High Availability — Complete Guide

Introduction

High Availability — 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 high availability with real banking transfers, ERP GL reports, or legacy stored procedure modernization — not toy animal demos. This article delivers production depth on Payments.

After this article you will

• Explain High Availability in plain English and in SQL Server / ADO.NET terms
• Implement high availability in ShopNest.DataAccess — Enterprise High-Performance Data Platform (Payments)
• 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 91 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 89 — Secrets Management — Complete Guide
• Time: 28 min reading + 30–45 min hands-on in SSMS

Concept deep-dive

Level 1 — Analogy

High Availability on ShopNest.DataAccess adds high-performance SQL Server data access for enterprise high availability modules.

Level 2 — Technical

High Availability deploys ShopNest data tier — Azure SQL elastic pools, DACPAC pipelines, geo-restore, and Key Vault connection secrets.

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 — Payments

Write High Availability in ShopNest.DataAccess for Payments: 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 — High Availability on ShopNest (Payments)
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

// High Availability — ShopNest.DataAccess (Payments)
await using var conn = new SqlConnection(_connectionString);
await conn.OpenAsync(ct);

SQL performance and connection management — High Availability

• 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 — ICICI-Style NEFT/IMPS Fund Transfer

MANDATORY enterprise scenario (Indian Banking): High Availability in ShopNest.DataAccess Payments.

Business problem

Core banking must process 50,000+ transfers per hour during salary day. EF Core change tracking adds overhead; ADO.NET with stored procedures and explicit SqlTransaction gives predictable latency and full SQL control for auditors.

Architecture

[Mobile Banking API] → [TransferRepository (ADO.NET)]
  → SqlConnection (pooled) → usp_TransferFunds @FromAcct, @ToAcct, @Amount
  → SqlTransaction (Serializable for ledger rows)
  → AuditLog INSERT via usp_WriteAuditTrail
Connection string: ShopNestPaymentsDb; CommandTimeout: 30s; Retry on deadlock 1205.

Production ADO.NET code

// ShopNest.DataAccess/Payments/TransferRepository.cs
public async Task<TransferResult> TransferAsync(TransferRequest req, CancellationToken ct)
{
    await using var conn = new SqlConnection(_connectionString);
    await conn.OpenAsync(ct);
    await using var tx = (SqlTransaction)await conn.BeginTransactionAsync(IsolationLevel.ReadCommitted, ct);

    try
    {
        await using var cmd = new SqlCommand("usp_TransferFunds", conn, tx)
        {
            CommandType = CommandType.StoredProcedure,
            CommandTimeout = 30
        };
        cmd.Parameters.Add("@FromAccount", SqlDbType.VarChar, 20).Value = req.FromAccount;
        cmd.Parameters.Add("@ToAccount", SqlDbType.VarChar, 20).Value = req.ToAccount;
        cmd.Parameters.Add("@Amount", SqlDbType.Decimal).Value = req.Amount;
        cmd.Parameters.Add("@IdempotencyKey", SqlDbType.UniqueIdentifier).Value = req.IdempotencyKey;

        var rows = await cmd.ExecuteNonQueryAsync(ct);
        await tx.CommitAsync(ct);
        return TransferResult.Success(req.IdempotencyKey);
    }
    catch (SqlException ex) when (ex.Number == 1205) // deadlock
    {
        await tx.RollbackAsync(ct);
        throw new TransientDatabaseException("Deadlock — retry with Polly", ex);
    }
}

Outcome

P99 transfer latency 45ms vs 180ms with EF Core tracked entities; RBI audit passed with stored procedure versioning.

Real-World Example 2 — Insurance Claims Batch Processing

MANDATORY enterprise scenario (Insurance (LIC-style)): High Availability in ShopNest.DataAccess Payments.

Business problem

Nightly batch validates 500K claims against policy rules. SqlBulkCopy inserts staging rows; stored procedure usp_ValidateClaims runs set-based SQL; invalid claims roll back in one transaction per batch of 5,000.

Architecture

CSV ingest → SqlBulkCopy → Staging.Claims
  → BEGIN TRANSACTION per batch
  → EXEC usp_ValidateClaims @BatchId
  → COMMIT or ROLLBACK + ErrorLog

Production ADO.NET code

using var bulk = new SqlBulkCopy(conn, SqlBulkCopyOptions.TableLock, tx)
{
    DestinationTableName = "Staging.Claims",
    BatchSize = 5000,
    BulkCopyTimeout = 600
};
await bulk.WriteToServerAsync(dataTable, ct);
await using var validate = new SqlCommand("usp_ValidateClaims", conn, tx) { CommandType = CommandType.StoredProcedure };
validate.Parameters.Add("@BatchId", SqlDbType.Int).Value = batchId;
await validate.ExecuteNonQueryAsync(ct);

Outcome

Batch window reduced from 6 hours to 90 minutes; zero partial batches after explicit transaction boundaries.

ADO.NET with ASP.NET Core — High Availability

Register IPaymentsRepository 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 High Availability in ADO.NET Core?
A: High Availability on ShopNest Payments: 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 Payments — show SqlConnection, SqlCommand, SqlParameter, async disposal, and one xUnit integration test.

Summary & next steps

• Article 90: High Availability — Complete Guide
• Module: Module 9: Cloud and DevOps · Level: ADVANCED
• Applied to ShopNest.DataAccess — Payments

Previous: Secrets Management — Complete Guide
Next: Banking Transaction System — Complete Guide

Practice: Run today's SQL in SSMS with execution plan — commit with feat(adonet): article-090.

FAQ