Add the documentation and the agent instructions
README for humans, AGENTS.md for agents and contributors, and the docs that outlive any one platform: architecture, secrets, connectivity, triggering a deploy from another repository. CLAUDE.md, GEMINI.md and .claude/skills/ are pointers rather than copies, so every agent and every human reads the same text. Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
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Lucas Winther
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Claude Opus 5
Claude Opus 5
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# AGENTS.md
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Instructions for LLMs and AI agents working in this repository. Humans should
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read `README.md` first; this file assumes you already know what the repo is for.
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This is the one instruction file. `CLAUDE.md` and `GEMINI.md` are pointers to it,
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and `.claude/skills/` points at `skills/`. Put content here or in `skills/`,
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never in a pointer — the whole arrangement exists so that every agent, and every
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human, reads the same text.
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## What this repository is
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A pattern for deploying Docker Compose stacks to a small number of Linux hosts
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with Ansible, published as a working example. Every service on every host is a
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Compose stack living in a directory under `/srv/stacks/` on the target. This repo
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holds the files that get copied there, and the playbooks that copy them.
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| Host | Address | Runs |
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| ------ | ------------------ | ----------------------------------- |
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| `edge` | `edge.example.com` | The reverse proxy, metrics |
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| `app` | `app.example.com` | The applications behind it, metrics |
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Both use SSH user `deploy` on port 22.
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Everything under `server/` and every host name is an **example**. `example.com`
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is reserved by RFC 2606 and can never resolve to a real service. Replace them
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with yours; the framework around them is the part meant to be reused.
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## Vocabulary
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- **host** — one machine (`edge`, `app`). The top-level directories under
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`server/` and the inventory names must always agree, because the role resolves
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a payload as `server/{{ inventory_hostname }}/{{ stack_name }}`.
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- **stack** — one Docker Compose deployment on one host (`metrics`, `webapp`).
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- **payload** — the files under `server/<host>/<stack>/`, copied to the host
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verbatim. Payloads are *data*. Do not template them, do not restructure them,
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and do not lint them as Ansible content.
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## Skills
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Step-by-step guides for recurring jobs live in `skills/<name>/SKILL.md`, outside
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any vendor directory.
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| Skill | Use when |
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| --- | --- |
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| `adding-a-stack` | Adding a new service, moving one to a different host, or bringing a hand-deployed service under Ansible and CI |
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## Playbook rules
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These are the conventions. Follow them; do not invent per-stack variations.
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1. **One playbook per stack**, at `ansible/playbooks/<stack>.yml`. If the same
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stack runs on several hosts it is still one playbook with a host group, not
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one per host. `metrics.yml` is the example.
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2. **Self-contained.** Every playbook must run standalone as
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`ansible-playbook playbooks/<stack>.yml` with no required extra vars — `pull`
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is the one optional flag. It declares its own `hosts`, its own vars, and
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everything the stack needs (directories, networks, compose files). Never rely
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on another playbook having run first. Where that means two playbooks create
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the same directory, that is correct and the repetition is deliberate.
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3. **Tag every play** with the stack name, so `site.yml --tags metrics` works.
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4. **Reuse goes in `roles/compose_stack`, not in copy-pasted tasks.** If two
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playbooks need the same logic, extend the role's variable contract rather than
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adding a second role. Keep the role count at one unless there is a genuinely
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different shape of work.
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5. **Modules, not shell.** `community.docker.docker_compose_v2` rather than
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`docker compose up`; `ansible.builtin.file` rather than `mkdir -m 777`. Reach
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for `command`/`shell` only when no module exists, and then set `changed_when`
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explicitly.
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6. **Fully-qualified collection names** everywhere (`ansible.builtin.copy`, not
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`copy`).
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7. **Name every task**, sentence case, imperative: `Sync the metrics payload`,
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not `sync files`.
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8. **Idempotent.** A second run must report zero changes, and `--check --diff`
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must not error.
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9. **No secrets in the repository.** `.env` files stay on the hosts. Never commit
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a key, a password, or a token — including inside a compose file, where no
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exclude can help you. See `docs/secrets.md`.
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### The `compose_stack` role contract
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Playbooks configure the role through these variables. Extend this list when a
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stack needs something new; do not work around it with loose tasks.
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| Variable | Default | Purpose |
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| ---------------- | ----------------------------------------------------- | ------------------------------------------- |
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| `stack_name` | *required* | Stack identifier, used for logs and tags |
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| `stack_src` | `server/{{ inventory_hostname }}/{{ stack_name }}` | Payload directory in this repository |
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| `stack_dest` | `{{ stack_root }}/{{ stack_name }}` | Destination directory on the host |
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| `stack_dirs` | `[]` | Directories to pre-create (see below) |
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| `stack_networks` | `[]` | External Docker networks to ensure exist |
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| `stack_files` | `[docker-compose.yml]` | Compose files passed to the module |
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| `stack_exclude` | `[.env]` | Paths never synced to the host |
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| `stack_prune` | `false` | Delete host files absent from the payload |
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| `stack_env` | `{}` | Extra env for compose; `PWD` is always set |
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| `stack_pull` | `{{ pull \| default('policy') }}` | Per-invocation override of the pull flag |
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| `stack_build` | `policy` | `always` for stacks built from a Dockerfile |
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| `stack_state` | `present` | `present`, `absent`, or `restarted` |
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Worked examples for each: `static-site.yml` (the minimum), `reverse-proxy.yml`
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(`stack_dirs`, reload), `metrics.yml` (`stack_src`, per-host maps, ownership),
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`webapp.yml` (two phases, `stack_files`, `stack_build`), `webapp-staging.yml`
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(`stack_src` + `stack_dest` + `stack_env`), `banner.yml` (not a stack at all).
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#### `stack_dirs`
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Entries take a `path` plus optional `mode`, `owner`, `group` and `recurse`. All
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four are applied **only when given**, so a directory that already exists on the
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host is never re-chmodded — several bind mounts are created root-owned by Docker
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on first start, and Postgres refuses to start if its data directory is not
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`0700`/`0750`.
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The role escalates **only when an entry sets `owner` or `group`.** Everything
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else is created as the deploy user, which is the whole point of pre-creating a
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bind mount: a root-owned directory is exactly what you are trying to avoid. A
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stack that genuinely needs another uid says so with `owner`/`group` — see
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`metrics.yml`, which hands `grafana-data` to `472` with `recurse: true` and no
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`mode`.
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Put that in `stack_dirs` rather than a `pre_tasks` chown. `pre_tasks` run before
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the role creates the stack directory, and `ansible.builtin.file` stamps the
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attributes it was given onto every parent directory it creates on the way down —
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so an escalated chown of a child leaves the stack directory itself owned by that
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uid, and the payload sync, which runs as the deploy user, cannot write into it.
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#### `compose_stack_synced`
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After the sync the role sets this fact. Use it to fire a reload for payloads that
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are bind-mounted configuration, which Compose never recreates a container for:
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```yaml
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post_tasks:
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- name: Reload nginx so changed vhost and conf files take effect
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ansible.builtin.command:
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cmd: docker exec reverse-proxy nginx -s reload
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when: compose_stack_synced | default(false)
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changed_when: true
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```
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Without the `when`, every run reports a change and the stack is never idempotent.
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#### Doing maintenance between sync and up
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`roles: - role: compose_stack` syncs and then starts, which is what almost every
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stack wants. When something has to happen in between — a backup, a schema
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migration — use the two entry points rather than inventing a second mechanism:
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```yaml
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tasks:
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- name: Sync the payload
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ansible.builtin.include_role:
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name: compose_stack
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tasks_from: sync
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- name: Migrate the database
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# ... gated on an update run
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- name: Start the stack
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ansible.builtin.include_role:
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name: compose_stack
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tasks_from: up
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```
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Declare the `stack_*` vars at **play** level so both phases see them; inside
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`roles:` they are scoped to that invocation and the second include will not see
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them. `webapp.yml` is the worked example.
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### Deploy mode versus update mode
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Every playbook serves two purposes and must support both without being edited:
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```bash
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ansible-playbook playbooks/webapp.yml # bring the stack up
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ansible-playbook playbooks/webapp.yml -e pull=always # pull newer images
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```
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`pull` is a single repo-wide extra-var passed straight through to
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`community.docker.docker_compose_v2`. Valid values are `policy` (the default —
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Compose decides, so nothing is fetched for images already present), `always`,
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`missing` and `never`. Read it as `pull | default('policy')` at every use site so
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the playbook still runs when the var is undefined.
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This is also the hook for the heavier work a stack needs only when its images
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actually move. Gate those tasks on the same flag rather than creating a separate
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`*-update.yml` playbook:
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```yaml
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- name: Back up the database before migrating
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ansible.builtin.command:
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cmd: docker compose -f docker-compose.yml -f docker-compose.migrate.yml run --rm backup
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chdir: /srv/stacks/webapp
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when: pull | default('policy') == 'always'
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changed_when: true
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```
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Two consequences worth stating plainly: **a default run must never destroy or
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migrate anything**, and **an update run must be safe to repeat.**
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## Safety
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- **Never run a playbook against a real host unless the user asks in that turn.**
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Run `--check --diff` first and say what it would change.
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- **Never read or print the contents of `.env` files, vault files, or SSH keys.**
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If a task genuinely needs one, ask.
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- Do not `git push`, open pull requests, or commit unless asked.
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## Verifying your work
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No Ansible tooling is assumed to be installed in a fresh checkout. One command
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runs everything CI runs:
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```bash
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make check # or: scripts/check.sh
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```
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It runs `yamllint` from the repository root, installs the Galaxy collections,
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runs `ansible-lint` at the `production` profile, and syntax-checks every
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playbook. Locally it reaches for `uvx` so the repo needs no virtualenv of its
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own; set `RUNNER=installed` if the tools are already on `PATH`.
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Two things worth knowing if you run the commands by hand instead:
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- **Run every ansible command from the `ansible/` directory.** `ansible.cfg` is
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only discovered in the current directory, and it is what puts `roles/` on the
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roles path. From the repository root you get
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`The role 'compose_stack' was not found`.
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- **Install the collections before linting**, or `community.docker` and
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`ansible.posix` resolve to nothing and the fully-qualified-name checks pass
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vacuously.
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Lint and syntax-check are the definition of "done" for a playbook. They are
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static and safe; run them yourself rather than asking the user to.
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