# Architecture

MobusStepwise is an ALF-backed workflow execution engine with two explicit profiles:

- `:stepwise` for ordered, wizard-like progression
- `:flow` for graph execution with explicit forks, joins, waits, and checkpointable parallel state

## Core Model

The engine separates **specification** (what the workflow looks like) from **runtime** (where the workflow is right now).

### `:stepwise` Spec (static definition)

```
%{
  profile:       :stepwise,
  initial_state: :step_one,
  steps:         [:step_one, :step_two, :step_three],
  states: %{
    step_one:   %{step_number: 1, ui: %{key: :step_one}, action: %{...}},
    step_two:   %{step_number: 2, ui: %{key: :step_two}},
    step_three: %{step_number: 3, ui: %{key: :step_three}}
  },
  transitions:   %{},          # optional explicit transitions
  breakpoints:   [],            # optional debug breakpoints
  subscriptions: []             # optional PubSub topics
}
```

Key fields:

| Field           | Purpose                                                       |
|-----------------|---------------------------------------------------------------|
| `profile`       | Always `:stepwise` — identifies this as a stepwise workflow   |
| `initial_state` | The step the engine starts on                                 |
| `steps`         | Ordered list of step identifiers (atoms or strings)           |
| `states`        | Per-step metadata: UI descriptor, actions, step number        |
| `transitions`   | Optional explicit event→state mappings for custom events      |
| `breakpoints`   | Optional debug breakpoints on events or state entries         |
| `subscriptions` | PubSub topic templates interpolated with runtime context      |

### `:stepwise` Runtime (live state)

```
%{
  execution_id:    "exec-001",
  tenant_id:       "tenant-123",
  spec:            %{...},           # normalized IR
  current_state:   :step_two,
  context:         %{name: "Alice"}, # accumulated user input
  artifacts:       %{},              # durable workflow-scoped data
  history:         [%{event: :next, from: :step_one, to: :step_two, at: ~U[...]}],
  trace:           [%{kind: :step, ...}],
  blocked_reasons: %{},
  breakpoint_hits: [],
  projection:      %Projection{...}  # canonical UI contract
}
```

## Engine Lifecycle

```
                     ┌─────────────────────────────────────────────┐
                     │                  SPEC                       │
                     │  profile, initial_state, steps, states      │
                     └─────────────────┬───────────────────────────┘
                                       │
                                       ▼
                              ┌─────────────────┐
                              │  Engine.init/2   │
                              │  normalize → IR  │
                              │  start pipeline  │
                              │  entry action    │
                              │  → projection    │
                              └────────┬────────┘
                                       │
                                       ▼
                              ┌─────────────────┐
                              │    RUNTIME       │◄──────────────┐
                              │  current_state   │               │
                              │  context         │               │
                              │  projection      │               │
                              └────────┬────────┘               │
                                       │                         │
                            event(:next, payload)                │
                                       │                         │
                                       ▼                         │
                     ┌─────────────────────────────────┐         │
                     │      ALF Pipeline               │         │
                     │                                 │         │
                     │  1. StepwiseContextMerge        │         │
                     │     merge payload → context     │         │
                     │                                 │         │
                     │  2. StepwiseAction              │         │
                     │     run capability (if defined) │         │
                     │                                 │         │
                     │  3. StepwiseAdvance             │         │
                     │     move to next/prev step      │         │
                     │                                 │         │
                     │  4. StepwiseEntryAction         │         │
                     │     run entry action on new     │         │
                     │     state (if state changed)    │         │
                     │                                 │         │
                     │  5. FsmBreakpoint               │         │
                     │     record breakpoint hits      │         │
                     │                                 │         │
                     │  6. StepwiseProjection          │         │
                     │     build UI projection         │         │
                     └────────────────┬────────────────┘         │
                                      │                          │
                                      ▼                          │
                              ┌─────────────────┐               │
                              │  updated RUNTIME │───────────────┘
                              │  new state       │
                              │  new projection  │
                              └─────────────────┘
```

### init/2

1. Extracts `tenant_id` and `execution_id` from runtime context
2. Starts the ALF pipeline (if not already running)
3. Normalizes the spec into internal representation (IR)
4. Builds initial runtime with `current_state` set to `initial_state`
5. Fires entry action for the initial state (capabilities with `:enter` trigger)
6. Computes initial projection

### handle_event/3

Sends an event (`:next`, `:back`, or custom) through the ALF pipeline with a payload. The pipeline stages execute in order:

1. **StepwiseContextMerge** — merges the event payload into `runtime.context`
2. **StepwiseAction** — if the current step defines an action (e.g. capability), executes it
3. **StepwiseAdvance** — advances or reverses `current_state` based on step ordering
4. **StepwiseEntryAction** — if the state changed, runs entry-triggered actions on the new state
5. **FsmBreakpoint** — records any breakpoint hits for debugging
6. **StepwiseProjection** — computes the canonical `Projection` struct

Returns `{:ok, runtime}`, `{:wait, runtime, wait_cfg}`, or `{:error, reason, runtime}`.

### get_state/1

Returns the current `Projection` struct from the runtime. If the projection is stale or missing, recomputes it by running the pipeline in projection-only mode.

### checkpoint/1 and restore/3

`checkpoint/1` strips the runtime down to a serializable map (no projection, no process references). `restore/3` re-hydrates a runtime from a checkpoint, re-normalizing the spec and recomputing the projection.

## Profiles

The engine uses `spec.profile` to select profile-specific behavior.

### `:stepwise`

- Ordered step progression from `steps` or `step_number`
- Single `current_state`
- ALF pipeline-backed step/action/projection flow
- Existing compatibility profile for all current consumers

### `:flow`

- Explicit `nodes` + `edges` graph model
- Token-based execution with multiple active branches
- Fork and join semantics with branch result aggregation
- Shared `%Projection{}` wrapper with graph data in `projection.extensions.flow`
- Versioned checkpoints that preserve active tokens, waits, join buffers, and branch results

## Events

Events drive state changes through the engine:

| Event             | Behavior                                                    |
|-------------------|-------------------------------------------------------------|
| `:next` / `"next"`| Advance to the next step in order                           |
| `:back` / `"back"`| Return to the previous step in order                        |
| Custom atoms      | Looked up in `spec.transitions` for explicit target state   |
| `:__enter__`      | Internal — fired on state entry to trigger entry actions    |

Events carry a `payload` map that is merged into `runtime.context` by `StepwiseContextMerge`, accumulating user input across steps.

For `:flow`, events are interpreted by the graph runtime. `:next` advances ready tokens, `:resume` resumes waiting tokens, and `:cancel` / `:timeout` can target specific tokens or branches through the payload.

## Projection

The `Mobus.Stepwise.Projection` struct is the canonical contract between the engine and the UI layer. It contains everything the UI needs to render the current step:

- `current_state` — which step we're on
- `available_events` — what the user can do (`:next`, `:back`)
- `ui` — `%{key: atom, assigns: map}` descriptor for component rendering
- `artifacts` — durable workflow-scoped data
- `blocked_reasons` — why certain events are blocked
- `errors` / `trace` — debugging information

For `:flow`, `current_state` is the focus node and graph-wide execution state lives under `projection.extensions.flow`.

## Capabilities

Steps can define **actions** that execute via a pluggable capability runner adapter:

```elixir
action: %{type: :capability, handle: "myapp.validate", triggers: [:next]}
```

- The adapter is configured via `config :mobus_stepwise, :capability_runner_adapter, Module`
- When no adapter is set, capability execution is a no-op (`{:ok, %{context: %{}}}`)
- Capabilities receive the full execution context and return context updates and/or artifacts
- Failed capabilities block the event and populate `blocked_reasons`

## Artifacts

Artifacts are durable, workflow-scoped data that survive refreshes, retries, and external callbacks. They are normalized into a canonical format with `kind`, `version`, `inserted_at`, and `data` fields. Capabilities can produce artifacts that are merged into the runtime.

## IR (Internal Representation)

Before the engine operates on a spec, it normalizes it via `Mobus.Stepwise.IR.normalize/1`. This:

- Coerces known profile strings to atoms
- Ensures `states`, `transitions`, and `nodes` exist in normalized shapes
- Normalizes the profile identifier

## Integration

A typical host application integrates MobusStepwise by:

1. Building a stepwise spec from its own domain model (steps, states, UI keys)
2. Calling `Engine.init/2` with the spec and a runtime context containing `tenant_id` and `execution_id`
3. On user interaction, calling `Engine.handle_event/3` with `:next` and the form payload
4. Reading `Engine.get_state/1` to get the `Projection` for rendering
5. Using `checkpoint/1` / `restore/3` for persistence across page refreshes

The flow:

```
Groove Definition
       │
       ▼
  Build stepwise spec (steps, states, UI keys)
       │
       ▼
  Engine.init(spec, %{tenant_id: t, execution_id: e, sync: true})
       │
       ▼
  Render projection.ui.key as LiveComponent
       │
       ▼
  User submits form → Engine.handle_event(runtime, :next, form_data)
       │
       ▼
  Re-render with updated projection
       │
       ▼
  Repeat until final step
```