Sajal Halder builds Java microservices at Rakuten scale, and now brings AI into banking.

1. Spring RAG AI

   AI

   Read the write-upView Spring RAG AI on GitHub

   • Java 25
   • Spring Boot 4
   • Spring AI
   • pgvector
   • OpenAI

   A retrieval-augmented generation system where agents route each question, retrieve context and check the answer before it is returned.

   • Multi-query expansion and conversation-aware retrieval over PostgreSQL pgvector
   • Query router for intent classification, plus an evaluator that scores answer quality
   • Ingests PDF, Word, PowerPoint, Excel and HTML with structure-aware chunking; streams answers over SSE

2. Expense Tracker AI

   AI

   Read the write-upView Expense Tracker AI on GitHub

   • TypeScript
   • Next.js 16
   • React 19
   • Supabase
   • Vercel AI SDK

   A full-stack expense tracker that reads receipt photos with AI, answers questions about spending in chat, and charts where the money goes.

   • AI reads a receipt photo, can pull several expenses out of one receipt, and asks for confirmation before saving
   • Dashboards for monthly trends, category split, payment methods and week-over-week change
   • Supabase authentication with Row Level Security, so each user only reaches their own data

3. Hybrid Fanout Feed System

   Read the write-upView Hybrid Fanout Feed System on GitHub

   • Java 17
   • Spring Boot
   • Cassandra
   • PostgreSQL
   • Redis

   A Twitter-style feed that switches strategy by audience size: fan-out on write below 10K followers, fan-out on read above.

   • PostgreSQL for relationships, Cassandra for posts and timelines, Redis for feed caching
   • Asynchronous batch fan-out with TTL-based cache expiry
   • Design target of feed generation under 100 ms at p95

4. Banking Microservices

   Read the write-upView Banking Microservices on GitHub

   • Java 17
   • Spring Boot
   • Maven
   • Docker
   • SonarQube

   A banking platform designed around six services (users, accounts, transactions, cards, payments and notifications) behind an API gateway.

   • Hexagonal architecture with domain, application and infrastructure layers
   • Eureka service discovery and a saga coordinator for distributed transactions
   • CI/CD stages for build, test, security scan and a SonarQube quality gate

5. Inventory Service

   Read the write-upView Inventory Service on GitHub

   • Java 17
   • Spring Data JPA
   • Gradle
   • JUnit 5

   A hexagonal-architecture microservice for inventory with concurrency-safe stock adjustments.

   • Optimistic locking for concurrent updates, with pagination and validation
   • Trace IDs generated and propagated per request; Actuator health and metrics
   • Test layout mirrors the main structure, written with JUnit 5

1. Building a RAG pipeline with Spring AI and pgvector

   How a retrieval-augmented generation pipeline fits together in Spring Boot: pgvector storage, structure-aware chunking, query rewriting, routing and answer evaluation.

   25 September 2026

2. Hybrid fan-out feed design: fan-out on write vs fan-out on read

   How a Twitter-style news feed can combine fan-out on write and fan-out on read with a 10K-follower threshold, using PostgreSQL, Cassandra and Redis.

   25 September 2026

3. Building an AI receipt scanner with Next.js, the Vercel AI SDK and Supabase

   How Expense Tracker AI turns a receipt photo into saved expenses with Next.js 16, the Vercel AI SDK and Supabase Row Level Security, and why a person confirms first.

   25 September 2026

All articles

1. 2026 — present

   Senior Consultant

   HLB Hong Leong Bank, Malaysia
   • Integrating AI capabilities into microservice architecture within core banking systems
2. Mar 2023 — May 2026

   Software Engineer

   Bangladesh Japan IT Ltd. (BJIT) · Dhaka
   • Contributed to microservices used by Rakuten that handle billions of requests daily
   • Collaborated with international teams and stakeholders to align on business objectives
   • Participated in peer code reviews, giving feedback to keep code quality high
3. 2017 — 2021

   B.Tech, Computer Science & Engineering

   Lovely Professional University · Punjab, India