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# ADR-002: Task State Machine Design
## Status
Accepted
## Context
Claudomator tasks move through a well-defined lifecycle: from creation through
queuing, execution, and final resolution. The lifecycle must handle asynchronous
execution (subprocess), user interaction (review, Q&A), retries, and cancellation.
## States
| State | Meaning |
|---|---|
| `PENDING` | Task created; not yet submitted for execution |
| `QUEUED` | Submitted to executor pool; goroutine slot may be waiting |
| `RUNNING` | Claude subprocess is actively executing |
| `READY` | Top-level task completed execution; awaiting user accept/reject |
| `COMPLETED` | Task is fully done (terminal) |
| `FAILED` | Execution error occurred; eligible for retry (terminal if retries exhausted) |
| `TIMED_OUT` | Task exceeded configured timeout; resumable or retryable (terminal if not resumed) |
| `CANCELLED` | Explicitly cancelled by user or API (terminal) |
| `BUDGET_EXCEEDED` | Exceeded `max_budget_usd` (terminal) |
| `BLOCKED` | Agent paused and wrote a question file; awaiting user answer |
Terminal states with no outgoing transitions: `COMPLETED`, `CANCELLED`, `BUDGET_EXCEEDED`.
## State Transition Diagram
```
┌─────────┐
│ PENDING │◄───────────────────────────────┐
└────┬────┘ │
POST /run │ POST /reject │
POST /cancel │ │
┌────▼────┐ ┌──────┴─────┐
┌────────┤ QUEUED ├─────────────┐ │ READY │
│ └────┬────┘ │ └──────┬─────┘
POST /cancel │ │ POST /accept │
│ pool picks up │ ▼
▼ ▼ │ ┌─────────────┐
┌──────────┐ ┌─────────┐ │ │ COMPLETED │
│CANCELLED │◄──┤ RUNNING ├──────────────┘ └─────────────┘
└──────────┘ └────┬────┘
│
┌───────────────┼───────────────────┬───────────────┐
│ │ │ │
▼ ▼ ▼ ▼
┌──────────┐ ┌──────────────┐ ┌─────────────┐ ┌─────────┐
│ FAILED │ │ TIMED_OUT │ │ BUDGET │ │ BLOCKED │
└────┬─────┘ └──────┬───────┘ │ _EXCEEDED │ └────┬────┘
│ │ └─────────────┘ │
retry │ resume/ │ POST /answer
│ retry │ │
└────────┬───────┘ │
▼ │
QUEUED ◄────────────────────────────────────┘
```
### Transition Table
| From | To | Trigger |
|---|---|---|
| `PENDING` | `QUEUED` | `POST /api/tasks/{id}/run` |
| `PENDING` | `CANCELLED` | `POST /api/tasks/{id}/cancel` |
| `QUEUED` | `RUNNING` | Pool goroutine starts execution |
| `QUEUED` | `CANCELLED` | `POST /api/tasks/{id}/cancel` |
| `RUNNING` | `READY` | Runner exits 0, no question file, top-level task (`parent_task_id == ""`) |
| `RUNNING` | `COMPLETED` | Runner exits 0, no question file, subtask (`parent_task_id != ""`) |
| `RUNNING` | `FAILED` | Runner exits non-zero or stream signals `is_error: true` |
| `RUNNING` | `TIMED_OUT` | Context deadline exceeded (`context.DeadlineExceeded`) |
| `RUNNING` | `CANCELLED` | Context cancelled (`context.Canceled`) |
| `RUNNING` | `BUDGET_EXCEEDED` | `--max-budget-usd` exceeded (signalled by runner) |
| `RUNNING` | `BLOCKED` | Runner exits 0 but left a `question.json` file in log dir |
| `READY` | `COMPLETED` | `POST /api/tasks/{id}/accept` |
| `READY` | `PENDING` | `POST /api/tasks/{id}/reject` (with optional comment) |
| `FAILED` | `QUEUED` | Retry (manual re-run via `POST /api/tasks/{id}/run`) |
| `TIMED_OUT` | `QUEUED` | `POST /api/tasks/{id}/resume` (resumes with session ID) |
| `BLOCKED` | `QUEUED` | `POST /api/tasks/{id}/answer` (resumes with user answer) |
## Implementation
**Validation:** `task.ValidTransition(from, to State) bool`
(`internal/task/task.go:93`) — called by API handlers before every state change.
**State writes:** `storage.DB.UpdateTaskState(id, state)` — single source of
write; called by both API handlers and the executor pool.
**Execution outcome → state mapping** (in `executor.Pool.execute` and `executeResume`):
```
runner.Run() returns nil AND parent_task_id == "" → READY
runner.Run() returns nil AND parent_task_id != "" → COMPLETED
runner.Run() returns *BlockedError → BLOCKED (question stored)
ctx.Err() == DeadlineExceeded → TIMED_OUT
ctx.Err() == Canceled → CANCELLED
any other error → FAILED
```
## Key Invariants
1. **`READY` is top-level only.** Subtasks (tasks with `parent_task_id != ""`) skip
`READY` and go directly to `COMPLETED` on success. This allows parent review
flows without requiring per-subtask acknowledgement.
2. **`BLOCKED` requires a `session_id`.** The executor persists `session_id` in
the execution record at start time. `POST /answer` retrieves it via
`GetLatestExecution` to resume the correct Claude session.
3. **`DELETE` is blocked for `RUNNING` and `QUEUED` tasks.** (`server.go:127`)
4. **`CANCELLED` is reachable from `PENDING`, `QUEUED`, and `RUNNING`.** For
`RUNNING` tasks, `pool.Cancel(taskID)` sends a context cancellation signal;
the state transition happens asynchronously when the goroutine exits. For
`PENDING`/`QUEUED` tasks, `UpdateTaskState` is called directly.
5. **Dependency waiting.** Tasks with `depends_on` stay conceptually in `QUEUED`
(goroutine running but not executing) while `waitForDependencies` polls until
all dependencies reach `COMPLETED`. If any dependency reaches a terminal
failure state, the waiting task transitions to `FAILED`.
6. **Retry re-enters at `QUEUED`.** `FAILED → QUEUED` and `TIMED_OUT → QUEUED`
are the only back-edges. Retry is manual (caller must call `/run` again); the
`RetryConfig.MaxAttempts` field exists but enforcement is left to callers.
## Side Effects on Transition
| Transition | Side effects |
|---|---|
| → `QUEUED` | `pool.Submit()` called; execution record created in DB with log paths |
| → `RUNNING` | `store.UpdateTaskState`; cancel func registered in `pool.cancels` |
| → `BLOCKED` | `store.UpdateTaskQuestion(taskID, questionJSON)`; session_id in execution |
| → `READY/COMPLETED/FAILED/TIMED_OUT/CANCELLED/BUDGET_EXCEEDED` | Execution end time set; `store.UpdateExecution`; result broadcast via WebSocket (`pool.resultCh → forwardResults → hub.Broadcast`) |
| `READY → PENDING` (reject) | `store.RejectTask(id, comment)` — stores `rejection_comment` |
| `BLOCKED → QUEUED` (answer) | `store.UpdateTaskQuestion(taskID, "")` clears question; new `storage.Execution` created with `ResumeSessionID` + `ResumeAnswer` |
## WebSocket Events
Task lifecycle changes produce WebSocket broadcasts to all connected clients:
- `task_completed` — emitted on any terminal or quasi-terminal transition (READY,
COMPLETED, FAILED, TIMED_OUT, CANCELLED, BUDGET_EXCEEDED, BLOCKED)
- `task_question` — emitted by `BroadcastQuestion` when an agent uses
`AskUserQuestion` (currently unused in the file-based flow; the file-based
mechanism is the primary BLOCKED path)
## Known Limitations and Edge Cases
- **`BUDGET_EXCEEDED` transition.** `BUDGET_EXCEEDED` appears in `terminalFailureStates`
(used by `waitForDependencies`) but has no outgoing transitions in `ValidTransition`,
making it permanently terminal. There is no `/resume` endpoint for it.
- **Retry enforcement.** `RetryConfig.MaxAttempts` is stored but not enforced by
the pool. The API allows unlimited manual retries via `POST /run` from `FAILED`.
- **`TIMED_OUT` resumability.** Only `POST /api/tasks/{id}/resume` resumes timed-out
tasks. Unlike BLOCKED, there is no question/answer — the resume message is
hardcoded: _"Your previous execution timed out. Please continue where you left off."_
- **Concurrent cancellation race.** If a task transitions `RUNNING → COMPLETED`
and `POST /cancel` is called in the same window, `pool.Cancel()` may return
`true` (cancel func still registered) even though the goroutine is finishing.
The goroutine's `ctx.Err()` check wins; the task ends in `COMPLETED`.
## Relevant Code Locations
| Concern | File | Lines |
|---|---|---|
| State constants | `internal/task/task.go` | 7–18 |
| `ValidTransition` | `internal/task/task.go` | 93–109 |
| State machine tests | `internal/task/task_test.go` | 8–72 |
| Pool execute | `internal/executor/executor.go` | 194–303 |
| Pool executeResume | `internal/executor/executor.go` | 116–185 |
| Dependency wait | `internal/executor/executor.go` | 305–340 |
| `BlockedError` | `internal/executor/claude.go` | 31–37 |
| Question file detection | `internal/executor/claude.go` | 103–110 |
| API state transitions | `internal/api/server.go` | 138–415 |
|
