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283 lines
12 KiB
Markdown
283 lines
12 KiB
Markdown
# s12: Task System — Break Big Goals into Small Tasks
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[中文](README.md) · [English](README.en.md) · [日本語](README.ja.md)
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s01 → ... → s10 → s11 → `s12` → [s13](../s13_background_tasks/) → s14 → ... → s20
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> *"Break big goals into small tasks, order them, persist"* — File-persisted task graph, the foundation for multi-agent collaboration.
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>
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> **Harness Layer**: Tasks — Persisted goals, recoverable progress.
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---
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## The Problem
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The agent receives a project: set up a database, write APIs, add tests. It uses s05's TodoWrite to create a checklist, then starts writing the API first, gets halfway through and realizes there are no database tables, goes back to fix them; when adding tests, discovers the API interface signatures have changed again...
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You can't build the roof before laying the foundation. Tasks have ordering. Task dependencies should form a Directed Acyclic Graph (DAG); the teaching version only demonstrates `blockedBy` checking, without cycle detection.
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s05's TodoWrite is an execution checklist for the current task, kept in session memory. What you need here is a **task system**: each task is a JSON file, tasks have `blockedBy` dependencies, and they persist across sessions on disk.
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---
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## The Solution
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Teaching code keeps a basic agent loop, omitting S11's full error recovery (RecoveryState, backoff, escalation, reactive compact, fallback model) to stay focused on the task system. Added: 5 new task tools + `.tasks/` directory for persistence + `blockedBy` dependency checking. The task system and error recovery are independent layers: in CC source, `utils/tasks.ts` only handles CRUD, while `query.ts`'s with_retry/RecoveryState handles error recovery, with no coupling between them.
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TodoWrite vs Task System:
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| | TodoWrite (s05) | Task System (s12) |
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|---|---|---|
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| Role | Execution checklist for the current task | Recoverable task system |
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| Storage | In-process / session state | `.tasks/{id}.json` |
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| Dependencies | None | `blockedBy` / `blocks` graph |
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| Lifecycle | Current session / current task | Cross-session |
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| Coordination | No task claiming | `owner` / claim |
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| Status | pending / in_progress / completed | pending / in_progress / completed |
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| Granularity | The agent's own steps | Tasks that can be claimed, tracked, and unblocked |
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---
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## How It Works
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### Task: Data Structure
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Each task is a JSON file, stored in the `.tasks/` directory:
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```python
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@dataclass
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class Task:
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id: str
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subject: str
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description: str
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status: str # pending | in_progress | completed
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owner: str | None # Agent name (multi-agent scenarios)
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blockedBy: list[str] # List of dependency task IDs
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```
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IDs are generated with `timestamp + random hex`, simple but sufficient. CC uses sequential IDs + a highwatermark file to prevent ID reuse, which is a more rigorous design.
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### create_task: Create Tasks
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```python
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def create_task(subject: str, description: str = "",
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blockedBy: list[str] | None = None) -> Task:
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task = Task(
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id=f"task_{int(time.time())}_{random_hex(4)}",
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subject=subject, description=description,
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status="pending", owner=None,
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blockedBy=blockedBy or [],
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)
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save_task(task)
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return task
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```
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Automatically calls `save_task` on creation to write `.tasks/{id}.json`. `blockedBy` declares dependencies, for example "write API" has `blockedBy: ["task_schema"]`.
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### can_start: Dependency Check
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A task can only start after all its `blockedBy` dependencies are **completed**:
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```python
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def can_start(task_id: str) -> bool:
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task = load_task(task_id)
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for dep_id in task.blockedBy:
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if not _task_path(dep_id).exists():
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return False # missing dependency = blocked
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dep = load_task(dep_id)
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if dep.status != "completed":
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return False
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return True
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```
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`can_start` is a prerequisite check for `claim_task`: if any `blockedBy` dependency is not completed, the task cannot be claimed. Missing dependencies are treated as blocked, avoiding crashes from referencing wrong IDs.
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### claim_task: Claim a Task
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When the agent starts working on a task, it calls `claim_task`: sets `owner`, changes status from `pending` → `in_progress`. The `owner` field records who is working on the task, preventing duplicate claims in multi-agent scenarios:
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```python
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def claim_task(task_id: str, owner: str = "agent") -> str:
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task = load_task(task_id)
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if task.status != "pending":
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return f"Task {task_id} is {task.status}, cannot claim"
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if not can_start(task_id):
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deps = [d for d in task.blockedBy
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if load_task(d).status != "completed"]
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return f"Blocked by: {deps}"
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task.owner = owner
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task.status = "in_progress"
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save_task(task)
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return f"Claimed {task_id} ({task.subject})"
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```
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If the task is already claimed by someone else (`status != "pending"`), or dependencies aren't met (`can_start` returns False), the claim is rejected.
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### complete_task: Complete and Unblock
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When a task is done, set it to `completed`. Simultaneously scan all other tasks to find downstream tasks that were **just unblocked**:
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```python
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def complete_task(task_id: str) -> str:
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task = load_task(task_id)
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task.status = "completed"
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save_task(task)
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# Find newly unblocked downstream tasks
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unblocked = [t.subject for t in list_tasks()
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if t.status == "pending" and t.blockedBy
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and can_start(t.id)]
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msg = f"Completed {task_id} ({task.subject})"
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if unblocked:
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msg += f"\nUnblocked: {', '.join(unblocked)}"
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return msg
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```
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After completing "schema", `can_start` returns True for "endpoints" and "docs"; they can begin.
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### get_task: View Full Details
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`list_tasks` only shows a one-line summary. `get_task` returns the full task JSON, including description and dependency details. When recovering across sessions, the agent needs to read the full description to continue work:
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```python
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def get_task(task_id: str) -> str:
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task = load_task(task_id)
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return json.dumps(asdict(task), indent=2)
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```
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### State Machine: Two Actions, Three States
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```
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pending ──claim──→ in_progress ──complete──→ completed
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```
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Here `claim` / `complete` are actions, while `pending` / `in_progress` / `completed` are states:
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- **claim_task**: `pending` → `in_progress`. Sets owner, begins work.
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- **complete_task**: `in_progress` → `completed`. Marks the task done and unblocks downstream.
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CC has no `in_progress → pending` release path. If a teammate terminates or shuts down, CC unassigns its unfinished tasks (clears owner) and resets status to `pending`, allowing other agents to reclaim them. The teaching version omits this recovery path.
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### Putting It Together
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```python
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# Create tasks with dependencies
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schema = create_task("setup database schema")
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endpoints = create_task("create API endpoints", blockedBy=[schema.id])
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tests = create_task("write tests", blockedBy=[endpoints.id])
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docs = create_task("write docs", blockedBy=[schema.id])
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# Agent claims the first available task
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claim_task(schema.id) # ✓ Claimed (no dependencies)
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complete_task(schema.id) # ✓ Completed → unblocks endpoints, docs
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claim_task(endpoints.id) # ✓ Claimed (schema completed)
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complete_task(endpoints.id) # ✓ Completed → unblocks tests
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claim_task(docs.id) # ✓ Claimed (schema completed)
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complete_task(docs.id) # ✓ Completed
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claim_task(tests.id) # ✓ Claimed (endpoints completed)
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complete_task(tests.id) # ✓ Completed
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```
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Each `create_task` writes a JSON file, each `claim_task` / `complete_task` updates the file. Across sessions, the `.tasks/` directory persists — the agent reads the files to recover progress.
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---
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## Changes from s11
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| Component | Before (s11) | After (s12) |
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|-----------|-------------|-------------|
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| Task management | None | Task dataclass + 5 tools |
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| New types | — | Task (id, subject, description, status, owner, blockedBy) |
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| Storage | No persistence | `.tasks/{id}.json` cross-session |
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| Dependencies | None | `blockedBy` graph + `can_start` check |
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| Tools | bash, read_file, write_file (3) | + create_task, list_tasks, get_task, claim_task, complete_task (8) |
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| Lifecycle | — | pending → in_progress → completed (no release rollback) |
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---
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## Try It
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```sh
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cd learn-claude-code
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python s12_task_system/code.py
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```
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Try these prompts:
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1. `Create tasks: setup database schema, create API endpoints (depends on schema), write tests (depends on endpoints), write docs (depends on schema)`
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2. `List all tasks and their statuses`
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3. `Claim the first unblocked task and complete it`
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4. `List tasks again — which ones are now unblocked?`
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What to observe: Are JSON files generated in the `.tasks/` directory? After completing a task, are the blocked tasks unblocked?
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---
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## What's Next
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The task graph is in place. But some tasks take a long time — like running full test suites or deploying to a server. The agent calls the LLM billed by token, it can't afford to wait on a slow operation.
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s13 Background Tasks → Slow operations go to the background. The agent continues processing other tasks, and gets notified when the background work is done.
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<details>
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<summary>Deep Dive into CC Source</summary>
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> The following is a complete analysis based on CC source code `utils/tasks.ts` (862 lines), `tools/TaskCreateTool/TaskCreateTool.ts` (138 lines), `tools/TaskUpdateTool/TaskUpdateTool.ts` (406 lines), `tools/TaskGetTool/TaskGetTool.ts` (128 lines), `tools/TaskListTool/TaskListTool.ts` (116 lines), `hooks/useTaskListWatcher.ts` (221 lines).
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### 1. TaskRecord's Full Fields
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The tutorial only covers id, subject, status, owner, blockedBy. CC actually has 9 fields (`utils/tasks.ts:76-89`):
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| Field | Type | Purpose |
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|------|------|---------|
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| `id` | string | Incrementing integer ID |
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| `subject` | string | Short title |
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| `description` | string | Free-form description |
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| `activeForm` | string? | Present tense form, shown in spinner when in_progress |
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| `owner` | string? | Assigned agent ID |
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| `status` | pending/in_progress/completed | Lifecycle |
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| `blocks` | string[] | Task IDs blocked by this task (downstream) |
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| `blockedBy` | string[] | Task IDs blocking this task (upstream) |
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| `metadata` | Record? | Arbitrary extension key-value pairs |
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Storage location: `~/.claude/tasks/{taskListId}/{id}.json`. One file per task.
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### 2. Not a TodoWrite Upgrade — Two Independent Systems
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In CC, Task System and TodoWrite **coexist**, toggled by `isTodoV2Enabled()` (`utils/tasks.ts:133`) — interactive sessions default to Task (V2), non-interactive/SDK sessions default to TodoWrite. The `CLAUDE_CODE_ENABLE_TASKS` env var can force-enable Task. Task has what TodoWrite lacks: file-lock concurrency protection, dependency enforcement, ownership, fs.watch reactive monitoring, lifecycle hooks.
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### 3. Concurrent Claim Locking
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`claimTask()` (`utils/tasks.ts:541-612`) uses dual locking to prevent races:
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**Task file lock**: `proper-lockfile` locks `{taskId}.json` (up to 30 retries, exponential backoff 5-100ms). Inside the lock:
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1. Re-read task (prevent TOCTOU)
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2. Check already claimed by another → `already_claimed`
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3. Check already completed → `already_resolved`
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4. Check upstream not completed → `blocked`
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5. Set owner
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**List-level lock** (agent busy check): `.lock` file, atomic scan of all tasks to check if the agent already has other open tasks.
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Note: The teaching version combines claiming and starting work into one step (claim = set owner + in_progress); real CC's `claimTask` primarily resolves owner competition — it only sets owner without changing status. Status updates are handled by `TaskUpdate`.
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### 4. High-Water Mark to Prevent ID Reuse
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The `.highwatermark` file records the highest task ID ever assigned. Even if a task is deleted, its ID won't be reused.
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### 5. Four Task Tools
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CC's task system has four tools (not the tutorial's single generic Task tool): `TaskCreate`, `TaskGet`, `TaskUpdate`, `TaskList`. All set `isConcurrencySafe: true` and `shouldDefer: true` (tool schemas aren't in the initial prompt; only visible after ToolSearch).
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The teaching version's `create_task(blockedBy=...)` declares dependencies at creation time, which is a reasonable simplification. Real CC's `TaskCreate` only accepts subject/description/activeForm/metadata — dependencies are maintained via `TaskUpdate`'s `addBlocks/addBlockedBy`.
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</details>
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<!-- translation-sync: zh@v1, en@v1, ja@v1 -->
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