# HostKit

Elixir-native host infrastructure declarations, planning, and runtime control.

HostKit is intended to be used from a normal Mix project with `.exs` infrastructure files. The DSL compiles to plain inspectable structs; Mix tasks are wrappers around the runtime API.

For naming, block shape, defaults, and reference style, see [DSL design guidelines](dsl-guidelines.md).

## Design

- Core owns systemd/systemdkit persistent units.
- Core owns unitctl transient runtime primitives.
- Integrations such as Caddy, Forgejo, object storage, and monitoring are providers.
- DSL evaluation never applies changes to a host.
- Planning and rendering are available as runtime APIs.

## Example

```elixir
use HostKit.DSL

project :toys do
  roots source: "/opt/toys/src",
        data: "/srv/toys",
        state: "/var/lib/toys",
        config: "/etc/toys"

  prefixes user: "toys-", unit: "toys-"

  host :elixir_toys, at: "elixir.toys" do
    ssh do
      user "dannote"
      sudo true
    end
  end

  service :exograph do
    account system: true
    storage :data, mode: 0o755
    storage :state, mode: 0o750

    daemon do
      description "Exograph search"
      after_target :network_online
      wants :network_online
      working_directory root_path(:source)
      exec ["/usr/local/bin/mix", "exograph.index.hex", "--web", "--port", "4200"]
      restart :on_failure
      restart_sec 10

      isolate do
        writable :data
        writable :state
        network :loopback
      end
    end
  end
end
```

## Plans and down plans

Rollback is represented as another HostKit plan. A plan change already carries `before` and `after` state, so HostKit can derive a down plan from the exact plan that was applied:

```elixir
{:ok, plan} = HostKit.plan(project, target: prod)
{:ok, down_plan} = HostKit.down(plan)

HostKit.format_plan(down_plan)
HostKit.apply(down_plan, confirm: true)
```

Partial rollback uses the same plan model:

```elixir
{:ok, down_plan} =
  HostKit.down(plan, only: [{:file, "/etc/gatehouse/config.exs"}])
```

Command-like operations need semantic down steps because HostKit cannot infer the opposite of an arbitrary command:

```elixir
command :migrate,
  exec: {"bin/app", ["eval", "App.Release.migrate()"]},
  phase: :before_start,
  down: {"bin/app", ["eval", "App.Release.rollback()"]}

command :warm_cache,
  exec: {"bin/app", ["eval", "App.Cache.warm()"]},
  down: :noop
```

The down command is emitted as an ordinary command change in the down plan. `down: :irreversible` records an explicit warning and omits the command from the down plan.

Created resources use conservative rollback policies. File-like resources can be deleted by a down plan, but directories are kept unless explicitly opted in:

```elixir
directory "/tmp/demo", rollback: :delete_if_created
directory "/srv/app", rollback: :keep
account :app, system: true, rollback: :keep
package :caddy, rollback: :keep
```

CLI usage mirrors this:

```sh
mix host_kit.plan infra/config.exs --host prod --out up.plan.json
mix host_kit.down up.plan.json --out down.plan.json
mix host_kit.apply --plan down.plan.json --confirm
```

## Run tracking

Tracked applies write minimal run records under the project-configured HostKit runs root:

```sh
mix host_kit.apply --track --plan up.plan.json --confirm
mix host_kit.runs --host prod infra/config.exs
mix host_kit.runs --host prod --verbose infra/config.exs
mix host_kit.runs --host prod --latest --verbose infra/config.exs
mix host_kit.down --host prod --run 20260614-101148-demo-up --out down.plan.json infra/config.exs
```

Run records are intentionally compact: they identify the run, project, direction, timestamp, and applied change statuses. They do not replace plan artifacts; use plan artifacts for inspectable up/down plan contents. When a tracked apply is started from `--plan`, HostKit copies that up-plan artifact under the runs root and records the copied path so `mix host_kit.down --last` can work from the tracked run.

Tracked applies also write backup payloads for previous file-like state when that state was captured in the plan. Backup payloads live under `hostkit_backups/<run-id>/` or the `--backups-root` override. `mix host_kit.down --last` and `mix host_kit.down --run RUN_ID` rewrite supported previous file-like state to `%HostKit.BackupRef{}` entries so generated down plans restore from backup payloads instead of embedding prior content. Backup-backed restore currently covers ordinary files plus rendered file resources such as env files, Caddy sites, proxy config, firewall/egress files, and systemd unit files when their previous rendered content was captured. Use `mix host_kit.runs --verbose`, `--latest`, or `--id RUN_ID` to inspect copied plan artifacts and backup payload paths.

Source updates are intentionally not inferred as reversible by default: a previous Git remote/ref may no longer be reachable. Treat source rollback as an explicit lifecycle operation or pair it with a backup/source-bundle strategy.

Run retention is explicit. Use `mix host_kit.runs --prune --keep N` to remove older run records plus their copied plan artifact and backup payload directories.

## Elixir app lifecycle helpers

The Elixir app recipe can emit lifecycle commands for common BEAM deployment operations. Ecto migrations are represented as normal commands with explicit down commands:

```elixir
elixir_app :shop do
  source github: "acme/shop", path: ".", ref: "main"
  phoenix host: "shop.example.com", secret_key_base: secret_env("SECRET_KEY_BASE")

  ecto release: "Shop.Release"
end
```

This emits a `:before_start` migration command that runs through the built release and a matching down command that calls `Shop.Release.rollback()`.

For multiple repos, HostKit emits one ordered command per repo. Down plans reverse that order:

```elixir
elixir_app :shop do
  source github: "acme/shop", path: ".", ref: "main"
  phoenix host: "shop.example.com", secret_key_base: secret_env("SECRET_KEY_BASE")

  ecto release: "Shop.Release" do
    repo "Shop.Repo"
    repo "Shop.AnalyticsRepo"
  end
end
```

The default expressions are:

```elixir
Shop.Release.migrate(Shop.Repo)
Shop.Release.rollback(Shop.Repo)
```

Use `:migrate` and `:rollback` for custom release functions when the defaults do not fit.

## Providers

Providers can contribute DSL modules, resource types, renderers, validators, and read/plan/apply lifecycle operations. Systemd and Unitctl are core primitives, not providers; integrations such as Caddy should be providers.

```elixir
use HostKit.DSL, providers: [HostKit.Providers.Caddy]

project :demo do
  provider :caddy, HostKit.Providers.Caddy do
    set :sites_dir, "/etc/caddy/sites"
  end

  service :web do
    daemon do
      exec ["/opt/web/bin/server"]
      listen :http, port: 4000
    end

    caddy_site "example.com", path: "web.caddy" do
      encode [:zstd, :gzip]
      reverse_proxy :http
    end
  end
end
```

## Instances and nested hosts

Top-level `host` declarations describe existing connection targets. `instance` declarations describe lifecycle-managed compute boundaries with backend-selected lifecycle and normal HostKit contents nested inside.

```elixir
use HostKit.DSL

project :demo do
  instance :demo_vm do
    backend :incus
    image "images:ubuntu/24.04"
    kind :container
    lifecycle :ephemeral

    expose :ssh, host: 2222, guest: 22
    expose :web, host: 18080, guest: 80

    host :guest, at: "127.0.0.1" do
      ssh do
        user "root"
        password "hostkit-demo"
        port 2222
        accept_hosts true
      end
    end

    service :web do
      package :caddy

      daemon do
        exec ["/usr/bin/env", "true"]
        listen :http, port: 80
      end
    end
  end
end
```

The instance owns compute lifecycle metadata (`backend`, `image`, `kind`, `lifecycle`, `expose`). The nested host owns connection metadata. Nested services/resources are ordinary HostKit declarations scoped to the instance contents. Plans emit the instance lifecycle resource first, then nested content resources annotated with the nested host target so read/apply operations run through that endpoint.

Backend implementations are intentionally separate from the generic DSL. Incus is implemented as a backend for `instance`, not as a user-facing `incus_machine` DSL. The Incus backend maps `expose` declarations to Incus proxy devices.

Backend configuration stays on the `backend` declaration instead of leaking backend-specific flags into generic plan/apply commands:

```elixir
instance :demo_vm do
  backend :incus, sudo: true, project: "hostkit"
end
```

For multi-line configuration, use backend options:

```elixir
instance :demo_vm do
  backend :incus do
    option :sudo, true
    option :project, "hostkit"
  end
end
```

## Host bootstrap packages and mise-managed runtimes

HostKit can install OS packages through the target package manager. The DSL is distribution-neutral by default and can be pinned to a manager when needed.

```elixir
bootstrap do
  package :ca_certificates
  package :build_essential, as: "build-essential", update: true
end
```

HostKit can also bootstrap `mise` and install system-wide tool versions. This is intended for host bootstrap and workspace agents; application services should still prefer packaged release artifacts where possible.

```elixir
bootstrap do
  mise do
    tool :erlang, "29.0.2"
    tool :elixir, "1.20.1"
  end
end
```

This applies through the `mise` CLI contract: it installs the binary with `mise.run` when missing, then runs `mise install --system` with `MISE_SYSTEM_DATA_DIR` set.

Package planning resolves semantic package names through Repology and caches responses in `.host_kit/cache/repology` for 24 hours by default. Use locks for deterministic apply:

```sh
mix host_kit.plan --write-package-lock host_kit.package.lock infra/config.exs
mix host_kit.apply --package-lock host_kit.package.lock --confirm infra/config.exs
```

Plan/apply artifacts make remote changes inspectable before apply. Prefer declaring the remote host in normal `.exs` HostKit config and selecting it with `--host`:

```elixir
use HostKit.DSL

project :infra do
  host :prod, at: "host.example" do
    ssh do
      user "root"
      identity_file Path.expand("~/.ssh/id_ed25519")
      password secret_env("HOSTKIT_SSH_PASSWORD")
      accept_hosts true
      retry attempts: 3, base_delay: 250, max_delay: 2_000
    end
  end
end
```

```sh
mix host_kit.plan --host prod \
  --package-lock host_kit.package.lock \
  --out host_kit.plan.json infra/config.exs

mix host_kit.apply --host prod \
  --plan host_kit.plan.json --confirm infra/config.exs
```

`ssh retry: ...` is an SSH transport policy. It retries connection establishment for transient SSH startup/network failures; it does not blindly rerun arbitrary deployment commands after a command has been sent to the remote host. Use `retry: 3` as shorthand for three attempts, `retry: false` to disable, or keyword options with `:attempts`, `:base_delay`/`:base_delay_ms`, and `:max_delay`/`:max_delay_ms`. Retry progress is emitted as apply events and mirrored to Logger for collection.

Plan artifacts are JSON and intended to be inspectable. Secret references are stored as references, not values, for example:

```json
{
  "$type": "struct",
  "module": "Elixir.HostKit.Secret",
  "fields": {
    "source": {
      "$type": "tuple",
      "items": [
        {"$type": "atom", "value": "env"},
        "HOSTKIT_SSH_PASSWORD"
      ]
    }
  }
}
```

`secret_env/1` records an environment-backed secret reference and resolves it only at the control-plane boundary that needs the value. Use it for HostKit's own credentials, such as SSH passwords or future provider API tokens. Target application environment files use contextual `env` declarations. Inside `service`, `env :name do ... end` declares a managed env file at the service's config path. Inside `daemon`, `env :name` attaches that same file to the systemd unit:

```elixir
service :app do
  env :runtime do
    set :mix_env, :prod
    secret :database_url, env: "DATABASE_URL"
  end

  daemon do
    env :runtime
    exec ["/opt/app/bin/server"]
  end
end
```

Use `env_file path do ... end` only when you need an explicit path.

Raw SSH flags remain available as an escape hatch: `--remote`, `--user`, `--port`, `--identity-file`, `--password`, and `--password-env`.

For Linux integration testing, use Incus as the lightweight native container/VM backend:

```sh
HOSTKIT_INCUS_SUDO=true HOSTKIT_SSH_PUBLIC_KEY=$HOME/.ssh/id_ed25519.pub \
  scripts/incus_integration_vm.sh ensure
HOSTKIT_INCUS_SUDO=true scripts/incus_integration_vm.sh ip
```

Set `HOSTKIT_INCUS_TYPE=vm` to launch an Incus VM instead of the default container, and `HOSTKIT_INCUS_INSTANCE=name` to change the instance name. Run the remote CLI integration against Incus with `HOSTKIT_INTEGRATION_TOOL=incus`, or against a pre-existing host declared in `.exs` config with `HOSTKIT_INTEGRATION_TOOL=remote HOSTKIT_INTEGRATION_CONFIG=examples/integration_hosts.example.exs`.

A real remote validation can use the same host config and a shell-provided secret:

```sh
HOSTKIT_SSH_PASSWORD='...' \
HOSTKIT_INTEGRATION_TOOL=remote \
HOSTKIT_INTEGRATION_CONFIG=examples/integration_hosts.example.exs \
mix test test/integration/cli_remote_test.exs --include integration
```

## Project-local DSLs

Use `HostKit.ProjectDSL` in consuming projects to build local conventions without baking them into HostKit.
Load project-local DSL files explicitly through the runtime API or Mix task `--require` option:

```elixir
# infra/toys_infra.exs
defmodule ToysInfra do
  use HostKit.ProjectDSL

  root :source, "/opt/toys/src"
  root :data, "/srv/toys"
  root :state, "/var/lib/toys"
  root :config, "/etc/toys"

  prefix :user, "toys-"
  prefix :unit, "toys-"

  defservice :toy_service do
    let :service_user, do: prefixed(:user, service_name())
    let :unit_name, do: prefixed(:unit, service_name()) <> ".service"

    path :source_dir, root(:source), service_name()
    path :data_dir, root(:data), service_name()
    path :state_dir, root(:state), service_name()
    path :config_dir, root(:config), service_name()

    macro :standard_user do
      account service_user(), system: true, home: state_path("home")
    end
  end
end
```

```elixir
# infra/config.exs
use HostKit.DSL, providers: [HostKit.Providers.Caddy]
use ToysInfra

project :toys do
  toy_service :exograph do
    standard_user()

    systemd_service unit_name() do
      working_directory source_dir()
      read_write_paths [data_dir(), state_dir(), source_dir()]
    end
  end
end
```

## Runtime API

```elixir
{:ok, project} = HostKit.load("infra/config.exs", require: ["toys_infra.exs"])
{:ok, plan} = HostKit.plan(project)
#=> %HostKit.Plan{changes: [%HostKit.Change{action: :create, ...}]}

prod = HostKit.Target.ssh(:prod, host: "elixir.toys", user: "dannote", sudo: true)
{:ok, remote_plan} = HostKit.plan(project, target: prod, reader: HostKit.Remote)

HostKit.format_plan(plan)
{:ok, results} = HostKit.apply(plan, dry_run: true)

# Supported apply resources: accounts, directories, files, systemd services, and systemd timers.
{:ok, results} = HostKit.apply(plan, confirm: true, sudo: true)

# Command and filesystem operations are routed through a runner boundary.
{:ok, results} = HostKit.apply(plan, confirm: true, runner: HostKit.Runner.Local)

prod = HostKit.Target.ssh(:prod, host: "elixir.toys", user: "dannote", sudo: true)

{:ok, results} = HostKit.apply(plan, target: prod, confirm: true)

{:ok, conn} = HostKit.Runner.SSH.Connection.open(host: "elixir.toys", user: "dannote")
try do
  prod = HostKit.Target.ssh(:prod, runner: {HostKit.Runner.SSH.Connection, conn: conn}, sudo: true)
  {:ok, remote_plan} = HostKit.plan(project, target: prod, reader: HostKit.Remote)
after
  HostKit.Runner.SSH.Connection.close(conn)
end

{:ok, unit} = HostKit.Render.render(project, {:systemd_service, "toys-exograph.service"})
```

## Storage volumes

HostKit models storage as named metadata instead of repeated path strings:

```elixir
volume =
  HostKit.Storage.volume(:repositories,
    path: "/srv/toys/forgejo/repositories",
    owner: "toys-forgejo",
    group: "toys-forgejo",
    mode: 0o750,
    backup: true
  )

directory HostKit.Storage.directory(volume)
read_write_paths HostKit.Storage.read_write_paths([volume])
```

Service conventions can derive these paths without project-specific macros and later reuse the same volume metadata for systemd sandboxing, Unitctl transient runtimes, and backups.

```elixir
project :toys do
  roots data: "/srv/toys", config: "/etc/toys"
  prefixes user: "toys-", unit: "toys-"

  service :forgejo do
    storage :repositories, under: :data, path: "repositories", mode: 0o750, backup: true
    storage :config, under: :config, owner: "root", group: service_user(), writable: false, secret: true

    daemon unit_name() do
      run user: service_user(), read_write_paths: writable_storage_paths()
    end
  end
end
```

## HostKit agent

HostKit can run as a supervised OTP application. The supervision tree currently starts agent state and a monitor worker:

```elixir
HostKit.Agent.status()
HostKit.Agent.configure(project: project, target: HostKit.Target.local(:prod))
HostKit.Agent.run_plan()
HostKit.Agent.run_monitor()
```

HostKit can also declare its own outer systemd supervisor unit:

```elixir
HostKit.Agent.Systemd.service(
  exec_start: ["/opt/host_kit/bin/host_kit", "agent", "--config", "/etc/host_kit/config.exs"]
)
```

State snapshots can be written for audit/drift history:

```elixir
HostKit.State.write(plan, "/var/lib/host_kit/state/latest-plan.json")
HostKit.State.read("/var/lib/host_kit/state/latest-plan.json")
```

This gives a clean two-layer supervision model: OTP inside the BEAM and systemd outside it.

## Firewall policy

HostKit can declare project- or host-scoped firewall policy:

```elixir
firewall do
  allow tcp: 22, from: :any
  allow tcp: [80, 443], from: :any
  allow tcp: 9100, from: {10, 44, 0, 0, 24}
  deny :all
end
```

Host-scoped policy lives inside `host`:

```elixir
host :prod, at: "elixir.toys" do
  firewall do
    allow tcp: 22, from: :any
    deny :all
  end
end
```

Extract, render, plan, and apply policies with:

```elixir
HostKit.Firewall.policies(project)
HostKit.Firewall.Nftables.render(policy)
HostKit.plan(project, reader: HostKit.Local)
HostKit.apply(plan, confirm: true, nft_reload: true)
```

Firewall policy is written to `/etc/nftables.d/hostkit.nft` by default and validated with `nft -c -f` before optional reload.

## Workspace inside monitoring

Workspace services can declare checks that are intended to run inside the sandbox later via a workspace agent:

```elixir
workspace :blog, owner: :alice do
  service :preview do
    inside do
      monitor :mix, task: "test", every: "5m"
      monitor :port, port: 4000
      monitor :git, clean: true
    end
  end
end
```

Extract them with:

```elixir
HostKit.Workspace.inside_monitors(project)
```

## Workspace execution and tenants

Tenants can own workspaces:

```elixir
tenant :alice, quota: [memory: "4G"] do
  agent port: 4173
end
```

Workspace command specs can be built for transient execution:

```elixir
HostKit.Workspace.exec_spec(project, :alice, :blog, ["mix", "test"])
HostKit.Workspace.exec(project, :alice, :blog, ["mix", "test"])
```

Inside monitors currently return `:pending_workspace_agent`, reserving execution for the sandbox agent boundary.

## OpenTelemetry Collector config

Telemetry declarations can be converted to an OpenTelemetry Collector config map:

```elixir
HostKit.OtelCollector.config(project, endpoint: "otel.example:4317")
```

## Workspace sandbox profiles

Systemd-backed isolation profiles can be applied inside daemons:

```elixir
workspace :blog, owner: :alice do
  service :preview do
    daemon do
      exec ["mix", "phx.server"]

      isolate :vibe_dev do
        writable root_path(:data)
        network :loopback
      end
    end
  end
end
```

Profiles include `:vibe_dev`, `:strict_app`, and `:untrusted`, and can be overridden inside `isolate`:

```elixir
isolate :untrusted do
  memory_max "256M"
  private_network false
end
```

## Workspace preview helper

Workspace services can expose a preview route with a named listener and Caddy site:

```elixir
workspace :blog, owner: :alice do
  service :preview do
    daemon unit_name() do
      run exec_start: ["mix", "phx.server"]
    end

    preview :http, port: 4000, domain: "alice-blog.dev.example.com"
  end
end
```

This expands to `listen :http`, a Caddy reverse proxy to that listener, an HTTP monitor, telemetry metadata, and Caddy access-log metadata.

## Workspace agent helper

Workspaces can declare the default sandbox agent service as ordinary HostKit resources:

```elixir
workspace :blog, owner: :alice do
  agent port: 4173
end
```

This expands to a service with an account, workspace directory, systemd daemon, loopback listener, logs, telemetry, systemd monitor, and loopback-only network policy.

## Workspace scope

`workspace` scopes ordinary HostKit DSL for user sandboxes while keeping resources inspectable:

```elixir
workspace :blog, owner: :alice do
  service :preview do
    directory root_path(:data), mode: :private_dir

    daemon unit_name() do
      run exec_start: ["mix", "phx.server"]
      listen :http, port: 4000, on: :loopback
    end
  end
end
```

Inside a workspace, services get workspace metadata plus separate path and identity names:

```elixir
root_path(:data) # .../alice/blog/preview
unit_name()      # prefix-alice-blog-preview.service
```

## Named listeners

Services can declare named listeners and reuse them from provider declarations:

```elixir
daemon unit_name() do
  run exec_start: ["/usr/bin/env", "true"]
  listen :http, port: 3000, on: :loopback
end

caddy_site "web.example.com" do
  reverse_proxy :http
end
```

Named listeners are stored as service metadata and render Caddy upstreams like `127.0.0.1:3000` at the provider boundary.

## Network addresses and policy

Network addresses can use Elixir tuple forms and semantic aliases:

```elixir
listen 3000, on: :loopback
listen 4000, on: {127, 0, 0, 1}
network_policy deny: :all, allow: [:loopback, {10, 44, 0, 0, 24}]
```

Systemd services compile network policy to:

```ini
IPAddressDeny=any
IPAddressAllow=localhost 10.44.0.0/24
RestrictAddressFamilies=AF_INET AF_INET6 AF_UNIX
```

## Log management intent

Log management can be declared globally, per service, or on individual resources:

```elixir
observability do
  logs driver: :journald,
       retention: "14d",
       ship: true,
       attributes: [deployment_environment: :prod]
end
```

Systemd service log declarations also add unit directives:

```elixir
daemon unit_name() do
  run exec_start: ["/usr/bin/env", "true"]
  logs identifier: service_name(), stdout: :journal, stderr: :journal
end
```

Extract log intent with:

```elixir
HostKit.Logs.configs(project)
```

Read recent journald logs through local or remote targets:

```elixir
HostKit.Logs.read("toys-forgejo.service", target: prod, since: "1h")
HostKit.Logs.tail("toys-forgejo.service", target: prod, lines: 100)
```

## OpenTelemetry collection intent

Observability defaults can be enabled once at project or service scope and inherited by resources:

```elixir
observability do
  telemetry logs: true,
            metrics: true,
            traces: false,
            attributes: [deployment_environment: :prod]
end
```

Resource-level overrides are still available:

```elixir
daemon unit_name() do
  run exec_start: ["/usr/bin/env", "true"]
  telemetry logs: :journald, metrics: false, service_name: service_name()
end
```

Extract collection intent with:

```elixir
HostKit.Telemetry.signals(project)
```

Systemd services and Caddy sites get default collection intent even without global defaults:

```elixir
# systemd: logs: :journald, metrics: :systemd
# caddy: logs: :access, metrics: :http
```

## Monitoring metadata

Declarations can carry monitoring intent for a later monitoring service or config generator:

```elixir
daemon do
  exec ["/usr/bin/env", "true"]
  listen :http, port: 4000
  monitor :systemd, expect: [state: :active], severity: :critical
end

caddy_site "web.example.com" do
  reverse_proxy :http
  monitor :http, url: "https://web.example.com", expect: [status: 200]
end
```

Extract or run checks with:

```elixir
HostKit.Monitor.checks(project)
HostKit.Monitor.run(project, target: prod)
```

Initial execution supports systemd state, HTTP status, and filesystem existence checks.

## File modes

Mode values can be raw octal, semantic aliases, tuples, keywords, or capability lists:

```elixir
mode: :secret_group_file
mode: {:rw, :r, nil}
mode: [owner: :rw, group: :r]
mode: [:setgid, :owner_rwx, :group_rwx, :other_rx]
```

Resources store normalized integer modes, so plan/apply remains simple.

## Env files and secrets

HostKit has a Dotenvy-validated env file resource. Secret values are resolved at apply time. Drift detection compares metadata and non-secret `set` entries; secret entry values are not read into plan artifacts for comparison.

```elixir
service :web do
  env :runtime do
    set :MIX_ENV, :prod
    set :PORT, 4000
    secret :SECRET_KEY_BASE, env: "SECRET_KEY_BASE"
  end

  daemon do
    env :runtime
    exec ["/opt/web/bin/server"]
  end
end
```

## Runtime isolation

HostKit uses shared runtime isolation structs for persistent systemd units and future transient Unitctl workloads:

```elixir
sandbox = HostKit.Runtime.Sandbox.new(:strict_web)
resources = HostKit.Runtime.Resources.new(memory_max: "512M", cpu_quota: "50%")

service sandbox |> HostKit.Runtime.Sandbox.to_systemd_service_options()
service resources |> HostKit.Runtime.Resources.to_systemd_service_options()
```

Built-in profiles include `:web_service`, `:strict_web`, `:strict_app`, `:small`, `:medium`, and `:large`.

The daemon DSL exposes a human-oriented isolation block for common service hardening:

```elixir
service :api do
  storage :data, mode: 0o750

  daemon do
    exec ["/opt/api/bin/server"]

    isolate do
      memory_max "512M"
      writable :data
      network :loopback
    end
  end
end
```

`daemon do ... end` derives the unit name from the enclosing service and enables it for `multi-user.target` by default. Use explicit systemd directives only when you need non-default unit behavior.

## Runtime controls

HostKit exposes Unitctl as its core transient runtime layer:

```elixir
{:ok, spec} =
  HostKit.Runtime.Spec.new(
    name: "demo-check",
    command: ["/usr/bin/env", "true"],
    sandbox: %{no_new_privileges: true, private_tmp: true}
  )

{:ok, instance} = HostKit.Runtime.start(spec)
{:ok, state} = HostKit.Runtime.status(instance)
:ok = HostKit.Runtime.stop(instance)
```

## Mix tasks

```sh
mix host_kit.dump --require toys_infra.exs infra/config.exs
mix host_kit.plan --require toys_infra.exs infra/config.exs
mix host_kit.plan --require toys_infra.exs infra/config.exs --local
mix host_kit.plan --require toys_infra.exs infra/config.exs --local --ignore systemd_service:toys-exograph.service
mix host_kit.plan --require toys_infra.exs infra/config.exs --remote elixir.toys --user dannote --sudo
mix host_kit.apply --require toys_infra.exs infra/config.exs --local --dry-run
mix host_kit.render --require toys_infra.exs infra/config.exs systemd_service toys-exograph.service
```