Key technologies.

The sensible default database for almost every product-design interview — ACID, rich SQL, JSONB, full-text and geo extensions, and far more scale on one node than candidates assume.

Reach for it when: The default for product-design interviews — user records, orders, relationships, anything transactional

A fully-managed, horizontally-scaling key-value and document store with single-digit-millisecond latency at any scale — when your access patterns are known and key-based, it removes sharding and capacity planning entirely.

Reach for it when: Access patterns are known up front and are key-based (get/put by id, query by partition key)

A masterless, linearly-scalable wide-column store built for enormous write throughput and multi-region availability — when you have a write firehose and known partition-key queries, it scales by simply adding nodes.

Reach for it when: Write-heavy workloads at scale — time-series, event logs, sensor/IoT data, messaging history, feeds

An in-memory, single-threaded data-structure store — the most versatile tool in an interview because one technology covers caching, locks, leaderboards, rate limits, geo, queues, and pub/sub.

Reach for it when: You need a cache in front of a slower store and want sub-millisecond reads

A distributed search and analytics engine built on Lucene — the default answer for full-text search, faceted filtering, and log/observability analytics when SQL LIKE and a B-tree index fall over.

Reach for it when: Full-text search is a real feature — relevance ranking, typo tolerance, autocomplete

A store that indexes high-dimensional embedding vectors for fast similarity search — the retrieval engine behind semantic search, recommendations, and RAG over your own data.

Reach for it when: Semantic search — match by meaning, not keywords (Q&A, "find similar", dedup)

A distributed, partitioned, replicated append-only log — the default backbone for high-throughput event streaming, decoupling services, and feeding many consumers from one durable stream.

Reach for it when: You need a durable, replayable event stream that many independent consumers read

A distributed stream-processing engine for stateful, low-latency computation over unbounded data — windowed aggregations, stream joins, and real-time analytics with exactly-once guarantees.

Reach for it when: Real-time windowed aggregations — counts, sums, percentiles per interval (ad clicks, metrics)

Effectively infinite, cheap, durable object storage for large unstructured files — images, video, backups, logs — that should never live in your database.

Reach for it when: You store large files — images, video, audio, PDFs, user uploads, ML data

The single front door to a backend: one entry point that authenticates, rate-limits, routes, and shapes every request so individual services do not each re-implement cross-cutting concerns.

Reach for it when: You have multiple backend services and want one client-facing entry point

The component that spreads incoming traffic across a pool of servers, removes unhealthy ones, and gives clients one stable endpoint — the foundation of horizontal scaling and availability.

Reach for it when: You run more than one server and need to spread traffic across them

A globally distributed cache that serves content from edge locations near users — cutting latency, absorbing read traffic off your origin, and shielding it from spikes.

Reach for it when: You serve static assets or media (images, video, bundles) to a global audience

Strongly-consistent coordination services that solve the hard distributed problems — leader election, distributed locks, configuration, and service discovery — so you never have to implement consensus yourself.

Reach for it when: You need leader election or a single active coordinator across a cluster

A buffer between producers and workers that decouples them, absorbs bursts, and processes work asynchronously with built-in retries and dead-letter handling — the simple task-queue answer when you do not need a full event log.

Reach for it when: Background / asynchronous jobs — email, transcoding, image processing, report generation