Event-Driven Systems — Complete Guide

Introduction

Event-Driven Systems — Complete Guide is essential for .NET developers building ShopNest.DataAccess — Enterprise High-Performance Data Platform — Toolliyo's 100-article ADO.NET Core master path covering SqlConnection, stored procedures, transactions, connection pooling, ASP.NET Core integration, Azure SQL, and ten enterprise portfolio projects. Every article includes minimum two enterprise real-world examples (ICICI banking, TCS ERP reporting, insurance batch, legacy modernization).

In Indian delivery projects (TCS, Infosys, Wipro), interviewers expect event-driven systems with real banking transfers, ERP GL reports, or legacy stored procedure modernization — not toy animal demos. This article delivers production depth on Reporting.

After this article you will

• Explain Event-Driven Systems in plain English and in SQL Server / ADO.NET terms
• Implement event-driven systems in ShopNest.DataAccess — Enterprise High-Performance Data Platform (Reporting)
• Compare SQL-concat / sync anti-patterns vs production-ready parameterized async ADO.NET
• Answer fresher, mid-level, and senior ADO.NET and SQL Server interview questions confidently
• Connect this lesson to Article 68 and the 100-article ADO.NET Core roadmap

Prerequisites

• Software: .NET 8 SDK, SQL Server Express / LocalDB or Docker, SSMS or Azure Data Studio
• Knowledge: C# basics and SQL Server
• Previous: Article 66 — Distributed Systems — Complete Guide
• Time: 28 min reading + 30–45 min hands-on in SSMS

Concept deep-dive

Level 1 — Analogy

Event-Driven Systems on ShopNest.DataAccess adds high-performance SQL Server data access for enterprise event-driven systems modules.

Level 2 — Technical

Event-Driven Systems scales ShopNest enterprise data — read replicas, warehouse ETL, background workers, and strangler-fig legacy modernization on Reporting.

Level 3 — Data platform view

[ASP.NET Core API / MVC Controller]
       ▼
[Application Service — IOrderRepository interface]
       ▼
[ADO.NET Repository — SqlConnection + SqlCommand + SqlParameter]
       ▼
[SQL Server — Tables · Indexes · Stored Procedures · Transactions]
       ▼
[Connection Pool · Read Replica · Azure SQL · Serilog + SQL Profiler]

Project structure

ShopNest.DataAccess/
├── src/
│   ├── ShopNest.DataAccess.Api/       ← ASP.NET Core Web API
│   ├── ShopNest.DataAccess.Core/      ← Repository interfaces + DTOs
│   ├── ShopNest.DataAccess.AdoNet/    ← SqlConnection, SPs, transactions
│   ├── ShopNest.DataAccess.Reports/   ← Streaming readers, GL reports
│   └── ShopNest.DataAccess.Tests/     ← Testcontainers SQL integration
├── sql/
│   ├── migrations/
│   └── stored-procedures/             ← usp_Orders_*, usp_Payments_*
└── docker-compose.yml                 ← SQL Server 2022

Hands-on implementation — Reporting

Write Event-Driven Systems in ShopNest.DataAccess for Reporting: SqlConnection/SqlCommand with parameters, async calls, and verify in SSMS with execution plan.

1. Open ShopNest.DataAccess repository for this lesson module.
2. Use SqlConnection with await using and connection string from IConfiguration.
3. Add SqlParameter for every user input — never string concatenation.
4. Use ExecuteReaderAsync for reads; transactions for multi-statement writes.
5. Verify in SSMS — check execution plan, row counts, and connection pool metrics.

Anti-pattern (SQL concat, sync calls, DataSet for huge reports)

// ❌ BAD — SQL concat, sync call, no disposal
public List<Order> GetOrders(string status) {
    var conn = new SqlConnection(_connStr);
    conn.Open();
    var cmd = new SqlCommand("SELECT * FROM Orders WHERE Status = '" + status + "'", conn);
    var reader = cmd.ExecuteReader(); // sync, blocks thread pool
    // connection never disposed — pool exhaustion under load
    return Parse(reader);
}

Production-style ADO.NET data access

// ✅ CORRECT — Event-Driven Systems on ShopNest (Reporting)
public async Task<IReadOnlyList<OrderDto>> GetByStatusAsync(string status, CancellationToken ct) {
    await using var conn = new SqlConnection(_config.GetConnectionString("ShopNestDb"));
    await conn.OpenAsync(ct);
    await using var cmd = new SqlCommand("usp_Orders_GetByStatus", conn) {
        CommandType = CommandType.StoredProcedure
    };
    cmd.Parameters.Add("@Status", SqlDbType.NVarChar, 20).Value = status;
    var list = new List<OrderDto>();
    await using var reader = await cmd.ExecuteReaderAsync(ct);
    while (await reader.ReadAsync(ct))
        list.Add(new OrderDto(reader.GetInt32(0), reader.GetDecimal(1), reader.GetString(2)));
    return list;
}

Complete example

// Event-Driven Systems — ShopNest.DataAccess (Reporting)
await using var conn = new SqlConnection(_connectionString);
await conn.OpenAsync(ct);

SQL performance and connection management — Event-Driven Systems

• Connection pooling — default enabled; never disable without load testing; watch pool exhaustion (error 10053/10054)
• Parameterized queries — always use SqlParameter; prevents SQL injection and enables plan cache reuse
• Async — ExecuteReaderAsync/ExecuteNonQueryAsync free thread pool under load
• CommandBehavior.SequentialAccess — stream large BLOB/text columns without loading full row into memory
• Indexes — align with WHERE/JOIN columns; use SQL Server DMVs to find missing indexes

Real-World Example 1 — Razorpay-Style Payment Reconciliation

MANDATORY enterprise scenario (Payment Gateway): Event-Driven Systems in ShopNest.DataAccess Reporting.

Business problem

End-of-day reconciliation matches 1M gateway transactions against internal ledger. ADO.NET table-valued parameters feed set-based MERGE in SQL Server — impossible to express efficiently in LINQ.

Architecture

Gateway CSV → TVP dbo.TransactionBatch → usp_ReconcilePayments
  → MERGE Payments.Ledger → Output mismatches to ReconciliationExceptions

Production ADO.NET code

var tvp = new SqlParameter("@Batch", SqlDbType.Structured)
{
    TypeName = "dbo.TransactionBatchType",
    Value = BuildDataTable(transactions)
};
cmd.Parameters.Add(tvp);
await cmd.ExecuteNonQueryAsync(ct);

Outcome

Reconciliation completes in 12 minutes; EF Core prototype timed out at 45 minutes on same hardware.

Real-World Example 2 — TCS ERP Monthly GL Reporting

MANDATORY enterprise scenario (Enterprise ERP): Event-Driven Systems in ShopNest.DataAccess Reporting.

Business problem

Finance teams run month-end General Ledger reports across 200+ cost centers. Report queries join 12 tables and return 2M rows — EF Core materializes entire graphs into memory. ADO.NET SqlDataReader streams rows to CSV/PDF generators with constant memory.

Architecture

[Report Scheduler] → [GlReportRepository]
  → EXEC usp_GenerateMonthlyGL @Year, @Month, @CostCenterId
  → SqlDataReader forward-only stream → IAsyncEnumerable
  → Bulk copy to staging → SSRS / Excel export
Read uncommitted avoided; NOLOCK only on read replica for analytics.

Production ADO.NET code

public async IAsyncEnumerable<GlLineDto> StreamGlReportAsync(int year, int month, [EnumeratorCancellation] CancellationToken ct)
{
    await using var conn = new SqlConnection(_readReplicaConnectionString);
    await conn.OpenAsync(ct);
    await using var cmd = new SqlCommand("usp_GenerateMonthlyGL", conn)
    {
        CommandType = CommandType.StoredProcedure
    };
    cmd.Parameters.Add("@Year", SqlDbType.Int).Value = year;
    cmd.Parameters.Add("@Month", SqlDbType.Int).Value = month;

    await using var reader = await cmd.ExecuteReaderAsync(CommandBehavior.SequentialAccess, ct);
    while (await reader.ReadAsync(ct))
    {
        yield return new GlLineDto(
            reader.GetString(0),
            reader.GetDecimal(1),
            reader.GetDateTime(2));
    }
}

Outcome

Memory flat at 80MB for 2M-row report vs 1.2GB EF Core ToList(); report runtime cut from 14 min to 3 min.

ADO.NET with ASP.NET Core — Event-Driven Systems

Register IReportingRepository as Scoped in DI. Never hold SqlConnection across requests. Use IConfiguration for connection strings; User Secrets locally, Azure Key Vault in production.

builder.Services.AddScoped<IOrderRepository, OrderRepository>();
builder.Services.AddHealthChecks().AddSqlServer(connectionString);

Stored procedures and SQL safety

Enterprise ShopNest modules use versioned stored procedures (usp_ prefix). Never concatenate user input — always SqlParameter. Log slow queries (>500ms) with Serilog and review execution plans in SSMS.

Common errors & fixes

• SQL built with string concatenation from user input — Use SqlParameter with typed values for every dynamic value.
• Not disposing SqlConnection / SqlDataReader — Use await using for connection, command, and reader — return connections to pool.
• Loading million-row reports into DataTable — Stream with SqlDataReader and yield batches; avoid DataSet for large data.
• Hard-coding connection strings in repository classes — IConfiguration + User Secrets locally; Azure Key Vault in production.

Best practices

• 🟢 SqlParameter for every dynamic value — zero string concatenation
• 🟢 await using for SqlConnection, SqlCommand, SqlDataReader — return to pool
• 🟡 Async ADO.NET end-to-end on ASP.NET Core request paths
• 🟡 Stream large reports with SqlDataReader; avoid DataSet for millions of rows
• 🔴 SqlTransaction for multi-statement financial writes with explicit rollback
• 🔴 Connection strings in Key Vault — never committed to Git

Interview questions

Fresher level

Q1: What is Event-Driven Systems in ADO.NET Core?
A: Event-Driven Systems on ShopNest Reporting: SqlConnection lifecycle, SqlCommand with parameters, async execution, and disposal for connection pool health.

Q2: ADO.NET vs EF Core — when to use which?
A: ADO.NET for stored procedures, bulk load, streaming reports, and legacy SQL; EF Core for rapid CRUD and migrations. ShopNest uses both.

Q3: How do you prevent SQL injection in ADO.NET?
A: Always SqlParameter with typed SqlDbType — never string concatenation, even for "trusted" internal tools.

Mid / senior level

Q4: Explain connection pooling and why disposal matters.
A: SqlConnection.Close/Dispose returns the physical connection to the pool. Leaked connections exhaust Max Pool Size and cause timeouts.

Q5: How do you handle transactions in ADO.NET?
A: SqlTransaction with try/commit/catch/rollback; choose isolation level (ReadCommitted default); retry deadlocks with Polly.

Q6: How would you optimize a slow stored procedure report?
A: Check execution plan in SSMS, add covering indexes, avoid SELECT *, stream with SqlDataReader, consider read replica for analytics.

Coding round

Implement a parameterized ADO.NET repository method for ShopNest Reporting — show SqlConnection, SqlCommand, SqlParameter, async disposal, and one xUnit integration test.

Summary & next steps

• Article 67: Event-Driven Systems — Complete Guide
• Module: Module 7: Advanced Enterprise Topics · Level: ADVANCED
• Applied to ShopNest.DataAccess — Reporting

Previous: Distributed Systems — Complete Guide
Next: Background Services — Complete Guide

Practice: Run today's SQL in SSMS with execution plan — commit with feat(adonet): article-067.

FAQ