# Single-Instance Multi-Tenant Architecture Exploration
Ramblers head office is in discussions about adopting NGX Ramblers as a solution to roll out to groups nationally. **The primary driver is the Brevo email integration** — Ramblers Head Office wants to offer groups a ready-made email solution (newsletters, walk notifications, booking confirmations) without each group having to set up and manage their own email infrastructure. One of their preferences is that a single NGX instance could serve multiple groups rather than the current model of one deployment per group. **Scale target: up to 500 groups.**
## Target Architecture — outlined more detail in the following articles
```mermaid
flowchart TD
D1@{ icon: "ngx:user", label: "group-a-ramblers.org.uk", pos: "b", h: 48 }
D2@{ icon: "ngx:user", label: "group-b-ramblers.org.uk", pos: "b", h: 48 }
DN@{ icon: "ngx:user", label: "...up to 500 domains", pos: "b", h: 48 }
D1 --> CF
D2 --> CF
DN --> CF
CF@{ icon: "ngx:cloudflare", label: "Cloudflare DNS", pos: "b", h: 48 }
CF --> FLY
subgraph FLY["Fly.io Application - 2 to 4 instances"]
TM["Tenant Resolution Middleware
Host header to groupCode"]
CACHE["Domain-to-group cache
refreshed periodically"]
DBR["Database Router
groupCode to mongoose.useDb"]
TM --> CACHE
TM --> DBR
end
DBR --> MDB1@{ icon: "ngx:mongodb", label: "Group A DB", pos: "b", h: 48 }
DBR --> MDB2@{ icon: "ngx:mongodb", label: "Group B DB", pos: "b", h: 48 }
DBR --> MDBN@{ icon: "ngx:mongodb", label: "Group N DB", pos: "b", h: 48 }
MDB1 --> S3
MDB2 --> S3
MDBN --> S3
S3@{ icon: "ngx:aws", label: "Single S3 Bucket
/{groupCode}/images/", pos: "b", h: 48 }
style FLY fill:#E8F5EE,stroke:#9BC8AB,stroke-width:2px,rx:12,ry:12,color:#404143
```
_____
This series of articles explores what would be required to re-architect NGX Ramblers so that any number of groups run off a single application instance, while maintaining the same segregated user experience — where navigating to a group's domain shows only that group's branding, members, walks, and content.
## Platform Overview
What NGX Ramblers provides beyond email — walks, bookings, galleries, CMS, and more
## Current Architecture
How the per-group deployment model works today — one Docker image, 13 independent apps
## Proposed Architecture
Single-instance multi-tenant design — tenant resolution, database switching, request lifecycle
## Brevo Email at Scale
The primary rollout driver — outbound email, deliverability, and multi-tenant email isolation
## Billing and Costs
Fly.io free-tier analysis, cost comparison at 500 groups, and billing risk assessment
## Assessment and Recommendation
Honest pros and cons, effort estimates (AI-assisted vs manual), and recommended approach
