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title: How to implement a TAPPaaS module
description: Walk-through of adding a new module to the TAPPaaS repository
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# How to implement a TAPPaaS module

From empty directory to merged pull request

<!--
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TODO: 30-second framing — who is in the room, what they walk out able to do.
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---

## Who this is for

- You want to add an **app** or a **foundation** capability to TAPPaaS
- You can read shell, JSON, and a little Nix
- You have a TAPPaaS test environment you are allowed to break

**Prerequisites**

- A clone of `TAPPaaS/TAPPaaS`, branch `TODO: ADR007 or stable`
- SSH access to a test site
- `TODO: tooling list`

---

## Vocabulary in 60 seconds

| Term | Means |
| --- | --- |
| **Module** | One installable unit under `src/apps/` or `src/foundation/` |
| **Service** | A part of a module with its own lifecycle scripts |
| **Manager** | Owns desired state for a domain |
| **Controller** | Applies desired state to one concrete technology |

`TODO: tighten against ADR-007 taxonomy; drop the terms that do not matter here`

---

## Anatomy of a module

```text
src/apps/<module>/
├── README.md          # what it is, why it exists
├── DESIGN.md          # how it is put together
├── INSTALL.md         # operator-facing install notes
├── UPGRADE.md         # operator-facing upgrade notes
├── AUTHORS.md         # who to ask
├── LICENSE
├── <module>.json      # declaration: the module's contract
├── <module>.nix       # runtime definition
├── install.sh         # create it
├── update.sh          # move it forward
├── test.sh            # prove it works
└── services/<svc>/    # per-service install/update/test/delete
```

---

## The path from nothing to merged

```mermaid
flowchart LR
  A[Scaffold from<br/>00-Template] --> B[Declare<br/>module.json]
  B --> C[Define runtime<br/>module.nix]
  C --> D[install.sh]
  D --> E[services/]
  E --> F[update.sh]
  F --> G[test.sh]
  G --> H[Docs]
  H --> I[PR + CI]
```

---

## Step 1 — Scaffold

```bash
cp -r src/apps/00-Template src/apps/<module>
```

- Rename `template.json` → `<module>.json`, `template.nix` → `<module>.nix`
- Read `README-template.md`, then delete it
- `TODO: is there a scaffold script? if not, should there be?`

---

## Step 2 — Declare the module

`<module>.json`

```json
{
  "TODO": "smallest working declaration"
}
```

- What the module *is*, not how it is built
- `TODO: required keys, optional keys, validation`

---

## Step 3 — Define the runtime

`<module>.nix`

```nix
# TODO: minimal example
```

- `TODO: what belongs in nix vs. what belongs in install.sh`

---

## Step 4 — `install.sh`

- Idempotent: safe to run twice
- Fails loudly, never half-way
- `pre-install.sh` when something must exist first

```bash
# TODO: skeleton with the house conventions
```

---

## Step 5 — Services

```text
services/<svc>/install-service.sh
services/<svc>/update-service.sh
services/<svc>/test-service.sh
services/<svc>/delete-service.sh
```

- One service = one thing with its own lifecycle
- `TODO: when to split into services vs. keep it in the module`

---

## Step 6 — `update.sh`

- Upgrade in place, no data loss
- Version pinning: `TODO`
- Rollback story: `TODO`

---

## Step 7 — `test.sh`

- Runs against a real environment, not a mock
- Exit non-zero on failure, quiet on success
- `TODO: what CI runs, and what only a human can check`

---

## Step 8 — Documentation

| File | Answers |
| --- | --- |
| `README.md` | What is this and why would I install it? |
| `DESIGN.md` | How is it built, what were the trade-offs? |
| `INSTALL.md` | How do I install it, step by step? |
| `UPGRADE.md` | What breaks between versions? |

Docs are synced to the docs site — a missing README is a missing page.

---

## Try it on a test site

```bash
# TODO: the real loop
```

1. Deploy to the test environment
2. Run `test.sh`
3. Break it on purpose, run it again

---

## Submit

- Branch: `TODO: naming convention`
- One module per pull request
- CI must be green
- Review checklist:
  - [ ] Idempotent install
  - [ ] `test.sh` passes on a clean site
  - [ ] Docs complete
  - [ ] `TODO`

---

## Pitfalls

- `TODO: the three mistakes reviewers actually see`
- `TODO`
- `TODO`

---

## Where to go next

- `src/apps/00-Template/` — the canonical starting point
- `docs/ADR/ADR-007 - TAPPaaS Taxonomy.md`
- `TODO: contribution guide link`

**Questions?**