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Your order-processing Lambda starts sending duplicate confirmation emails. Not always — maybe one order in twenty. CloudWatch shows more invocations than messages published. The function code hasn't changed in weeks. What changed is that someone added a fraud check that pushed processing time from 25 seconds to around 45, and your SQS queue is still running the default 30-second visibility timeout. That combination is the whole bug. When a Lambda pulls a message from SQS, the message isn't deleted — it's hidden for the duration of the visibility timeout. If the function is still working when that window closes, SQS assumes the consumer died and hands the same message to another invocation. Now two Lambdas are processing the same order, both will "succeed," and both will send the email. Nothing errors. Nothing retries. There is no log line that says "this message was delivered twice because your timeouts are misconfigured." Infrawise ( npm ) flags this exact mismatch as a high-severity finding before it costs you an afternoon of staring at idempotency-free handler code. This post walks through why the bug is so hard to see, how the detection works, and how to keep an AI assistant from reintroducing it. Why you never catch this one yourself Three things make this misconfiguration nearly invisible: It passes every test. In local tests and staging, your handler processes a synthetic message in two seconds. The 30-second visibility window never comes close to expiring. The bug only exists under production conditions — real payload sizes, real downstream latency, cold starts stacking on top of slow dependencies. The defaults set the trap. SQS queues default to a 30-second visibility timeout. Lambda functions routinely get their timeout bumped to 60, 120, or 900 seconds as they grow. Nobody bumps the queue at the same time, because the two settings live in different consoles, different IaC resources, and usually different pull requests. The failure signature points elsewhe

Modern C# Features: A Deep Dive into Records, Pattern Matching, Async, and Performance A practical guide to the C# language features that have reshaped how we write .NET code — records, pattern matching, async/await improvements, nullable reference types, LINQ enhancements, Span<T> , and performance optimizations. Table of Contents Introduction Records Pattern Matching Async/Await Improvements Nullable Reference Types LINQ Enhancements Span<T> and Memory<T> Performance Optimizations Quick Reference Table Conclusion Introduction C# has evolved significantly since C# 8. Each release (9, 10, 11, 12, 13) has focused on three consistent themes: Conciseness — write less boilerplate to express the same intent. Safety — catch bugs at compile time instead of runtime (especially around null ). Performance — give developers low-level control without leaving the managed, safe world of .NET. This guide walks through the features that matter most in day-to-day development, with working code examples you can drop into a dotnet run project. 1. Records Introduced in C# 9 , record types give you immutable, value-based data models with almost no ceremony. Why records exist Before records, representing an immutable data object meant hand-writing a constructor, Equals , GetHashCode , ToString , and often a With -style copy method. Records generate all of this for you. // Before: a "plain" immutable class public class PersonClass { public string FirstName { get ; } public string LastName { get ; } public PersonClass ( string firstName , string lastName ) { FirstName = firstName ; LastName = lastName ; } public override bool Equals ( object ? obj ) => obj is PersonClass p && p . FirstName == FirstName && p . LastName == LastName ; public override int GetHashCode () => HashCode . Combine ( FirstName , LastName ); public override string ToString () => $"PersonClass {{ FirstName = { FirstName }, LastName = { LastName } }} " ; } // After: the same thing as a record public record Person ( stri