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    CQRS & Event Sourcing

    When the audit trail is the product: every decision is an event in Postgres, the write model enforces invariants, and Redis (or a flat Postgres table) serves queries. Rebuild a projection any time the UI changes. Use this where “what happened” matters more than a mutable row.

    2K – 100Kusers supported
    $250 – $2,200/month infrastructure

    Architecture Diagram

    Interactive — hover over any node to see its role and description.

    UserCommand APIQuery APIAggregateWrite modelEvent StorePostgreSQLProjectorRead ModelRedisAudit UIUserAPIBackendDatabaseQueueCacheFrontend

    Use Cases

    Fintech ledgers and wallet movements
    Healthcare workflows with a legal audit trail
    Complex booking or allocation engines
    Multiplayer game economies (item provenance)
    Any domain where undo/replay is a requirement

    Technology Stack

    frontend

    ReactNext.js

    backend

    Node.jscommand handlers

    database

    PostgreSQL event storeRedis read models

    infrastructure

    RDSElastiCacheworkers

    Scalability Roadmap

    Stage 1Single bounded context users· One Postgres

    One aggregate type. Projectors in-process. Prove replay works.

    Stage 2Several streams users· Async projectors

    Workers, checkpoints, and Redis for the busiest screens.

    Stage 3High write users· Partitioned store

    Partition events by stream. Snapshot aggregates that get long.

    Stage 4Global users· Regional projectors

    Event store in the home region. Redis read models per region. Catch-up on failover.

    Cost Breakdown

    Development Cost

    $20,000 – $55,000 (10–20 weeks) — only if the domain needs it

    Infrastructure Cost

    $250 – $2,200/month

    Maintenance Cost

    $2,000 – $5,000/month for projection bugs and schema evolution

    Security Considerations

    Events are immutable; corrections are new events, not edits
    PII in events encrypted or tokenised at append time
    Rebuild of projections is a privileged operation

    More Architectures

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    pgvector Semantic Search

    Embeddings, HNSW indexes, and hybrid SQL + vector search in Postgres

    Data Infrastructure

    Redis Caching Platform

    Cache-aside, write-through, sessions, and hot-key protection

    Data Infrastructure

    Multi-Region Database

    Aurora Global, regional failover, and reads close to the user

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