Java 21 + Spring Boot
Receives requests, applies payment rules, schedules reconciliation.
Inspect source ↗02 / INTERVIEW GUIDE
Learn the whole system.
Explain the reason for every tool, the path of a request, and what changes when a component fails.
THE 60 SECOND ANSWER
Tell the engineering story.
PayGuard handles provider timeouts after a simulated charge. The API persists an UNKNOWN payment keyed for idempotency. A worker later queries the provider and reconciles it to SUCCESS. I tested the request flow in Java integration tests and on a GitHub runner, then deployed locally with Compose and Minikube and instrumented it with metrics and logs.
TECHNOLOGY → REASON → CODE
Open the implementation.
PostgreSQL + Flyway
Stores payment state and retries across restarts; migrations version the schema.
Inspect source ↗Idempotency key
Associates retry with the same payment ID; prevents duplicate payment creation.
Inspect source ↗Fake provider
Reproduces timeout after charge without any external gateway or money.
Inspect source ↗Docker Compose
Runs the local API and observability dependencies.
Inspect source ↗Kubernetes + Minikube
Probes, restart, scaling, storage, rollout, and rollback.
Inspect source ↗Prometheus + Grafana
Scrapes measurements and renders dashboards; evaluates SLO rules.
Inspect source ↗Alloy + Loki
Ships PayGuard container logs into a searchable store.
Inspect source ↗Terraform + Ansible
Declares isolated cluster resources and checks the live result.
Inspect source ↗GitHub Actions + Jenkins
Hosted tests and live demo; separate self-managed pipeline definition.
Inspect source ↗PRACTICE QUESTIONS
Questions to answer at interview.
Why is a timeout not a failed payment?
The provider could process the charge before losing the response. UNKNOWN preserves uncertainty until a status check settles it.
What does the idempotency key protect?
A repeated request with the same key maps to the existing payment record and returns the same ID.
What if an application pod dies?
Kubernetes starts a replacement. Payment state remains in PostgreSQL, whose persistent volume survives the application pod restart.
What is the difference between Terraform and Ansible here?
Terraform describes isolated Kubernetes resources. Ansible reads the live cluster and verifies expected operational state.
Can GitHub Pages host the payment API?
No. Pages serves these static explanations; GitHub Actions runs a temporary API for repeatable proof. The local workstation can run the complete stack; GitHub Pages shows dated Grafana and Prometheus captures produced by temporary GitHub runners.