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Expected answer

The event loop is Node.js's mechanism for handling async I/O despite being single-threaded. It has phases: timers, pending callbacks, idle/prepare, poll (I/O), check (setImmediate), and close callbacks. When async operations (file I/O, network) start, they're delegated to libuv's thread pool or OS async APIs. When complete, their callbacks are queued and executed in the appropriate phase. This allows Node.js to handle thousands of concurrent connections without threads.

Expected answer

Each service gets its own NestJS app with clearly defined bounded contexts. Communication via message broker (RabbitMQ/Kafka) for async events and gRPC for sync calls. Shared types via a common package. Each service has its own database (database per service pattern). API Gateway (NestJS + GraphQL federation) for client-facing API. Health checks, circuit breakers (using nestjs-terminus), distributed tracing (OpenTelemetry), and centralized logging.

Expected answer

Streams process data in chunks rather than loading everything into memory. Four types: Readable, Writable, Duplex, Transform. Use them for: processing large files (CSV parsing), HTTP request/response bodies, real-time data processing, piping between sources. Example: reading a 10GB log file line-by-line with createReadStream + readline uses constant memory vs. readFile which would crash.

Expected answer

Layered approach: 1) Async wrapper middleware to catch promise rejections, 2) Route-level validation with Zod/Joi, 3) Service-level try/catch with custom error classes (NotFoundError, ValidationError), 4) Global error handling middleware that maps error types to HTTP codes, 5) Unhandled rejection/exception handlers for graceful shutdown. Log errors with structured logging (pino) and correlate with request IDs.

Expected answer

process.nextTick() executes in the current phase before the event loop continues — it's a microtask. setImmediate() executes in the check phase of the next event loop iteration. nextTick can starve I/O if called recursively (the event loop never advances). setImmediate is safer for deferring work because it allows I/O to process between calls. Rule of thumb: use setImmediate for deferring, nextTick only when you need something to happen before any I/O.