High Availability — Complete Guide

High Availability — Complete Guide: free step-by-step lesson with examples, common mistakes, and interview tips — part of ADO.NET Core Tutorial on Toolliyo Academy.

8 min read Updated 8/21/2026

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ADO.NET Core Tutorial · Lesson 90 of 100

High Availability

Foundations ✓ → SQL & safety ✓ → Production ✓ → Projects

Projects · 4 — Portfolio builds · ~10 min · Module 9: Cloud and DevOps

What is this?

HA means redundant SQL (AG/failover groups) plus ADO.NET retries and multi-instance APIs.

Why should you care?

ShopNest uptime is API farm + database redundancy together.

See it live — copy this example

Use a .NET console or API project with SQL Server LocalDB. Run dotnet run after pasting.

services.AddSingleton<SqlRetryPolicy>(); // transient errors
// CS with failover partner
// multiple API replicas behind LB

What happened?

DB HA ≠ app HA.
Retries for transients.
Idempotent writes.
Load-balanced APIs.

Practice next

Enable failover group.
Retry policy in client.
Scale API replicas.
Circuit breaker on SQL.
Graceful degrade read-only.

Remember

HA SQL + retries. Scale APIs. Idempotent writes.

ShopNest HA stack

Failover + retries keep checkout alive.

Outcome: Regional blip is a blip.

Interview prep for this lesson

Practice these questions aloud after reading—each links to a full structured answer.

Junior PDF Detailed

What is ADO.NET?

Short answer: ADO.NET (Active Data Objects .NET) is a data access technology in the .NET framework that enables applications to interact with databases and other data sources. Explain a bit more It provides a set of clas…

Mid PDF Detailed

Memory Efficiency:?

Short answer: A DataReader is a forward-only, read-only cursor, meaning it streams data from the database and does not store the entire result set in memory. DataSet, on the other hand, loads the entire result set into m…

Mid PDF Detailed

Optimistic Concurrency Control:?

Short answer: Optimistic Concurrency assumes that conflicts will be rare and allows multiple users to read and modify data without locking it. Explain a bit more When updating data, you compare the current data in the da…

Mid PDF Detailed

Using ExecuteReader (for a single row, multiple columns):?

Short answer: SqlCommand command = new SqlCommand("SELECT CustomerName, ContactName FROM Customers WHERE CustomerID = @CustomerID", connection); command.Parameters.AddWithValue("@CustomerID", 1); conn…

Mid PDF Detailed

SqlCommand: Used for SQL Server databases. It represents a Transact-SQL?

Short answer: command or stored procedure. Example: SqlCommand command = new SqlCommand("SELECT * FROM Customers", connection); Example code command or stored procedure. Example: SqlCommand command = new SqlCom…

All course Q&A Practice exams

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