- Message Queues and Asynchronous Work
Why the line after a commit is the most dangerous line in the file, and what to do about it. Queues against logs, at-least-once delivery and the idempotency it forces, the transactional outbox, retries with exponential backoff, dead-letter queues, and how a worker claims work without two workers doing it twice. Every piece taken from a queue that runs, including the duplicate it produced.
- Consistency, Availability and CAP
CAP without the folklore — what the theorem says, what it does not say, and why PACELC is the more useful version day to day. Strong against eventual consistency, read-your-writes and monotonic reads, quorums, and the dual-write problem that appears the moment a second datastore enters the design. Shown against a live Postgres-to-Elasticsearch sink, including exactly where it goes wrong.
- Scaling the Database
The order to do things in, because most systems reach for sharding several steps too early. Indexes and query plans, connection pooling, read replicas and the replica lag that breaks read-your-writes, vertical against horizontal, partitioning versus sharding, choosing a shard key you will not regret, hot shards, and what resharding actually costs once you are live.
- Caching – Patterns, Invalidation and What Breaks
A cache-aside read on a real endpoint, measured cold and warm, then everything that makes caching hard rather than easy. Write-through against write-behind, TTL and eviction policies, the two invalidation strategies and why you want both, cache stampede and how to stop it, and the rule that decides whether a cache is an optimisation or a new single point of failure.
- Load Balancing and the Stateless Tier
Getting a request to a server that can answer it — and, better, not sending it at all. Layer 4 versus layer 7, round robin against least connections, health checks that actually detect a sick instance, and why sticky sessions are a trap rather than a feature. What "stateless" really requires of your application, how a JWT delivers it, and where a CDN removes the request entirely.