A practical tech blog about Spring Boot, Backend development, and technical interview prep.https://rhcwlq89.github.io/https://rhcwlq89.github.io/en/blog/aws-credential-federation-deep-dive/https://rhcwlq89.github.io/en/blog/aws-credential-federation-deep-dive/A foundational guide for engineers who followed a GitHub Actions OIDC tutorial but still aren't sure what STS really is, why it has no console page, what 'federation' actually means, or why the trust policy's sub condition matters so much. Covers IAM/STS/OIDC mechanics in depth, plus SAML, IAM Identity Center, and EKS IRSA as variants of the same pattern.Tue, 28 Apr 2026 01:00:00 GMThttps://rhcwlq89.github.io/en/blog/aws-private-ec2-guide-5/https://rhcwlq89.github.io/en/blog/aws-private-ec2-guide-5/Pull the monthly bill of the environment Parts 1–4 produced apart, line by line, and find where to cut. Real numbers for NAT Gateway, ALB, EC2, and data transfer; the levers that matter (S3 Gateway Endpoint, Graviton, Savings Plans); and budget templates for side-project, startup, and enterprise tiers.Mon, 27 Apr 2026 09:00:00 GMThttps://rhcwlq89.github.io/en/blog/aws-private-ec2-guide-4/https://rhcwlq89.github.io/en/blog/aws-private-ec2-guide-4/Build a deployment pipeline that puts no port 22 and no AWS access key on the path. OIDC federation removes static credentials, S3 acts as the artifact hub, and either SSM Run Command (simple) or CodeDeploy (zero-downtime) lands the code on the instance.Mon, 27 Apr 2026 06:00:00 GMThttps://rhcwlq89.github.io/en/blog/aws-private-ec2-guide-3/https://rhcwlq89.github.io/en/blog/aws-private-ec2-guide-3/How to land a shell on a Private EC2 without ever opening port 22. The mechanics of SSM Session Manager (the agent polls AWS), the three prerequisites (Agent / IAM / network path), the cost trade-off between NAT Gateway and VPC Endpoints, and the port-forwarding pattern that securely reaches RDS without a VPN.Mon, 27 Apr 2026 01:00:00 GMThttps://rhcwlq89.github.io/en/blog/aws-private-ec2-guide-2/https://rhcwlq89.github.io/en/blog/aws-private-ec2-guide-2/Turn the Part 1 architecture into real code. We build a 10.0.0.0/16 VPC, 2AZ Public/Private subnets, IGW/NAT Gateway, the SG-references-SG pattern, and ALB + EC2 — all in one main.tf that comes up with a single terraform apply.Sat, 25 Apr 2026 01:00:00 GMThttps://rhcwlq89.github.io/en/blog/aws-private-ec2-guide-1/https://rhcwlq89.github.io/en/blog/aws-private-ec2-guide-1/We unpack the 'why' behind the standard pattern of placing EC2 in a Private Subnet wrapped by ALB and NAT Gateway. Public IPv4 vs Elastic IP, sizing recommendations, HA basics, and the concrete risks of putting PII-handling servers in a Public Subnet. Part 1 of a Bastion-less AWS EC2 operations series.Fri, 24 Apr 2026 01:00:00 GMThttps://rhcwlq89.github.io/en/blog/rdb-migration-multitenant-guide/https://rhcwlq89.github.io/en/blog/rdb-migration-multitenant-guide/Changing schemas on a live service is as hard as designing them. ALTER TABLE lock traps, zero-downtime column changes with Expand-Contract, Flyway vs Liquibase, rollback strategies, and three multi-tenant schema strategies. The final part of the series.Thu, 16 Apr 2026 13:00:00 GMThttps://rhcwlq89.github.io/en/blog/rdb-antipattern-temporal-design-guide/https://rhcwlq89.github.io/en/blog/rdb-antipattern-temporal-design-guide/Covers repeatedly failing schema design patterns. EAV, God Table, Polymorphic Association, Soft Delete traps, and then the patterns you should actively use — Temporal Tables, Slowly Changing Dimensions, and Event Sourcing basics. Knowing what NOT to do is a design skill.Thu, 16 Apr 2026 09:00:00 GMThttps://rhcwlq89.github.io/en/blog/rdb-index-join-strategy-guide/https://rhcwlq89.github.io/en/blog/rdb-index-join-strategy-guide/Go beyond index basics into design-level thinking. Covering indexes, partial indexes, cardinality analysis, JOIN algorithms (Nested Loop, Hash, Merge), reading EXPLAIN ANALYZE, and the N+1 problem. Turn index creation into index design.Wed, 15 Apr 2026 09:00:00 GMThttps://rhcwlq89.github.io/en/blog/rdb-domain-schema-design-guide/https://rhcwlq89.github.io/en/blog/rdb-domain-schema-design-guide/Design the e-commerce order domain at production quality. 3-level hierarchy (order → delivery → item), state machines, payment idempotency, split payments, partial refunds, and cancellation audit trails. One domain, all the way down.Wed, 08 Apr 2026 09:00:00 GMThttps://rhcwlq89.github.io/en/blog/rdb-relationship-patterns-guide/https://rhcwlq89.github.io/en/blog/rdb-relationship-patterns-guide/Stop designing table relationships by gut feeling. From 1:1 split criteria to 1:N vs N:M judgment, self-referencing trees, and polymorphic associations — every relationship design pattern you'll face in practice.Wed, 08 Apr 2026 05:00:00 GMThttps://rhcwlq89.github.io/en/blog/rdb-constraints-integrity-guide/https://rhcwlq89.github.io/en/blog/rdb-constraints-integrity-guide/CHECK, UNIQUE, FK, DEFAULT, Generated Columns — constraints aren't 'annoying overhead,' they're 'defense code baked into the schema.' This guide provides practical criteria for DB vs app validation, whether to use FKs or not, and defensive schema design patterns.Tue, 07 Apr 2026 05:00:00 GMThttps://rhcwlq89.github.io/en/blog/rdb-normalization-guide/https://rhcwlq89.github.io/en/blog/rdb-normalization-guide/Normalization isn't 'always required,' and denormalization isn't 'a compromise for performance.' Using a single online store scenario, this guide explains 1NF through 3NF intuitively, then presents clear criteria for when breaking normalization is the right call — with MySQL/PostgreSQL differences.Sun, 05 Apr 2026 07:00:00 GMThttps://rhcwlq89.github.io/en/blog/rdb-schema-basics-guide/https://rhcwlq89.github.io/en/blog/rdb-schema-basics-guide/The first decisions you need to make when designing RDB tables. Naming conventions, data type selection, PK strategies (AUTO_INCREMENT vs UUID vs ULID), and NULL semantics — skip these now, pay for it later with painful migrations.Sun, 05 Apr 2026 05:00:00 GMThttps://rhcwlq89.github.io/en/blog/spec-driven-development-guide/https://rhcwlq89.github.io/en/blog/spec-driven-development-guide/Beyond Vibe Coding — write the spec first, let AI generate the code. Covers SDD concepts, workflow, tools, and its realistic limitations.Fri, 03 Apr 2026 13:00:00 GMThttps://rhcwlq89.github.io/en/blog/java-concurrent-practical-patterns/https://rhcwlq89.github.io/en/blog/java-concurrent-practical-patterns/ExecutorService, CompletableFuture, CountDownLatch, ConcurrentHashMap, BlockingQueue, Semaphore, ReentrantLock — 7 commonly used java.util.concurrent classes with real-world patterns and working code.Thu, 26 Mar 2026 05:00:00 GMThttps://rhcwlq89.github.io/en/blog/fcfs-load-test-behind-the-scenes/https://rhcwlq89.github.io/en/blog/fcfs-load-test-behind-the-scenes/Problems we hit while k6-testing 4 FCFS strategies — Rate Limiter conflicts, queue over-counting bugs. Plus 4 ways to isolate FCFS APIs from regular APIs in production: separate DataSource, Redis offloading, service separation, and Bulkhead pattern.Wed, 25 Mar 2026 05:00:00 GMThttps://rhcwlq89.github.io/en/blog/fcfs-load-test-comparison/https://rhcwlq89.github.io/en/blog/fcfs-load-test-comparison/DB Lock, Redis, Queue, Token — we load-tested all 4 FCFS strategies under identical conditions (100 stock, 100/500/1,000/2,000 concurrent users) with k6. Each scenario ran 10 times to get stable averages for TPS, P95/P99 response times, and we identify the optimal strategy for each situation.Tue, 24 Mar 2026 13:00:00 GMThttps://rhcwlq89.github.io/en/blog/fcfs-token-implementation/https://rhcwlq89.github.io/en/blog/fcfs-token-implementation/Implements a first-come-first-served system that issues entry tokens upfront — only token holders can purchase. Covers JWT-based token design, TTL expiration strategy, and multi-layered bot prevention with Rate Limiting + CAPTCHA.Tue, 24 Mar 2026 11:00:00 GMThttps://rhcwlq89.github.io/en/blog/fcfs-queue-implementation/https://rhcwlq89.github.io/en/blog/fcfs-queue-implementation/Implements a first-come-first-served system using Redis Sorted Set waiting queues and Kafka message queues. Covers queue UX (polling vs WebSocket), slot reallocation on timeout/abandonment, and traffic spike absorption strategies.Tue, 24 Mar 2026 09:00:00 GMThttps://rhcwlq89.github.io/en/blog/fcfs-redis-implementation/https://rhcwlq89.github.io/en/blog/fcfs-redis-implementation/Implements a first-come-first-served stock deduction system using Redis DECR atomic operations and Lua scripts. Builds it with Spring Boot + Redisson, directly compares TPS against DB locks under identical conditions (100 stock, 100 concurrent users), and covers Redis-DB consistency and failure recovery strategies.Tue, 24 Mar 2026 07:00:00 GMThttps://rhcwlq89.github.io/en/blog/fcfs-db-lock-implementation/https://rhcwlq89.github.io/en/blog/fcfs-db-lock-implementation/Implements a first-come-first-served stock deduction system using SELECT FOR UPDATE. Builds it with Spring Boot + JPA, validates data consistency with 100 concurrent requests, and identifies where DB locks hit their limits.Fri, 20 Mar 2026 07:00:00 GMThttps://rhcwlq89.github.io/en/blog/fcfs-system-comparison-guide/https://rhcwlq89.github.io/en/blog/fcfs-system-comparison-guide/Breaks down the core challenges of FCFS systems — concurrency, stock deduction, duplicate prevention — and compares 6 implementation strategies from DB locks to Redis, message queues, waiting queues, and token issuance.Fri, 20 Mar 2026 05:00:00 GMThttps://rhcwlq89.github.io/en/blog/db-deadlock-and-lock-strategy-guide/https://rhcwlq89.github.io/en/blog/db-deadlock-and-lock-strategy-guide/Real deadlock cases at each isolation level — Read Committed, Repeatable Read, and Serializable. Covers pessimistic vs optimistic locking, FOR UPDATE mechanics, and why you'll eventually need Redis.Tue, 17 Mar 2026 14:50:00 GMThttps://rhcwlq89.github.io/en/blog/db-isolation-level-guide/https://rhcwlq89.github.io/en/blog/db-isolation-level-guide/A practical guide to the 4 transaction isolation levels with real-world examples. Covers concurrency anomalies, MySQL vs PostgreSQL differences, and how to choose the right level.Tue, 17 Mar 2026 13:30:00 GMThttps://rhcwlq89.github.io/en/blog/tsdb-introduction-guide/https://rhcwlq89.github.io/en/blog/tsdb-introduction-guide/What is a Time Series Database (TSDB), why RDBs fall short for time series data, and a comparison of Prometheus, InfluxDB, and TimescaleDB.Tue, 17 Mar 2026 12:30:00 GMThttps://rhcwlq89.github.io/en/blog/http-custom-headers-guide/https://rhcwlq89.github.io/en/blog/http-custom-headers-guide/A comprehensive guide to commonly used HTTP custom headers like X-Request-ID and X-Forwarded-For. Covers the history of the X- prefix convention, CORS implications, and Spring Boot implementation.Mon, 16 Mar 2026 05:00:00 GMThttps://rhcwlq89.github.io/en/blog/claude-code-advanced-guide-3/https://rhcwlq89.github.io/en/blog/claude-code-advanced-guide-3/A practical guide to delegating tasks with sub-agents and orchestrating parallel collaboration with agent teams in Claude CodeSat, 14 Mar 2026 11:00:00 GMThttps://rhcwlq89.github.io/en/blog/claude-code-advanced-guide-2/https://rhcwlq89.github.io/en/blog/claude-code-advanced-guide-2/A practical guide to extending Claude Code with plugins, connecting external tools via MCP, and using Claude natively in VS CodeSat, 14 Mar 2026 10:00:00 GMThttps://rhcwlq89.github.io/en/blog/claude-code-advanced-guide-1/https://rhcwlq89.github.io/en/blog/claude-code-advanced-guide-1/A practical guide to Claude Code's memory system (CLAUDE.md, auto memory), 5 bundled skills, custom skills, and workflow automation with hooksSat, 14 Mar 2026 09:00:00 GMThttps://rhcwlq89.github.io/en/blog/cors-understanding-guide/https://rhcwlq89.github.io/en/blog/cors-understanding-guide/Understand how CORS (Cross-Origin Resource Sharing) works and learn how to resolve common CORS errors in practiceThu, 12 Mar 2026 06:00:00 GMThttps://rhcwlq89.github.io/en/blog/saml-authentication-guide/https://rhcwlq89.github.io/en/blog/saml-authentication-guide/A practical guide to SAML 2.0 protocol core concepts and implementing Microsoft Entra ID (Azure AD) SAML integration with Spring BootThu, 12 Mar 2026 01:00:00 GMThttps://rhcwlq89.github.io/en/blog/terraform-basics-guide/https://rhcwlq89.github.io/en/blog/terraform-basics-guide/A comprehensive guide covering IaC fundamentals, Terraform core concepts, workflow, state management, and modules for developers getting started with infrastructure as codeSun, 08 Mar 2026 05:00:00 GMThttps://rhcwlq89.github.io/en/blog/kind-loki-grafana-log-monitoring/https://rhcwlq89.github.io/en/blog/kind-loki-grafana-log-monitoring/A hands-on guide to setting up Loki, Promtail, and Grafana on a kind cluster to collect and query Spring Boot application logsSun, 08 Mar 2026 02:00:00 GMThttps://rhcwlq89.github.io/en/blog/kind-argocd-gitops-pipeline/https://rhcwlq89.github.io/en/blog/kind-argocd-gitops-pipeline/A hands-on guide to installing ArgoCD on a kind cluster and building a GitOps automated deployment pipeline with the marketplace-deploy projectSun, 08 Mar 2026 01:00:00 GMThttps://rhcwlq89.github.io/en/blog/kind-local-k8s-cluster-setup/https://rhcwlq89.github.io/en/blog/kind-local-k8s-cluster-setup/A hands-on guide to building a multi-node Kubernetes cluster locally using kind and deploying a Spring Boot applicationSun, 08 Mar 2026 00:00:00 GMThttps://rhcwlq89.github.io/en/blog/eks-argocd-gitops-pipeline-guide/https://rhcwlq89.github.io/en/blog/eks-argocd-gitops-pipeline-guide/A guide to installing ArgoCD on EKS and building a complete GitOps CI/CD pipeline with GitHub ActionsSat, 07 Mar 2026 02:00:00 GMThttps://rhcwlq89.github.io/en/blog/eks-production-setup-guide/https://rhcwlq89.github.io/en/blog/eks-production-setup-guide/A practical guide to building production-grade EKS clusters from eksctl to Terraform, including networking setupSat, 07 Mar 2026 01:00:00 GMThttps://rhcwlq89.github.io/en/blog/kubernetes-basics-guide/https://rhcwlq89.github.io/en/blog/kubernetes-basics-guide/A comprehensive guide to Kubernetes basics - architecture, core objects, networking, deployment strategies, and essential kubectl commandsSat, 07 Mar 2026 00:00:00 GMThttps://rhcwlq89.github.io/en/blog/claude-code-popular-plugins-guide/https://rhcwlq89.github.io/en/blog/claude-code-popular-plugins-guide/We handpicked 10 popular plugins that supercharge Claude Code, breaking down each one's core features, why developers love them, and the real-world drawbacks to watch out for.Thu, 05 Mar 2026 01:00:00 GMThttps://rhcwlq89.github.io/en/blog/oh-my-opencode-guide/https://rhcwlq89.github.io/en/blog/oh-my-opencode-guide/A practical guide to installing Oh My OpenCode plugin for OpenCode and configuring multi-agent orchestration, LSP/AST tools, and MCP integrationsSun, 08 Feb 2026 11:00:00 GMThttps://rhcwlq89.github.io/en/blog/spring-sso-integration-guide/https://rhcwlq89.github.io/en/blog/spring-sso-integration-guide/A practical guide to SSO integration with IdPs like Keycloak, Okta, and Azure AD using Spring SecurityThu, 05 Feb 2026 06:00:00 GMThttps://rhcwlq89.github.io/en/blog/aws-database-selection-guide/https://rhcwlq89.github.io/en/blog/aws-database-selection-guide/How to choose between running a DB directly on EC2 with Docker, RDS, Aurora, and Aurora Serverless v2 — a detailed comparison based on cost, performance, and operational burdenTue, 03 Feb 2026 06:30:00 GMThttps://rhcwlq89.github.io/en/blog/aws-cli-resource-guide/https://rhcwlq89.github.io/en/blog/aws-cli-resource-guide/A comprehensive guide to AWS CLI authentication methods (CloudShell, IAM Identity Center, access keys) and resource query commands for major servicesTue, 03 Feb 2026 02:00:00 GMThttps://rhcwlq89.github.io/en/blog/aws-bastion-host-guide/https://rhcwlq89.github.io/en/blog/aws-bastion-host-guide/A complete guide to configuring SSH access to Private EC2 instances through a Bastion HostTue, 03 Feb 2026 01:00:00 GMThttps://rhcwlq89.github.io/en/blog/aws-ec2-guide/https://rhcwlq89.github.io/en/blog/aws-ec2-guide/A complete guide from EC2 instance basics to production operationsMon, 02 Feb 2026 01:00:00 GMThttps://rhcwlq89.github.io/en/blog/springboot-practical-guide-7/https://rhcwlq89.github.io/en/blog/springboot-practical-guide-7/From SLF4J, Logback, Log4j2 comparison to structured logging, MDC, and centralized log managementMon, 02 Feb 2026 00:00:00 GMThttps://rhcwlq89.github.io/en/blog/springboot-practical-guide-6/https://rhcwlq89.github.io/en/blog/springboot-practical-guide-6/Real-time system monitoring with Prometheus, Grafana, and custom metricsSun, 25 Jan 2026 06:00:00 GMThttps://rhcwlq89.github.io/en/blog/springboot-practical-guide-5/https://rhcwlq89.github.io/en/blog/springboot-practical-guide-5/Handling large-scale data with index design, cursor-based pagination, and Read ReplicasSun, 25 Jan 2026 05:00:00 GMThttps://rhcwlq89.github.io/en/blog/springboot-practical-guide-4/https://rhcwlq89.github.io/en/blog/springboot-practical-guide-4/Building fault-tolerant systems with Circuit Breaker, Rate Limiter, and BulkheadSun, 25 Jan 2026 04:00:00 GMThttps://rhcwlq89.github.io/en/blog/springboot-practical-guide-3/https://rhcwlq89.github.io/en/blog/springboot-practical-guide-3/Building a reliable event system with Kafka and the Outbox patternSun, 25 Jan 2026 03:00:00 GMThttps://rhcwlq89.github.io/en/blog/springboot-practical-guide-2/https://rhcwlq89.github.io/en/blog/springboot-practical-guide-2/A complete caching guide from Cache-Aside pattern to cache problem resolution, ready to apply in productionSun, 25 Jan 2026 02:00:00 GMThttps://rhcwlq89.github.io/en/blog/springboot-practical-guide-1/https://rhcwlq89.github.io/en/blog/springboot-practical-guide-1/A practical guide to solving concurrency issues with atomic UPDATEs and idempotency keysSun, 25 Jan 2026 01:00:00 GMThttps://rhcwlq89.github.io/en/blog/spring-boot-pre-interview-assignment/https://rhcwlq89.github.io/en/blog/spring-boot-pre-interview-assignment/Spring Boot-based online marketplace REST API comprehensive assignment — from product, order, and member domain design to testing and deploymentFri, 23 Jan 2026 01:00:00 GMThttps://rhcwlq89.github.io/en/blog/spring-boot-pre-interview-guide-7/https://rhcwlq89.github.io/en/blog/spring-boot-pre-interview-guide-7/Advanced patterns and architecture — events, async processing, file handling, and multi-moduleWed, 21 Jan 2026 01:00:00 GMThttps://rhcwlq89.github.io/en/blog/spring-boot-pre-interview-guide-6/https://rhcwlq89.github.io/en/blog/spring-boot-pre-interview-guide-6/Deployment environment setup and CI/CD — Docker, GitHub Actions, and ActuatorMon, 19 Jan 2026 01:00:00 GMThttps://rhcwlq89.github.io/en/blog/spring-boot-pre-interview-guide-5/https://rhcwlq89.github.io/en/blog/spring-boot-pre-interview-guide-5/Authentication/authorization and security configuration — Spring Security, JWT, and role managementSat, 17 Jan 2026 01:00:00 GMThttps://rhcwlq89.github.io/en/blog/spring-boot-pre-interview-guide-4/https://rhcwlq89.github.io/en/blog/spring-boot-pre-interview-guide-4/Performance optimization and query tuning — N+1 problem, pagination, and caching strategiesThu, 15 Jan 2026 01:00:00 GMThttps://rhcwlq89.github.io/en/blog/spring-boot-pre-interview-guide-3/https://rhcwlq89.github.io/en/blog/spring-boot-pre-interview-guide-3/API documentation with Swagger/SpringDoc, SLF4J logging, and cross-cutting concerns with AOPTue, 13 Jan 2026 01:00:00 GMThttps://rhcwlq89.github.io/en/blog/spring-boot-pre-interview-guide-2/https://rhcwlq89.github.io/en/blog/spring-boot-pre-interview-guide-2/Environment-specific DB selection and ddl-auto policies, Memory Repository pitfalls, Test Pyramid annotation choices, choosing between test doubles (Dummy, Stub, Spy, Mock, Fake), and using Testcontainers to catch bugs that H2 dialect differences hide — the second most-flagged area in Spring Boot pre-interview submissions, covered in one post.Sun, 11 Jan 2026 01:00:00 GMThttps://rhcwlq89.github.io/en/blog/spring-boot-pre-interview-guide-1/https://rhcwlq89.github.io/en/blog/spring-boot-pre-interview-guide-1/The four-layer design points evaluators repeatedly flag in Spring Boot pre-interview assignments — Controller/Service/Repository/Domain responsibility split, Request → Command conversion, what @Transactional(readOnly=true) actually does, and the three-tier priority of GlobalExceptionHandler. Series Part 1.Fri, 09 Jan 2026 01:00:00 GMThttps://rhcwlq89.github.io/en/blog/docker-compose-guide/https://rhcwlq89.github.io/en/blog/docker-compose-guide/A guide for engineers who are new to Compose and unsure how it differs from a stack of docker run lines, or why their app crashes before the DB is ready even with depends_on. Covers the mental model behind services/networks/volumes, the real behavior of healthcheck and condition: service_healthy, dev/prod separation strategies, and macOS bind mount performance — all in one place.Wed, 07 Jan 2026 01:00:00 GMT