FabledSteward/docs/plugins/writing-a-plugin.md

Writing a Plugin

This guide walks through building a complete plugin from scratch. The Traefik plugin (plugins/traefik/) is the reference implementation — read it alongside this guide.

---

Step 1: Create the Directory

plugins/
└── myplugin/
    └── __init__.py    ← start here

The directory name is the plugin's identity. It must match the name field in plugin.yaml and is used as the URL prefix (/plugins/myplugin/) and the Python import name.

---

Step 2: Write plugin.yaml

name: myplugin
version: "1.0.0"
description: "A short description of what this plugin monitors"
author: "Your Name or GitHub username"
license: "MIT"          # any SPDX identifier, e.g. MIT, Apache-2.0, GPL-3.0

# Optional: prevents loading on older app versions
min_app_version: "0.1.0"

# Optional: where the plugin's source lives
repository_url: "https://github.com/yourname/yourrepo"
homepage: "https://github.com/yourname/yourrepo/tree/main/myplugin"
tags:
  - monitoring
  - http

# Default config values — users override these via the Settings UI
# or by writing to the app_settings DB table under "plugin.myplugin"
config:
  target_url: "http://localhost:9090/metrics"
  scrape_interval_seconds: 60

---

Step 3: Define Models (if needed)

If your plugin stores data, define SQLAlchemy models using the shared Base from steward.models.base.

# plugins/myplugin/models.py
from __future__ import annotations
import uuid
from datetime import datetime
from sqlalchemy import String, Float, DateTime
from sqlalchemy.orm import Mapped, mapped_column
from steward.models.base import Base


class MyPluginMetric(Base):
    __tablename__ = "myplugin_metrics"

    id: Mapped[str] = mapped_column(String, primary_key=True, default=lambda: str(uuid.uuid4()))
    resource_name: Mapped[str] = mapped_column(String, nullable=False, index=True)
    scraped_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False)
    my_value: Mapped[float] = mapped_column(Float, nullable=False)

---

Step 4: Write Migrations

Create plugins/myplugin/migrations/ with the standard Alembic layout. Copy env.py and script.py.mako from plugins/traefik/migrations/ as a starting point — the env.py is boilerplate and rarely needs changes.

Generate the initial migration:

# From the project root
alembic --config alembic.ini revision \
    --autogenerate \
    --head=steward@head \
    --branch-label=myplugin \
    -m "myplugin initial"

Edit the generated file to set depends_on:

# In the generated revision file:
depends_on = ("0004_core_head_id",)   # the core migration head ID
down_revision = None
branch_labels = ("myplugin",)

# Prefix the revision ID with the plugin name:
revision = "myplugin_001_initial"

---

Step 5: Write Scheduled Task Logic

Keep task logic in a separate file so __init__.py stays clean.

# plugins/myplugin/scheduler.py
from __future__ import annotations
import logging
from steward.core.scheduler import ScheduledTask
from steward.core.alerts import record_metric

logger = logging.getLogger(__name__)


def make_task(app) -> ScheduledTask:
    interval = int(app.config["PLUGINS"]["myplugin"]["scrape_interval_seconds"])

    async def scrape():
        await _do_scrape(app)

    return ScheduledTask(
        name="myplugin_scrape",
        coro_factory=scrape,
        interval_seconds=interval,
        run_on_startup=True,
    )


async def _do_scrape(app) -> None:
    from .models import MyPluginMetric
    from datetime import datetime, timezone

    url = app.config["PLUGINS"]["myplugin"]["target_url"]

    try:
        value = await _fetch_value(url)
    except Exception:
        logger.exception("myplugin scrape failed (url=%s)", url)
        return

    now = datetime.now(timezone.utc)

    async with app.db_sessionmaker() as session:
        async with session.begin():
            # Write to plugin's own history table
            session.add(MyPluginMetric(
                resource_name="my-resource",
                scraped_at=now,
                my_value=value,
            ))

            # Emit to plugin_metrics so alert rules can fire
            await record_metric(
                session=session,
                source_module="myplugin",
                resource_name="my-resource",
                metric_name="my_value",
                value=value,
            )


async def _fetch_value(url: str) -> float:
    import httpx
    async with httpx.AsyncClient() as client:
        resp = await client.get(url)
        resp.raise_for_status()
        return float(resp.text.strip())

---

Step 6: Write Routes (if needed)

# plugins/myplugin/routes.py
from quart import Blueprint, current_app, render_template
from steward.auth.middleware import require_role
from steward.models.users import UserRole
from .models import MyPluginMetric

myplugin_bp = Blueprint("myplugin", __name__, template_folder="templates")


@myplugin_bp.get("/")
@require_role(UserRole.viewer)
async def index():
    async with current_app.db_sessionmaker() as db:
        from sqlalchemy import select
        result = await db.execute(
            select(MyPluginMetric).order_by(MyPluginMetric.scraped_at.desc()).limit(50)
        )
        rows = result.scalars().all()
    return await render_template("myplugin/index.html", rows=rows)


@myplugin_bp.get("/widget")
@require_role(UserRole.viewer)
async def widget():
    """HTMX fragment for the dashboard widget."""
    async with current_app.db_sessionmaker() as db:
        from sqlalchemy import select
        result = await db.execute(
            select(MyPluginMetric).order_by(MyPluginMetric.scraped_at.desc()).limit(1)
        )
        latest = result.scalar_one_or_none()
    return await render_template("myplugin/widget.html", latest=latest)

---

Step 7: Write Templates

Templates live in plugins/myplugin/templates/myplugin/ (the extra nesting avoids naming collisions with core templates).

{# plugins/myplugin/templates/myplugin/index.html #}
{% extends "base.html" %}
{% block title %}My Plugin — Steward{% endblock %}
{% block content %}
<div class="page-title">My Plugin</div>
{% for row in rows %}
  <p>{{ row.resource_name }} — {{ row.my_value }}</p>
{% endfor %}
{% endblock %}

{# plugins/myplugin/templates/myplugin/widget.html — HTMX fragment, no extends #}
{% if latest %}
  <div class="ping-row">
    <span>{{ latest.resource_name }}</span>
    <span>{{ latest.my_value }}</span>
  </div>
{% else %}
  <p class="empty">No data yet.</p>
{% endif %}

---

Step 8: Wire Up __init__.py

# plugins/myplugin/__init__.py
from __future__ import annotations

_app = None


def setup(app) -> None:
    global _app
    _app = app
    from .models import MyPluginMetric  # noqa: registers model with Base.metadata


def get_scheduled_tasks() -> list:
    from .scheduler import make_task
    return [make_task(_app)]


def get_blueprint():
    from .routes import myplugin_bp
    return myplugin_bp

---

Step 9: Enable the Plugin

Plugin config is stored in the app_settings DB table. The easiest way to enable a plugin is via the Settings UI, or by inserting directly:

INSERT INTO app_settings (key, value_json, updated_at)
VALUES ('plugin.myplugin', '{"enabled": true, "target_url": "http://localhost:9090/metrics"}', now());

On next startup, the plugin will be loaded, its migrations applied, and its blueprint and tasks registered.

---

Using record_metric()

record_metric() is how plugins feed data into the alert pipeline. Any metric you write here can be the target of an alert rule created in the UI.

from steward.core.alerts import record_metric

# Must be inside an active transaction
async with session.begin():
    await record_metric(
        session=session,
        source_module="myplugin",       # matches alert rule source_module
        resource_name="my-server",      # matches alert rule resource_name
        metric_name="response_time_ms", # matches alert rule metric_name
        value=42.3,
    )

record_metric() writes to plugin_metrics and evaluates all matching alert rules inline. Notifications are deferred outside the transaction. It propagates SQLAlchemyError on DB failure — don't swallow it.

---

Auth in Routes

Use the @require_role decorator from steward.auth.middleware:

from steward.auth.middleware import require_role
from steward.models.users import UserRole

@myplugin_bp.get("/admin-only")
@require_role(UserRole.admin)
async def admin_page():
    ...

@myplugin_bp.get("/read-only")
@require_role(UserRole.viewer)   # viewer, operator, and admin can all access
async def read_page():
    ...

Role hierarchy: admin > operator > viewer. Requiring viewer grants access to all three roles.

---

Audit Log Entries

A change made through a plugin's pages is audited with steward.core.audit.log_audit, naming the thing it changed by entity_type and entity_id. So the audit page shows that thing's current name, linked to its page, export get_audit_entities() from __init__.py:

# plugins/myplugin/__init__.py
def get_audit_entities() -> dict:
    from .models import Target
    return {"myplugin_target": (Target, "/plugins/myplugin/targets/{}")}

The model needs id and name columns. An entity deleted since keeps the name the entry recorded in its detail (detail={"name": ...}). Add the action's wording to ACTIONS in steward/audit/service.py so the row reads as a sentence.

---

Agent Tools (MCP)

Steward serves an MCP endpoint at /mcp that AI clients reach with a personal API token created on the AI access page. A plugin adds tools to it by exporting an optional register_mcp_tools(register) from __init__.py:

# plugins/myplugin/__init__.py
def register_mcp_tools(register) -> None:
    from .mcp_tools import list_targets, rescan_target
    from steward.models.users import UserRole
    register(list_targets, role=UserRole.viewer, read_only=True)
    register(rescan_target, role=UserRole.operator, read_only=False)

# plugins/myplugin/mcp_tools.py
from steward.mcp._context import current_caller, mcp_app


async def list_targets(limit: int = 50) -> dict:
    """What this tool is for, when to use it over its siblings, what each
    argument's empty value means, and what comes back. This docstring is the
    tool's description: it is all the agent knows about it."""
    app = mcp_app()                 # Quart's current_app does not exist here
    async with app.db_sessionmaker() as db:
        ...
    return {"targets": [...], "total": n, "truncated": n > limit}

• role and read_only are required. A tool is listed and callable only for callers whose role reaches role, and only while the plugin is enabled.
• Tools run outside Quart's request pipeline: use mcp_app() and current_caller() from steward.mcp._context, never current_app or session.
• Refuse by raising ValueError("what was wrong, and what to do instead"). The agent reads that text. Any other exception reaches it only as a generic failure.
• Tool names are global across core and every plugin; a name already taken fails registration and the plugin contributes no tools (logged).
• Hot-reloading the plugin replaces its tools.
• Follow the response conventions the core tools use (steward/mcp/tools/_shape.py): times via iso() plus age_s(), lists through bounded() so a cut is never silent, series through summarize(), and lookups by name or id through pick().

Contributing to get_overview

get_overview is the agent's first call: what is wrong right now, ranked. A plugin adds its own problems to it by exporting an optional mcp_overview(db) that returns findings built with steward.mcp.tools.overview.finding:

# plugins/myplugin/__init__.py
async def mcp_overview(db) -> list[dict]:
    from steward.mcp.tools.overview import finding
    return [
        finding("warning", "target_unreachable", t.name, f"{t.name} has not answered for {t.misses} scans",
                since=t.last_ok_at, ids={"target_id": t.id}, next=f'get_target(target="{t.id}")')
        for t in await _unreachable_targets(db)
    ]

• severity is "critical" (something is down or about to be) or "warning" (degraded, or heading that way). Report problems only; a healthy plugin returns [].
• It is called only while the plugin is enabled, with its own session. If it raises, the overview still returns, and names the plugin under unavailable.

What happened on a host

get_anomaly shows, beside a metric anomaly, what else happened on its host around each occurrence. A plugin that records events on a host adds them by exporting an optional mcp_events(db, host_id, host_name, since, until) that returns events built with steward.mcp.tools._events.event:

# plugins/myplugin/__init__.py
async def mcp_events(db, host_id, host_name, since, until) -> list[dict]:
    from steward.mcp.tools._events import event
    return [event(r.at, "backup_job", f"Backup {r.name}: {r.state}", until=r.finished_at)
            for r in await _jobs_on(db, host_id, since, until)]

• Return what happened between since and until, oldest first: something that lasted gives until. Keep it to tens of rows; the caller keeps the first 40.
• It is called only while the plugin is enabled, with its own session. If it raises, the anomaly is still returned without the plugin's events. The host agent (maintenance jobs, reboots) and Docker (container starts, deaths, OOM kills) are the examples to read.

---

Shipping a Plugin

Steward never downloads plugins. There is no catalog and no install button: the app runs with no outbound network.

• First-party plugins live in this repository under plugins/ and ship inside the image, so they version with core.
• A third-party plugin is a folder you put in the external plugin root, /data/plugins by default (STEWARD_PLUGIN_DIR moves it). Restart Steward and the plugin appears in Settings → Plugins, where you enable it. A bundled plugin of the same name wins over an external one.

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Checklist

• plugins/myplugin/ directory created
• plugin.yaml with correct name (matches directory), author, license, tags
• __init__.py exports setup() and get_scheduled_tasks()
• Models import inside setup() to register with metadata
• Migrations use depends_on pointing to core head, not down_revision
• Revision IDs prefixed with plugin name
• record_metric() called inside session.begin()
• Routes use @require_role decorator
• MCP tools (if any) registered via register_mcp_tools(register) with a role and read_only
• Templates namespaced under templates/myplugin/
• Plugin enabled in app settings