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architecture-docs/architecture.md

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+# Fediversity Implementation and planning
+
+## Actors
+
+- Maintainers
+
+  The group developing and maintaining this project.
+  We are creating the deployment workflows and service configurations, and curate changes proposed by contributors.
+
+- Developers
+
+  People with the technical background to engage with our work, and may contribute back, build on top of, remix, or feel inspired by our work to create something better.
+
+- Hosting provider
+
+  They provide and maintain the physical infrastructure, and run the software in this repository, through which operators interact with their deployments.
+  Hosting providers are technical administrators for these deployments, ensuring availability and appropriate performance.
+
+  We target small- to medium-scale hosting providers with 20+ physical machines.
+
+- Operator
+
+  They select the applications they want to run.
+  They don't need to own hardware or deal with operations.
+  Operators administer their applications in a non-technical fashion, e.g. as moderators.
+  They pay the hosting provider for registering a domain name, maintaining physical resources, and monitoring deployments.
+
+- User
+
+  They are individuals using applications run by the operators, and e.g. post content.
+
+## Glossary
+
+- [Fediverse](https://en.wikipedia.org/wiki/Fediverse)
+
+  A collection of social networking applications that can communicate with each other using a common protocol.
+
+- Application
+
+  User-facing software (e.g. from Fediverse) configured by operators and used by users.
+
+- Configuration
+
+  A collection of settings for a piece of software.
+
+  > Example: Configurations are deployed to VMs.
+
+- Provision
+
+  Make a resource, such as a virtual machine, available for use.
+
+- Deploy
+
+  Put software onto computers.
+  The software includes technical configuration that links software components.
+
+- Migrate
+
+  Move service configurations and deployments (including user data) from one hosting provider to another.
+
+- Run-time backend
+
+  A type of digital environment one can run operating systems such as NixOS on, e.g. bare-metal, a hypervisor, or a container run-time.
+
+- Provider
+
+  An interface against which we deploy to a run-time backend.
+
+- Provider configuration
+
+  A configuration that specifies resources made available to deploy to and how to access these.
+
+- Resource
+
+  A resource is any external entity that we need for our set-up
+  This may include e.g. hypervisors, file systems, DNS entries, VMs or object storage instances.
+
+## Technologies used
+
+This is an incomplete and evolving list of core components planned to be used in this project.
+It will grow to support more advanced use cases as the framework matures.
+
+### Nix and [NixOS](https://nixos.org/)
+
+NixOS is a Linux distribution with a [vibrant](https://repology.org/repositories/graphs), [reproducible](https://reproducible.nixos.org/) and [security-conscious](https://tracker.security.nixos.org/) ecosystem.
+As such, we see NixOS as the only viable way to reliably create a reproducible outcome for all the work we create.
+
+Considered alternatives include:
+
+- containers: do not by themselves offer the needed reproducibility
+
+### [Proxmox](https://proxmox.com/)
+
+Proxmox is a hypervisor, allowing us to create VMs for our applications while adhering to our goal of preventing lock-in.
+In addition, it has been [packaged for Nix](https://github.com/SaumonNet/proxmox-nixos) as well, simplifying our requirements to users setting up our software.
+
+Considered alternatives include:
+
+- OpenNebula: seemed less mature
+
+### [Garage](https://garagehq.deuxfleurs.fr/)
+
+Garage is a distributed object storage service.
+For compatibility with existing clients, it reuses the protocol of Amazon S3.
+
+Considered alternatives include:
+
+- file storage: less centralized for backups
+
+## Architecture
+
+At the core of Fediversity lies a NixOS configuration module for a set of selected applications.
+
+- We will enable using it with **different run-time environments**, such as a single NixOS machine or a ProxmoX hypervisor.
+- Depending on the targeted run-time environment, deployment may involve [NixOps4](https://nixops.dev) or [OpenTofu](https://opentofu.org/) as an **orchestrator**.
+- We further provide demo front-end for **configuring applications** and configuring **run-time backends**.
+
+To ensure reproducibility, all software will be packaged with Nix.
+
+To reach our goals, we aim to implement the following interactions.
+
+The used legend is as follows:
+
+- Circle: [actor](#actors)
+- Angled box: type
+- Rectangle: value
+- Rounded box: function
+- Diamond: state
+- Arrow: points towards dependant
+
+For further info on components see the [glossary](#glossary).
+
+![](https://git.fediversity.eu/Fediversity/meta/raw/branch/main/architecture-docs/interactions-migration.svg)
+### Configuration data flow
+
+This data flow diagram refines how a deployment is obtained from an operator's application configuration and a hosting provider's runtime setup.
+
+An **application module** specifies operator-facing **application options**, and a **configuration mapping** which determines the application's underlying implementation. Application modules can be supplied by external developers, which would curate application modules against that interface.
+
+For its runtime setup, a hosting provider has to supply a **resource mapping** that would take their self-declared **provider configuration** (which determines the *available* resources) and the output of an application's resource mapping (which determine resource *requirements*) and produce a **configuration**. This configuration ships with a mechanism to be *deployed* to the infrastructure (which is described by the environment, and features the required resources), where it will accumulate **application state**.
+
+Applications and runtime environments thus interface through **resources**, the properties of which are curated by Fediversity maintainers.
+
+![](https://git.fediversity.eu/Fediversity/meta/raw/branch/main/architecture-docs/interactions-fediversity.svg)
+
+
+### Service portability
+
+The process of migrating one's applications to a different host encompasses:
+
+1. Domain registration: involves a (manual) update of DNS records at the registrar
+1. Deploy applications: using the reproducible configuration module
+1. Copy application data:
+    - Run back-up/restore scripts
+    - Run application-specific migration scripts, to e.g. reconfigure connections/URLs
+
+### Data model
+
+Whereas the bulk of our configuration logic is covered in the configuration schema, [implemented here](https://git.fediversity.eu/Fediversity/Fediversity/src/branch/main/deployment/data-model.nix) and [tested here](https://git.fediversity.eu/Fediversity/Fediversity/src/branch/main/deployment/data-model-test.nix), our reference front-end applications will store data.
+The data model design for the configuration front-end needed support the desired functionality is as follows, using the crow's foot notation to denote cardinality:
+
+<img src="https://git.fediversity.eu/Fediversity/meta/raw/branch/main/architecture-docs/panel-data-model.svg" alt="" style="max-width:600px;"/>
+
+### Host architecture
+
+Whereas the core abstraction in Fediversity is a NixOS configuration module, a more full-fledged example architecture of the web host use-case we aim to support as part of our exploitation would be as follows, where virtual machines in question run Fediversity to offer our selected applications:
+
+![](https://git.fediversity.eu/Fediversity/meta/raw/branch/main/architecture-docs/host-architecture.png)