### Solstice CI — Orchestrator Scheduling, Image Map Config, and Libvirt/Zones Backends (MVP)

This document summarizes the initial implementation of Orchestrator scheduling, a YAML-based image map configuration, cloud image preparation, and a hypervisor abstraction with Linux/KVM (libvirt) and illumos zones scaffolding.

#### What’s included (MVP)
- Scheduler and capacity
  - Global max concurrency (`MAX_CONCURRENCY`, default 2) with backpressure by aligning AMQP `prefetch` to concurrency.
  - Optional per-label capacity via `CAPACITY_MAP` (e.g., `illumos-latest=2,ubuntu-22.04=4`).
  - Ack-on-accept: AMQP message is acked after basic validation and enqueue to scheduler; errors during provisioning are handled internally.
- YAML image map configuration
  - Loaded at startup from `--config` / `ORCH_CONFIG`; defaults to `examples/orchestrator-image-map.yaml`.
  - Keys: `default_label`, `aliases`, optional `sizes` presets, and `images` map with backend (`zones` or `libvirt`), `source` URL, `local_path`, `decompress` (`zstd` or none), `nocloud` (bool), and per-image default resources.
  - Default mapping provided:
    - `default_label: illumos-latest`
    - Alias: `illumos-latest → openindiana-hipster`
    - `openindiana-hipster` image points to current OI cloud image: `https://dlc.openindiana.org/isos/hipster/20250402/OI-hipster-cloudimage.img.zstd`, marked `nocloud: true` and `backend: zones`.
  - Size presets (not yet consumed directly by jobs): `small` (1 CPU, 1 GiB), `medium` (2 CPU, 2 GiB), `large` (4 CPU, 4 GiB).
- Image preparation (downloader)
  - On startup, the orchestrator ensures each configured image exists at `local_path`.
  - If missing, downloads from `source` and optionally decompresses with Zstd into the target path.
- Hypervisor abstraction
  - `Hypervisor` trait and `RouterHypervisor` dispatcher.
  - Backends:
    - `libvirt` (Linux/KVM): skeleton that connects to libvirt in `prepare`; domain XML/overlay/NoCloud seed wiring to follow.
    - `zones` (illumos/bhyve): stub scaffold (not yet functional); will integrate with `zone` crate + ZFS clones in a follow-up.
  - `NoopHypervisor` for development on hosts without privileges.
- Orchestrator MQ wiring
  - Consumes `JobRequest` messages and builds `VmSpec` from resolved label and image defaults.
  - Injects minimal cloud-init user-data content (NoCloud) into the spec for future seeding.

#### Configuration (CLI/env)
- `--config`, `ORCH_CONFIG` — path to YAML image map (default `examples/orchestrator-image-map.yaml`).
- `--max-concurrency`, `MAX_CONCURRENCY` — global VM concurrency (default 2).
- `--capacity-map`, `CAPACITY_MAP` — per-label capacity (e.g., `illumos-latest=2,ubuntu-22.04=4`).
- AMQP: `AMQP_URL`, `AMQP_EXCHANGE`, `AMQP_QUEUE`, `AMQP_ROUTING_KEY`, `AMQP_PREFETCH` (defaulted to `MAX_CONCURRENCY`).
- Libvirt (Linux): `LIBVIRT_URI` (default `qemu:///system`), `LIBVIRT_NETWORK` (default `default`).

#### Local usage (dev)
1. Ensure RabbitMQ is running (docker-compose service `rabbitmq`).
2. Start the Orchestrator:
   ```bash
   cargo run -p orchestrator -- \
     --config examples/orchestrator-image-map.yaml \
     --max-concurrency 2
   ```
   On first run, the OI cloud image will be downloaded and decompressed to the configured `local_path`.
3. In another terminal, enqueue a job (Forge Integration webhook or CLI `enqueue`). The orchestrator will resolve `runs_on` (or default label) and schedule a VM using the configured backend.

Note: The current libvirt/zones backends are partial; actual VM boot is a follow-up. The scheduler and config wiring are complete and ready for backend integration.

#### What’s next (planned)
- Libvirt backend:
  - Create qcow2 overlays, generate domain XML (virtio devices), attach NoCloud ISO seed, define, start, shutdown, and destroy.
  - Ensure libvirt default network is active at startup if necessary.
- Illumos zones backend:
  - Integrate `oxidecomputer/zone` and ZFS clone workflow; set bhyve attributes (`vcpus`, `ram`, `bootdisk`), networking, and SMF.
- Lifecycle and runner coordination:
  - gRPC Orchestrator↔Runner for logs/status, job completion handling, and cleanup.
- Persistence and recovery:
  - Store job/VM state in Postgres; graceful recovery on restart.
- Tests and docs:
  - Unit tests for config parsing and scheduler; feature-gated libvirt smoke test; expand docs.