GuideTechnology
13 min read
Starting from a payment system full of if-else branches, this article shows why interfaces and polymorphism exist, how abstract classes differ from interfaces, what default methods are for, and why composition usually beats inheritance. It also previews the idea behind dependency injection.
GuideTechnology
11 min read
How one Java class can build on another, how overriding changes behaviour at runtime, how overloading is chosen at compile time, and why inheritance is easy to misuse. With the constructor-order trap, field hiding and the exact differences interviewers ask about.
GuideTechnology
12 min read
Why object-oriented programming exists and how Java expresses it: classes as blueprints, objects as instances, constructors, access modifiers, static versus instance members, and encapsulation as a way to protect the rules of your data. Built around a bank account that cannot go wrong.
GuideTechnology
13 min read
How to make a Java program choose and repeat: if, switch (old statement and modern expression), the loop forms, and how to organise code into methods. Includes the pass-by-value rule that interviewers love and the scope rules that cause mysterious compile errors.
GuideTechnology
15 min read
How Java stores and combines data: primitive types versus references, what happens when a number overflows, why integer division surprises people, how casting and conversion work, when to use var, and how arrays hold a fixed number of values.
GuideTechnology
15 min read
What Java actually is, how a .java file becomes a running program, what the JDK, JRE and JVM each do, and how to compile and run your first program. Plus the full roadmap of this 41-article course, from zero to a tested Spring Boot API.
CourseTechnology
41 lessons · 11 h
A free 41-article course that takes you from your first Java program to a tested, secured Spring Boot and PostgreSQL backend, with three projects, real verified output and interview preparation. Modern Java 21 and later, with older styles labelled.
GuideTechnology
14 min read
Turn the course into interview performance: an eight-week practice plan, how to run timed solo drills and mock interviews, a self-scoring rubric that separates SDE-1 and SDE-2 expectations, a drill sheet for the twelve classic problems, ten new practice problems to test transfer, common failure patterns, and the complete job-ready checklist.
GuideTechnology
17 min read
An educational design for searching a billion web pages: a polite, distributed crawler with a URL frontier and robots.txt, deduplication, building an inverted index with batch processing, document-partitioned shards and replicas, scatter-gather query serving with result caching and tail-latency control, ranking from relevance and link signals, freshness tiers, failures and safety.
GuideTechnology
16 min read
Design an online store from product pages to delivery: a catalogue and search served from caches and event-fed indexes, carts in a key-value store, inventory reservations with expiry, a checkout saga across inventory and payment, oversell prevention, and a flash sale that survives a thousandfold spike with a waiting room, bucketed stock and isolation from normal shopping.
GuideTechnology
17 min read
Design a food delivery platform where every order coordinates three parties in real time: restaurant discovery and menus as cached read models, an idempotent checkout, an order state machine driven by a saga with timeouts and compensations, dispatching delivery partners timed to food readiness, batching, live tracking, notifications, dinner-peak load, failures and the trade-offs between them.
GuideTechnology
18 min read
Design the core of a ride-hailing app: ingesting hundreds of thousands of driver location updates per second, a geospatial index of available drivers, matching riders to drivers with offers and timeouts, never assigning one driver twice, a trip state machine, live tracking for riders, city-based partitioning, payments, failures, privacy and fraud.
GuideTechnology
15 min read
Design an on-demand video platform: resumable uploads straight to object storage, a parallel transcoding pipeline that splits videos into chunks and renditions, adaptive bitrate streaming with HLS-style segments and manifests, CDN delivery at terabits per second, storage tiering, view counting, failures, content safety and the metrics that matter to viewers.
GuideTechnology
19 min read
Design the payment platform behind checkout and refunds, where correctness matters more than speed: payment intents and attempts, an explicit state machine, idempotency at every hop, asynchronous confirmation through verified webhooks, a double-entry ledger with a runnable example, reconciliation against provider reports, refunds, provider failover, fraud and PCI scope, and an audit trail.
GuideTechnology
16 min read
Design ticketing for concerts and matches where thousands of people want the same seats at the same moment: seat inventory modelling, holds with expiry, atomic all-or-nothing seat claims, idempotent payment and confirmation, what happens when payment succeeds after a hold expires, virtual waiting rooms for big on-sales, hot inventory, bots, failures and the invariant that a seat is never sold twice.
GuideTechnology
17 min read
Design a service that runs millions of one-time and recurring jobs at the right time, at least once, across many machines: time-bucketed storage, dispatchers that own partitions through leases, atomic claims that prevent double dispatch, queues and workers with heartbeats, retries and dead letters, misfire policies after outages, top-of-the-hour spikes, time zones, and safe HTTP callbacks.
GuideTechnology
14 min read
Design the suggestions that appear as users type in a search box: requirements and estimates per keystroke, why querying the database per keystroke fails, an offline pipeline that turns search logs into prefix-to-top-K tables or tries, in-memory serving, client debouncing and CDN caching, trending updates, sharding and replication, privacy filtering and failures.
GuideTechnology
16 min read
Design a home feed that shows posts from accounts a user follows: estimates for posting, reading and fan-out, fan-out on read versus fan-out on write, the hybrid model that handles celebrity accounts, precomputed feed storage, hydration, ranking with a chronological fallback, cursor pagination, deletions and privacy, and what changes at social-media scale.
GuideTechnology
18 min read
Design one-to-one and group chat for tens of millions of concurrent users: WebSocket connection gateways, a session registry for routing messages to the right server, per-conversation sequence numbers for ordering, acknowledgements and deduplication, offline delivery with push and sync, multi-device cursors, presence, partitioned message storage, failures and security.
GuideTechnology
17 min read
Design a notification platform that other services use to reach users by push, SMS, email and in-app inbox: requirements and estimates, a 202-accepted API with idempotency keys, preferences and templates, priority queues per channel, provider rate limits and failover, retries and dead-letter queues, deduplication, delivery tracking, campaign fan-out, failures, security and observability.
GuideTechnology
15 min read
Design an in-memory cache service shared by many applications: requirements and estimates, the get/set/delete API, a single-node design with TTLs and LRU eviction, then partitioning with consistent hashing or hash slots, client-side versus proxy routing, replication and failover, rebalancing, hot keys, cache stampedes, and what consistency a cache can and cannot promise.
GuideTechnology
19 min read
Design a rate limiter that enforces limits across many API servers: requirements, estimates, where the limiter sits, token bucket, leaky bucket, fixed window, sliding log and sliding window counter algorithms with a runnable comparison, atomic Redis implementations, local plus global hybrids, failure behaviour (fail open or fail closed), multi-region limits and response headers.
GuideTechnology
20 min read
The classic first system design problem, worked end to end with the course's method: requirements, estimates, APIs, data model, a simple baseline, short-code generation and collisions, 301 versus 302 redirects, caching a read-heavy path, expiry, analytics off the critical path, sharding, failures, abuse prevention, and the Pastebin variant with object storage.
GuideTechnology
14 min read
A practical, repeatable way to run a 45-minute system design interview: what interviewers typically evaluate, a 21-item checklist grouped into seven phases, a time plan, phrases for clarifying, estimating and stating trade-offs, how to handle hints and pushback, how SDE-1 and SDE-2 expectations differ, and a compact worked example.
GuideTechnology
16 min read
The security decisions that shape an architecture: trust boundaries and least privilege, authentication with passwords, MFA and identity providers, sessions versus tokens and JWTs, object-level authorisation, service-to-service identity, encryption in transit and at rest, secrets management, input validation and abuse prevention, SSRF, data privacy, audit logs and payment data.
GuideTechnology
14 min read
When services communicate by publishing facts instead of calling each other: commands versus events, event notification versus event-carried state transfer, designing and versioning events, ordering, replay, and the two patterns often paired with events: CQRS read models and event sourcing, with honest guidance on when each is worth its complexity.
GuideTechnology
14 min read
An honest comparison of monoliths, modular monoliths and microservices: what each gives and costs, why a modular monolith is often the best start, how to draw service boundaries and own data, synchronous versus asynchronous communication, API gateways, service discovery, service meshes, the distributed-monolith trap, and migrating with the strangler fig pattern.
GuideTechnology
14 min read
If you cannot observe a system, you cannot operate it. Learn the three signals (structured logs, metrics and distributed traces), the golden signals and RED and USE methods, percentiles and histograms, trace context propagation and OpenTelemetry, SLIs, SLOs and error-budget alerts, health checks and synthetic monitoring, and how to observe asynchronous systems without drowning in data.
GuideTechnology
14 min read
How systems survive failures of servers, availability zones and whole regions, and recover from data disasters. Covers failure domains, redundancy and failover, availability arithmetic with correlated failures, RPO and RTO, the four disaster-recovery strategies from backup-and-restore to active-active, multi-region write strategies, backups that actually restore, and safe deployments.
GuideTechnology
16 min read
How one slow dependency takes down a whole system, and the techniques that stop it: timeouts and deadlines, retry budgets, circuit breakers, bulkheads, rate limiting, load shedding, backpressure and graceful degradation, with a runnable circuit breaker and a degradation plan for a real page.
