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Eight concurrent workers sharing one idempotency key turned a $49 order into $392 of real charges on PostgreSQL 17.6, and adding a FOR UPDATE row lock changed nothing at all. This walks through what actually prevents the double charge, measured at every step, and what Stripe's and Shopify's own documentation says rather than what gets repeated about them.

Reading a 2 GB file all at once doesn't run out of memory — it hits V8's 512 MB string limit first. Streams process it in flat memory instead. Here's the pipeline pattern, custom Transform streams, and backpressure, with the numbers measured on Node 22.

The EventEmitter pattern lets components in the same process react to the same event without being directly coupled — no message broker needed.

One file to guard every route — plus the Next.js 16 rename that moves it off the Edge runtime, the request-vs-response header trap that leaks user IDs to the browser, and the CVE that explains why this can never be your only auth layer.

Node.js runs on one CPU core by default. The cluster module lets you fork N workers across all cores — here's the pattern, graceful shutdown, IPC, and when PM2 makes it unnecessary.

Parallel routes let a single layout render multiple independent pages at once. Combine them with intercepting routes and you get URL-aware modals with zero hacks.

Server Actions collapse form handling into a single server function—no API route, no manual fetch, no useState for loading. Here's the pattern that replaces 80% of your mutation boilerplate.

Four caching layers, three revalidation APIs, and the Next.js 16 rename that turns everything you learned in 15 into the "previous model". Here's the complete mental model for data fetching in the App Router.

Most Node.js services drop in-flight requests on every deploy. Here's the complete pattern: SIGTERM handling, HTTP draining, queue cleanup, and Kubernetes integration.