementation in .NET Core

---

1️⃣ Order Service (Local Transaction)

Order Entity

public class Order
{
    public Guid Id { get; set; }
    public string Status { get; set; } // Pending, Confirmed, Cancelled
}

Order Controller

[HttpPost("create")]
public async Task<IActionResult> CreateOrder()
{
    var order = new Order { Id = Guid.NewGuid(), Status = "Pending" };
    _db.Orders.Add(order);
    await _db.SaveChangesAsync();
    return Ok(order.Id);
}

[HttpPost("cancel/{orderId}")]
public async Task<IActionResult> CancelOrder(Guid orderId)
{
    var order = await _db.Orders.FindAsync(orderId);
    order.Status = "Cancelled";
    await _db.SaveChangesAsync();
    return Ok();
}

---

2️⃣ Payment Service

[HttpPost("charge")]
public async Task<IActionResult> Charge(Guid orderId)
{
    // Simulate payment
    if (Random.Shared.Next(1, 10) < 3)
        return BadRequest("Payment Failed");

    _db.Payments.Add(new Payment { OrderId = orderId, Status = "Paid" });
    await _db.SaveChangesAsync();
    return Ok();
}

[HttpPost("refund")]
public async Task<IActionResult> Refund(Guid orderId)
{
    var payment = await _db.Payments.FirstAsync(p => p.OrderId == orderId);
    payment.Status = "Refunded";
    await _db.SaveChangesAsync();
    return Ok();
}

---

3️⃣ Inventory Service

[HttpPost("reserve")]
public async Task<IActionResult> Reserve(Guid orderId)
{
    // Simulate failure
    throw new Exception("Inventory not available");
}

---

4️⃣ Saga Orchestrator (Core Logic)

Orchestrator Controller

[HttpPost("place-order")]
public async Task<IActionResult> PlaceOrder()
{
    Guid orderId = Guid.Empty;

    try
    {
        // 1. Create Order
        orderId = await _orderClient.CreateOrder();

        // 2. Payment
        await _paymentClient.Charge(orderId);

        // 3. Inventory
        await _inventoryClient.Reserve(orderId);

        // 4. Shipping
        await _shippingClient.Create(orderId);

        return Ok("Order Completed");
    }
    catch (Exception ex)
    {
        await Compensate(orderId);
        return StatusCode(500, ex.Message);
    }
}

---

5️⃣ Compensation Logic (ROLLBACK)

private async Task Compensate(Guid orderId)
{
    if (orderId == Guid.Empty) return;

    try
    {
        await _paymentClient.Refund(orderId);
    }
    catch { /* log */ }

    try
    {
        await _orderClient.CancelOrder(orderId);
    }
    catch { /* log */ }
}

👉 Each compensation is its own API call
👉 Each service performs local DB rollback

---

🔷 Important Production Considerations ⭐

---

1️⃣ Idempotency (VERY IMPORTANT)

Compensation might be called multiple times.

if (payment.Status == "Refunded")
    return Ok();

---

2️⃣ Retry with Polly

Transient failures happen.

Policy
  .Handle<HttpRequestException>()
  .WaitAndRetryAsync(3, retry => TimeSpan.FromSeconds(2));

---

3️⃣ Timeout & Circuit Breaker

Avoid hanging orchestrator.

.TimeoutAsync(TimeSpan.FromSeconds(5))
.CircuitBreakerAsync(3, TimeSpan.FromMinutes(1));

---

4️⃣ Saga State Table (Recommended)

SagaState
{
    SagaId,
    OrderId,
    StepCompleted,
    Status
}

So recovery can happen after crash.

---

5️⃣ Event-Based Orchestrator (Better Scalability)

Instead of HTTP:

• Azure Service Bus

• Kafka

• RabbitMQ

---

🔷 Why This Works

• No distributed transaction

• Each service owns its data

• Rollback via compensating transactions

• Eventual consistency

---

🔷 Interview-Ready Answer (Short)

“In orchestrator saga pattern, a central service controls the workflow. Each microservice performs a local transaction. If any service fails, the orchestrator triggers compensating transactions—like refunding payment or canceling the order—to logically rollback the system and achieve eventual consistency.”

---

🔷 When to Use Orchestrator Saga

Scenario	Use
Complex workflows	✅
Payment flows	✅
High observability needed	✅
Simple async flows	❌

---

If you want next, I can explain:

• Event-driven saga using Kafka/Azure Service Bus

• Saga persistence & recovery

• Real production pitfalls

• Comparison with choreography saga

Just tell me 👍

Promise vs Observable

 

console.log('start');

Promise.resolve().then(() => console.log('promise'));

console.log('end');

    const promise = new Promise((resolve,reject)=>{

      console.log("Promise executed");

      resolve(100);

      resolve(200);

      resolve(300);

    });

    promise.then((val)=>{

  console.log("Promise value: ", val);

 });

    const testobs = new Observable(sub=>{

      console.log("Observable executed");

      sub.next(200);

      sub.next(300);

      sub.next(400);

      sub.complete();

    });

    testobs.subscribe((val)=>{

      console.log("Observable value: ", val);

    });

 Below is a clear, interview-ready and practical explanation of the Circuit Breaker pattern in .NET Core, along with real-time examples and code using Polly (the standard library for resilience in .NET).

---

✅ Circuit Breaker Pattern in .NET Core

What it is

Circuit Breaker is a resilience pattern used to prevent your system from repeatedly calling a failing external service (API/database/message broker).
It protects the system from cascading failures.

---

🔌 States of Circuit Breaker

State	Meaning
Closed	Everything works normally (calls allowed).
Open	Calls blocked immediately because external service is failing.
Half-Open	Allows limited trial calls to see if the service recovered.

---

⭐ Why we use it

• To avoid hitting a failing service over & over

• To prevent resource consumption (threads, CPU, DB connections)

• To improve latency by quickly failing requests

• To give time for the external system to recover

---

📦 Circuit Breaker in .NET Core using Polly

Install package:

dotnet add package Polly.Extensions.Http

---

✔️ Simple Circuit Breaker Example

services.AddHttpClient("StockApiClient")
    .AddTransientHttpErrorPolicy(policy => 
        policy.CircuitBreakerAsync(
            handledEventsAllowedBeforeBreaking: 3,      // After 3 failures
            durationOfBreak: TimeSpan.FromSeconds(30),  // Keep circuit open for 30 sec
            onBreak: (result, ts) =>
            {
                Console.WriteLine("Circuit opened!");
            },
            onReset: () =>
            {
                Console.WriteLine("Circuit closed!");
            },
            onHalfOpen: () =>
            {
                Console.WriteLine("Circuit half-open: trying test call.");
            }
        ));

Meaning:

• If API fails 3 times → Circuit goes OPEN

• For the next 30 seconds → all calls fail immediately

• After 30 sec → Circuit goes HALF-OPEN

• If next call succeeds → RESET (Closed)

• If fails → OPEN again

---

✔️ Circuit Breaker + Retry (Best Practice)

services.AddHttpClient("PaymentService")
    .AddPolicyHandler(
        HttpPolicyExtensions
            .HandleTransientHttpError()
            .RetryAsync(3) // retry 3 times
    )
    .AddPolicyHandler(
        HttpPolicyExtensions
            .HandleTransientHttpError()
            .CircuitBreakerAsync(5, TimeSpan.FromSeconds(20))
    );

Retry handles intermittent failures.
Circuit Breaker handles persistent failures.

---

✔️ Using Circuit Breaker in Controller

public class StockController : ControllerBase
{
    private readonly IHttpClientFactory _factory;
    public StockController(IHttpClientFactory factory) => _factory = factory;

    [HttpGet("price/{symbol}")]
    public async Task<IActionResult> GetStockPrice(string symbol)
    {
        var client = _factory.CreateClient("StockApiClient");

        var response = await client.GetAsync($"/api/stock/{symbol}");

        if (!response.IsSuccessStatusCode)
            return StatusCode((int)response.StatusCode, "External service failed");

        var data = await response.Content.ReadAsStringAsync();
        return Ok(data);
    }
}

---

🎯 Real-Time Scenarios Where You Use Circuit Breaker

1. Payment Gateways

If Razorpay/Stripe API is failing, don’t call them continuously.
Circuit stops the calls.

2. Microservices Architecture

If Order Service relies on Inventory Service and Inventory is down →
stop hammering it; return fallback immediately.

3. Slow database or cache

If Redis/Azure SQL is timing out → circuit opens → API returns quick response instead of long delays.

4. Third-party APIs (SMS, Email, Notification)

Avoid getting throttled or blocked by calling continuously.

---

📘 How You Answer in Interviews (30-second answer)

“Circuit Breaker is a resilience pattern that prevents the system from making calls to a failing external service.
In .NET Core, we implement it using Polly.
After a defined number of failures, the circuit moves to Open state and blocks all calls for a specified time.
This prevents cascading failures and improves performance.
I generally combine Retry + Circuit Breaker in HttpClient when calling microservices or external systems.”

---