Internal architecture

This guide describes the core HostKit runtime shape. It is for contributors and advanced users who want to understand how declarations become inspectable plans, how plans are applied, and why rollback is represented as another plan.

Core flow

flowchart LR
  DSL[HostKit DSL / .exs] --> Project[HostKit.Project]
  Project --> Resources[Plain resource structs]
  Resources --> Plan[HostKit.Plan]
  Plan --> Changes[HostKit.Change before/after pairs]
  Changes --> Apply[HostKit.Apply]
  Apply --> Runner[HostKit.Runner Local/SSH]
  Apply --> Events[HostKit.Apply.Event]
  Apply --> RunRecord[HostKit.RunRecord]

  Plan --> Down[HostKit.down/2]
  Down --> DownPlan[Down Plan]
  DownPlan --> Apply

The key rule is: the plan is the operational unit. A rollback is not a separate migration system; it is a down plan derived from an existing plan and applied through the same apply engine.

Main entities

classDiagram
  class Project {
    name
    hosts
    services
    resources
    providers
    conventions
  }

  class Resource {
    <<protocol-by-convention>>
    id(resource)
  }

  class Plan {
    project
    resources
    changes
    summary
    diagnostics
    opts
  }

  class Change {
    action
    resource_id
    before
    after
    reason
  }

  class ApplyEvent {
    type
    resource_id
    action
    lifecycle
    details
    reason
  }

  class RunRecord {
    id
    project
    direction
    applied_at
    changes
  }

  Project --> Resource
  Plan --> Project
  Plan --> Change
  Change --> Resource : before/after
  ApplyEvent --> Change
  RunRecord --> Change

`HostKit.Project`

A project is the compiled form of a HostKit declaration. The DSL is only a builder; it should compile to plain structs that can be inspected without applying anything.

A project owns:

declared hosts,
services and their scoped resources,
top-level resources,
enabled providers,
provider config,
project conventions such as path roots and naming prefixes.

Resource structs

Resources describe desired state or an operational step. Examples:

%HostKit.Resources.File{}
%HostKit.Resources.EnvFile{}
%HostKit.Systemd.Service{}
%HostKit.Resources.Command{}
%HostKit.Resources.Readiness{}
%HostKit.Proxy{}
%HostKit.Ingress{}

Resources are intentionally ordinary structs. HostKit.Resource.id/1 gives each resource a stable resource id.

`HostKit.Plan`

A plan is a resolved, ordered view of resources and changes. It contains:

resources: resources after resolution/expansion,
changes: HostKit.Change entries,
diagnostics: warnings/errors from planning,
opts: target/planning metadata.

Planning can compare desired resources with actual host state when a reader is configured. That is what makes rollback meaningful: changes can carry both before and after.

`HostKit.Change`

A change is the smallest apply unit:

%HostKit.Change{
  action: :update,
  resource_id: {:file, "/etc/app.env"},
  before: old_file,
  after: new_file,
  reason: :drift
}

For ordinary updates:

after is the up direction,
before is the down direction.

Down plans

HostKit.down(plan) reverses supported changes into another %HostKit.Plan{}.

flowchart TD
  UpPlan[Up Plan] --> UpChange[Change: before -> after]
  UpChange --> Down[HostKit.down/2]
  Down --> DownChange[Change: after -> before]
  DownChange --> DownPlan[Down Plan]
  DownPlan --> Apply[HostKit.apply/2]

Commands are semantic operations, so HostKit cannot infer an opposite command. A command must declare its down behavior:

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

Supported command down policies:

down: %HostKit.Resources.Command{} — emit this command in the down plan,
down: :noop — explicitly no down action,
down: :irreversible — omit and warn,
down: nil — omit and warn.

`HostKit.Apply`

Apply executes changes through the configured runner. The same apply engine handles up plans and down plans.

Apply emits mailbox events when reporter: pid is configured:

HostKit.apply(plan, confirm: true, reporter: self())

Events are sent as:

{HostKit.Apply, %HostKit.Apply.Event{}}

`HostKit.Apply.Event`

Events are the primary user-facing progress API. They cover:

apply lifecycle,
change lifecycle,
command lifecycle metadata,
service restart/readiness progress,
HTTP health checks.

Lifecycle command events include:

%{
  phase: :before_start,
  operation: :migrate,
  direction: :up
}

Readiness events include service and health details, for example:

%HostKit.Apply.Event{
  type: :health_check_passed,
  resource_id: {:readiness, :app_ready},
  details: %{url: "http://127.0.0.1:4000/health"}
}

`HostKit.Resources.Readiness`

Readiness waits for generated or user-declared startup checks:

ready :app_ready, timeout: 60_000 do
  systemd("app.service", restart: true, kill: true)
  http("http://127.0.0.1:4000/health", body: "ok")
end

Recipes such as elixir_app can emit readiness automatically. During apply, readiness emits progress events for restarts, active services, waiting checks, pass/fail, and timeout.

`HostKit.RunRecord`

Run records are minimal tracking artifacts written when apply is called with track: true or mix host_kit.apply --track.

flowchart LR
  ApplyTrack[apply --track --plan up.plan.json] --> Copy[copy up plan under runs root]
  Copy --> Record[run record JSON]
  Record --> Runs[mix host_kit.runs]
  Record --> Last[mix host_kit.down --last]
  Last --> UpPlan[copied up plan artifact]
  UpPlan --> DownPlan[generated down plan]

They intentionally do not replace plans. They store compact metadata such as:

run id,
project,
direction,
applied timestamp,
resource ids/actions/statuses,
copied up/down plan artifact references when available.

The storage roots are based on project conventions:

roots hostkit_state: "/var/lib/hostkit"
# derived defaults:
# hostkit_runs: "/var/lib/hostkit/runs"
# hostkit_backups: "/var/lib/hostkit/backups"

Providers, recipes, and semantic resources

flowchart TD
  Provider[Provider] --> DSL[Provider DSL]
  Provider --> Render[Render/apply integration]
  Recipe[Recipe] --> Resources[Plain resources]
  Ingress[Semantic ingress] --> Caddy[Caddy resources]
  Ingress --> Gatehouse[Gatehouse proxy]
  ElixirApp[elixir_app recipe] --> Source[source]
  ElixirApp --> Mise[mise runtime]
  ElixirApp --> Commands[commands]
  ElixirApp --> Systemd[systemd service]
  ElixirApp --> Readiness[readiness]
  ElixirApp --> Ingress

Providers own integrations such as Caddy and Gatehouse.
Recipes compose high-level patterns into plain resources.
Semantic resources such as ingress are expanded during planning into provider-specific resources.

Targeting and runners

flowchart LR
  Host[Host declaration] --> Target[HostKit.Target opts]
  Target --> Reader[Local/Remote reader]
  Target --> Runner[Local/SSH runner]
  Reader --> Plan
  Runner --> Apply

A target controls how HostKit reads current state and applies changes. Mix tasks are thin wrappers around the runtime API.

Design constraints

DSLs compile to plain structs.
Runtime API is primary; Mix tasks wrap it.
Plans are inspectable and artifact-friendly.
Rollback is a down plan, not a separate migration system.
Apply progress is mailbox-driven via reporter: pid.
Telemetry may mirror events, but it is not the primary apply API.
Secrets should remain secret-safe in artifacts and logs.

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