# How it's built
### System Overview
Eli is built as a modern, cloud-native SaaS platform designed for global enterprise deployment. Our architecture prioritizes security, scalability, and compliance while maintaining high availability across multiple geographic regions.
### System Architecture Diagram
> **Note**: This diagram provides a simplified view of Eli's core architecture and its purpose is not to display all of its sub-processors. For a complete list of sub-processors and third-party services we work with, please refer to our [Who We Work With](/confidentialite-et-securite-en/service-overview/who-we-work-with.md) page.
### Application Architecture
#### Application layer
**Admin app (Front only) - `app.eliapp.io`**
* **Purpose**: Campaign management interface for organizational administrators, HR teams, and communication managers
* **Technology**: Next.js 15.2, React 19, TypeScript, Tailwind CSS
* **Platform**: Any browser, ES2022
* **Key Features**: Campaign creation, analytics dashboard, user management, content authoring
* **Authentication**: Multi-method support through Eli Authentication Service
* **API Integration**: Consumes Admin API
**Employee app (Front only) - `my.eliapp.io`**
* **Purpose**: Employee engagement platform for campaign participation, content consumption and employee interaction
* **Technology**: Next.js 15.2, React 19, TypeScript, Tailwind CSS
* **Platform**: Any browser, ES2022
* **Key Features**: Campaign participation, progress tracking, interactive challenges, online communities
* **Authentication**: Multi-method support through Eli Authentication Service
* **API Integration**: Consumes User API
**Mobile app - `m.eliapp.io`**
* **Purpose**: Employee engagement platform for campaign participation and content consumption and employee interaction
* **Technology**: Flutter framework for cross-platform deployment
* **Platforms**: Native iOS and Android applications
* **Key Features**: Campaign participation, progress tracking, enhanced interactive challenges, online communities, offline capabilities, push notifications, deep linking
* **Authentication**: Multi-method support through Eli Authentication Service
* **API Integration**: Consumes User API
**Auth app (Full-stack) - `auth.eliapp.io`**
* **Purpose**: Centralized authentication and session management for other Eli apps
* **Technology**: Next.js 15.2, React 19, Firebase Auth integration
* **Features**: SSO support, session management, security token generation
* **Integration**: Validates all requests across the platform ecosystem
* **Authentication methods**:
* Sign in With Google
* Sign in With Apple
* Email / password
* OTP via email
**API app (Back only) - `api.eliapp.io`**
* **Framework**: Express.js with Node.js runtime
* **API Design**: tRPC (Type-safe Remote Procedure Calls) for end-to-end type safety
* **Architecture**: Microservices pattern with three dedicated API endpoints:
* **User API** (`/trpc/user`): Employee-facing features and mobile app support. Cookie-based auth.
* **Admin API** (`/trpc/admin`): Administrative functions and campaign management. Cookie-based auth.
* **Service API** (`/trpc/service`): External integrations, webhooks, and system callbacks. 164-chars Bearer tokens auth.
### Data layer
**Our database: MongoDB Atlas**
* **Configuration**: 3-replica set for high availability and data redundancy
* **Location**: Google Cloud Platform, Belgium region (europe-west1)
* **Features**: Automatic failover, point-in-time recovery, encryption at rest
* **ORM**: Prisma for type-safe database access and schema management. Prisma schemas (and API schemas) are shared to all apps through internal packages for bullet-proof consistency and safety.
**Our file storage: Firebase storage**
* **Location**: Belgium (europe-west1)
* **Use Cases**: User profile pictures, images shared in posts,...
### Infrastructure & Hosting
**Our primary hosting platform: Vercel**
* **Location**: Paris, France (AWS eu-west-3 region)
* **Features**:
* Global CDN with edge computing capabilities
* Automatic scaling based on traffic demand
* Built-in DDoS protection and security filtering
* Zero-downtime deployments
**Our queue management system: Upstash Qstash**
* **Use Cases**: Email sending, Push Notifications, background processing, scheduled tasks
* **Features**: Automatic retries, dead letter queues, monitoring and observability
**Our email delivery system: Resend**
* **Location**: Ireland (AWS eu-west-1 region)
* **Use Cases**: Transactional emails, campaign notifications, OTP delivery, system alerts
* **Features**: High deliverability rates, webhook support, email analytics, API-first design
### Scalability & Performance
**Auto-scaling capabilities**
* **Application layer**: Automatic horizontal scaling on Vercel based on traffic demand
* **Database**: MongoDB Atlas auto-scaling with performance optimization
* **Queue system**: Distributed processing with automatic retry mechanisms
* **CDN**: Global edge locations for optimal content delivery all around the globe
### Data Residency & EU compliance at a glance
* **Web Services**: Hosted in Paris, France (AWS eu-west-3 CDG1)
* **Database**: Hosted in Belgium (GCP europe-west1)
* **Compliance**: GDPR-compliant by design with EU data residency
* **Security**: All communications over HTTPS/TLS 1.3 encryption
## Development & Deployment
**Version control**
We use Gitlab for all our projects.
**Continuous Integration & Deployment**
**Web apps (all of them)**
* Vercel CI pipeline for build automation and deployment
* Zero-warning linting policy at high standards
* Staging environments and rollback capabilities
**Android app**
* Gitlab CI pipeline for build automation and Play Store deployment
* Zero-warning linting policy
* Staging environments and rollback capabilities
**iOS app**
* Gitlab CI pipeline for build automation and App Store deployment
* Zero-warning linting policy
* Staging environments and rollback capabilities
## Technology Stack Summary
**Frontend & mobile**: Next.js 15.2, React 19, TypeScript, Tailwind CSS, Flutter\
**Backend**: Node.js, Express.js, tRPC, Prisma ORM\
**Database**: MongoDB Atlas (3-replica set)\
**Authentication**: Firebase Auth\
**Storage**: Firebase Storage\
**Queue**: Upstash QStash\
**Email**: Resend\
**Hosting**: Vercel (Paris), GCP (Belgium)\
**CI/CD**: Vercel, GitLab CI, XCode Cloud
***
**For technical architecture questions, contact:** [security@eli.app](mailto:security@eliapp.io)
**Last Updated:** October 2025
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