Our pipelines and Dockerfiles ran corepack enable before installing dependencies with pnpm. After we bumped the base image from Node.js 24 to 26, every build failed: corepack: command not found. Corepack is no longer bundled with Node.js from version 25. How we install it explicitly and keep versions pinned.
A handful of tests in our Python service failed only when the whole suite ran, and only in some orders. A session-scoped fixture returned a mutable settings object that one test modified. How we proved it with random ordering and bisection, and how fixture scopes, factories and monkeypatch keep tests isolated.
We added a logging middleware to a FastAPI service and a server-sent events endpoint stopped streaming smoothly: events arrived in bursts, timings were wrong and disconnects went unnoticed. The middleware used BaseHTTPMiddleware. Why it interferes, and how pure ASGI middleware fixes it.
A Go pricing service's goroutine count grew by thousands per hour. A helper that raced a slow lookup against a timeout left the lookup goroutine blocked forever on an unbuffered channel nobody would read. How we found it with goroutine profiles and fixed it with context and goleak.
A Go ingestion service that forwarded events to a partner API was OOM-killed every time traffic spiked. Each event started a goroutine that waited on the slow partner, so goroutines and their buffers grew without limit. How we found it with pprof and fixed it with a bounded worker pool.
A Node.js queue worker restarted a few times a day, dropping in-flight jobs. An analytics call was started without await or catch; when the analytics service failed, the rejection was unhandled and Node.js terminated the process, as it does by default since version 15. How we fixed it.
Every system that needs to tell another system that something happened ends up choosing between polling, webhooks, WebSockets and a message broker. A practical comparison by who connects to whom, delivery guarantees, firewalls, fan-out and cost, with the failure mode of each and the combination we use most.
A small refactor added __eq__ to our Money class so amounts could be compared. Soon a cache decorator and several set operations failed with TypeError: unhashable type: 'Money', because defining __eq__ without __hash__ sets __hash__ to None. How equality and hashing must agree.
Whenever someone exported a large CSV report, our Node.js API stopped answering everything else, including liveness probes, and pods restarted mid-export. Building 200,000 rows synchronously blocked the event loop for seconds. How we measured it and moved the work off the main thread.
During a launch our nginx reverse proxy returned errors and logged worker_connections are not enough, at well under the expected capacity. Every proxied request holds a client and an upstream connection, and the file descriptor limit was lower still. How we sized it properly.