Basistha Kumar

End-to-End Encrypted Messaging Backend

• ▸ Designed a passwordless, anonymous authentication system using Ed25519 key pairs and challenge-response verification, requiring only a username and public key at registration with no email, phone number, or password ever stored or transmitted.
• ▸ Built a stateless JWT security layer with dual-path invalidation: an explicit revoked-token table for logout and timestamp-based rejection for key rotation, ensuring prior sessions are invalidated without a per-token blocklist entry.
• ▸ Implemented a WebSocket/STOMP delivery pipeline pushing encrypted messages to recipients in real time while persisting ciphertext and nonces server-side with no ability to read message content.
• ▸ Engineered an account recovery system generating eight BCrypt-hashed one-time codes at registration, allowing users to reclaim access and install a new key pair without any server-stored password.
• ▸ Configured Spring Security with a custom OncePerRequestFilter for JWT validation, revocation checks, and key-rotation enforcement, and a separate STOMP channel interceptor for WebSocket authentication.

Encrypted File Storage & Management Backend

• ▸ Engineered file upload and download via AWS S3 presigned URLs, keeping file bytes out of the application server entirely — the backend issues a time-limited signed URL and the client transacts with S3 directly, decoupling storage I/O from API throughput.
• ▸ Designed a passwordless authentication system using Ed25519 key pairs and challenge-response verification with stateless JWT sessions, using dual-path invalidation: an explicit revoked-token table for logout and timestamp-based rejection for key rotation, ensuring prior sessions are invalidated without a per-token blocklist entry per user.
• ▸ Built a hierarchical folder system with recursive parent-folder references and a granular per-resource permission model, allowing owners to grant read or write access to individual recipients on both files and folders independently.
• ▸ Implemented per-user storage quota enforcement with atomic quota updates on every upload, deletion, and soft-restore, rejecting requests that would breach the allocated limit before a presigned URL is issued to the client.
• ▸ Implemented a two-tier scheduled cleanup: a fixedRate job every 5 minutes soft-deleting demo files older than 30 minutes from S3 and the database, and a nightly cron job performing a full hard-delete cascade of demo accounts — files, folders, shares, recovery keys, and auth challenges — in dependency order to avoid foreign-key violations.

Collaborative Document Editor & Project Management Backend

• ▸ Built WebSocket/STOMP real-time collaboration layer enabling low-latency document sync across concurrent users.
• ▸ Implemented CRDT-based conflict resolution using the YATA algorithm with deterministic ordering (counter @ clientId), ensuring all clients converge to identical document state regardless of operation order.
• ▸ Architected JWT authentication and role-based access control with Spring Security for multi-user document permissions.
• ▸ Designed PostgreSQL persistence via Spring Data JPA with JSON-serialized CRDT state; containerized full stack with Docker Compose.

E-Commerce REST API

• ▸ Architected a RESTful e-commerce backend using Spring Boot with stateless JWT authentication, BCrypt password hashing, and role-based access control separating public, user, and admin endpoints.
• ▸ Implemented transactional order processing with pessimistic write locking on stock fields to prevent overselling under concurrent requests, with automatic stock restoration on order cancellation.
• ▸ Built a token-bucket rate limiter using Bucket4j applied per client IP on authentication endpoints, rejecting excess requests with HTTP 429 before JWT parsing is attempted.
• ▸ Designed a schema migration strategy using a custom ApplicationRunner to perform idempotent DDL operations via JdbcTemplate post-Hibernate-initialisation, handling column drops and type conversions that ddl-auto=update cannot perform on existing tables.
• ▸ Integrated JavaMail for HTML order confirmation and cancellation emails with isolated exception handling ensuring mail failures never roll back the enclosing transaction.

Real-time Formula 1 Race Data Platform

• ▸ Built a real-time data pipeline to ingest high-frequency telemetry (weather, tyre degradation, safety car scenarios) and persist results in PostgreSQL for downstream analytics.
• ▸ Designed a stateless REST architecture with PostgreSQL connection pooling enabling horizontal scaling under concurrent simulation load.
• ▸ Integrated external race strategy prediction service via RESTful APIs, handling structured JSON payloads with full input validation.
• ▸ Owned all backend API integration, data pipeline design, and database schema end-to-end.

Resume-to-JD Fit Assessment API

• ▸ Used Spring AI's .entity(FitAssessment.class) for structured output: BeanOutputConverter reflects on the record's fields, generates a JSON schema, appends format instructions to the outgoing prompt, and deserializes the model's response directly into a typed Java record.
• ▸ Injected ChatClient.Builder (auto-configured by the OpenAI starter) and called .build() once at construction, following the Spring AI idiom that allows attaching defaults like system messages or advisors before the client is finalized.
• ▸ Loaded the prompt from a classpath resource file using PromptTemplate, substituting {resume} and {jobDescription} placeholders at request time, separating prompt content from code so wording can be iterated without recompiling.
• ▸ Wired a Groq provider profile using the OpenAI-compatible base-url override, demonstrating provider swapping with zero code changes: same service, different model, activated with -Dspring.profiles.active=groq.
• ▸ Validated requests with @NotBlank and @Size constraints, mapping validation failures to structured 400 responses and AI provider failures to 502/503 via @RestControllerAdvice.