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# API Reference

This document provides a complete reference for all Visual Notebook API endpoints, including request schemas, response schemas, status codes, and usage examples.

## Base URL

- Development: `http://localhost:8000`
- Production: Same origin (backend serves the frontend)
- All API endpoints are prefixed with `/api`
- The health endpoint is at `/health` (no prefix)

## Authentication

The API uses cookie-based session management. The `session_id` cookie is set automatically after the first CSV upload and sent with all subsequent requests. No explicit authentication is required.

## Common Response Patterns

### Error Responses

All error responses follow the RFC 7807 pattern with a `detail` field:

```json
{
    "detail": "Error description"
}
```

### Session Cookie

The first endpoint that creates a session (`POST /api/upload/csv`) sets an `httponly` cookie:

```
Set-Cookie: session_id=abc123def456; HttpOnly; Max-Age=604800; Path=/; SameSite=lax
```

---

## Endpoints

### POST /api/upload/csv

Upload a CSV file, register it in DuckDB, and create a new session.

**Request**: `multipart/form-data`

| Field | Type | Required | Description |
|-------|------|----------|-------------|
| `file` | File | Yes | CSV file to upload (must have `.csv` extension) |

**Response**: `200 OK`

```json
{
    "dataset_id": "a3b2c1d4",
    "table_name": "ds_a3b2c1d4",
    "row_count": 15000,
    "columns": [
        {
            "name": "order_id",
            "dtype": "INTEGER",
            "column_type": "numeric",
            "null_count": 0,
            "unique_count": 15000,
            "min": 1,
            "max": 15000,
            "mean": 7500.5,
            "std": 4330.1,
            "q25": 3750.75,
            "q50": 7500.5,
            "q75": 11250.25,
            "count": 15000
        },
        {
            "name": "status",
            "dtype": "VARCHAR",
            "column_type": "categorical",
            "null_count": 5,
            "unique_count": 3,
            "min": null,
            "max": null,
            "mean": null,
            "std": null,
            "q25": null,
            "q50": null,
            "q75": null,
            "count": 14995
        }
    ],
    "session_id": "a1b2c3d4e5f6"
}
```

**Error Responses**:

| Status | Condition |
|--------|-----------|
| 400 | File does not have `.csv` extension |
| 413 | File exceeds `MAX_UPLOAD_SIZE` |

---

### GET /api/schema

Retrieve the schema and statistics for the currently loaded dataset.

**Request**: No parameters.

**Response**: `200 OK`

```json
{
    "table_name": "ds_a3b2c1d4",
    "row_count": 15000,
    "columns": [
        {
            "name": "order_id",
            "dtype": "INTEGER",
            "column_type": "numeric",
            "null_count": 0,
            "unique_count": 15000,
            "min": 1,
            "max": 15000,
            "mean": 7500.5,
            "std": 4330.1,
            "q25": 3750.75,
            "q50": 7500.5,
            "q75": 11250.25,
            "count": 15000
        }
    ]
}
```

**Error Responses**:

| Status | Condition |
|--------|-----------|
| 400 | No dataset loaded in the current session |

---

### GET /api/rows

Retrieve paginated rows from the current dataset.

**Query Parameters**:

| Parameter | Type | Default | Description |
|-----------|------|---------|-------------|
| `offset` | integer | 0 | Row offset (0-based) |
| `limit` | integer | 50 | Maximum number of rows to return |

**Response**: `200 OK`

```json
{
    "rows": [
        {"order_id": 1, "status": "shipped", "amount": 99.99},
        {"order_id": 2, "status": "pending", "amount": 45.50}
    ],
    "columns": ["order_id", "status", "amount"],
    "offset": 0,
    "limit": 50,
    "total": 15000
}
```

**Error Responses**:

| Status | Condition |
|--------|-----------|
| 400 | No dataset loaded |

---

### GET /api/ops

List all available data operations.

**Request**: No parameters.

**Response**: `200 OK`

```json
{
    "operations": [
        {
            "key": "filter",
            "label": "Filter",
            "description": "Filter rows by condition",
            "param_schema": {
                "type": "object",
                "required": ["column", "operator"],
                "properties": {
                    "column": {"type": "string"},
                    "operator": {"type": "string", "enum": ["eq", "neq", "gt", "lt", "gte", "lte", "contains", "is_null", "is_not_null"]},
                    "value": {"type": ["string", "number"]}
                }
            }
        },
        {
            "key": "sort",
            "label": "Sort",
            "description": "Sort by column",
            "param_schema": {
                "type": "object",
                "required": ["column"],
                "properties": {
                    "column": {"type": "string"},
                    "descending": {"type": "boolean", "default": false}
                }
            }
        },
        {
            "key": "select",
            "label": "Select Columns",
            "description": "Keep only selected columns",
            "param_schema": {
                "type": "object",
                "required": ["columns"],
                "properties": {
                    "columns": {"type": "array", "items": {"type": "string"}}
                }
            }
        },
        {
            "key": "rename",
            "label": "Rename Columns",
            "description": "Rename columns using a mapping",
            "param_schema": {
                "type": "object",
                "required": ["mapping"],
                "properties": {
                    "mapping": {"type": "object", "additionalProperties": {"type": "string"}}
                }
            }
        },
        {
            "key": "groupby",
            "label": "Group By",
            "description": "Aggregate data by groups",
            "param_schema": {
                "type": "object",
                "required": ["group_by", "aggregations"],
                "properties": {
                    "group_by": {"type": "array", "items": {"type": "string"}},
                    "aggregations": {"type": "object", "additionalProperties": {"type": "array", "items": {"type": "string"}}}
                }
            }
        },
        {
            "key": "fill_nulls",
            "label": "Fill Nulls",
            "description": "Fill null values using a strategy",
            "param_schema": {
                "type": "object",
                "properties": {
                    "strategy": {"type": "string", "enum": ["forward", "backward", "mean", "zero", "constant"], "default": "forward"},
                    "columns": {"type": "array", "items": {"type": "string"}},
                    "value": {"type": ["string", "number"]}
                }
            }
        },
        {
            "key": "clip",
            "label": "Clip Values",
            "description": "Clip values to a range",
            "param_schema": {
                "type": "object",
                "required": ["column"],
                "properties": {
                    "column": {"type": "string"},
                    "lower": {"type": "number"},
                    "upper": {"type": "number"}
                }
            }
        }
    ]
}
```

---

### POST /api/ops/{op_key}

Apply a data operation to the current dataset.

**Path Parameters**:

| Parameter | Type | Description |
|-----------|------|-------------|
| `op_key` | string | Operation key (filter, sort, select, rename, groupby, fill_nulls, clip) |

**Request Body**:

```json
{
    "op_key": "filter",
    "params": {
        "column": "amount",
        "operator": "gt",
        "value": 100
    }
}
```

**Response**: `200 OK`

```json
{
    "state_id": "e5f6a7b8",
    "columns_schema": [
        {
            "name": "order_id",
            "dtype": "INTEGER",
            "column_type": "numeric",
            "null_count": 0,
            "unique_count": 5200,
            "min": 1,
            "max": 15000,
            "mean": 7800.3,
            "std": 4200.5,
            "q25": 3900.0,
            "q50": 7800.0,
            "q75": 11700.0,
            "count": 5200
        }
    ],
    "row_count": 5200,
    "sample_rows": [
        {"order_id": 5, "status": "shipped", "amount": 150.00},
        {"order_id": 12, "status": "pending", "amount": 220.50}
    ]
}
```

**Error Responses**:

| Status | Condition |
|--------|-----------|
| 400 | No dataset loaded |
| 404 | Operation key not found |
| 422 | Parameter validation failed |
| 500 | Operation execution failed |

**Operation Parameter Examples**:

Filter:
```json
{"column": "status", "operator": "eq", "value": "shipped"}
{"column": "amount", "operator": "gte", "value": 50}
{"column": "notes", "operator": "contains", "value": "urgent"}
{"column": "email", "operator": "is_null"}
```

Sort:
```json
{"column": "amount", "descending": true}
```

Select:
```json
{"columns": ["order_id", "amount", "status"]}
```

Rename:
```json
{"mapping": {"order_id": "id", "amount": "total"}}
```

GroupBy:
```json
{"group_by": ["status"], "aggregations": {"amount": ["mean", "sum", "count"]}}
```

Fill Nulls:
```json
{"strategy": "forward"}
{"strategy": "zero"}
{"strategy": "constant", "value": "N/A", "columns": ["status"]}
```

Clip:
```json
{"column": "amount", "lower": 0, "upper": 1000}
```

---

### POST /api/history/undo

Undo the last operation, reverting to the previous state.

**Request**: No body.

**Response**: `200 OK`

```json
{
    "state_id": "a3b2c1d4",
    "operation": "upload",
    "columns_schema": [
        {
            "name": "order_id",
            "dtype": "INTEGER",
            "column_type": "numeric",
            "null_count": 0,
            "unique_count": 15000,
            "min": 1,
            "max": 15000,
            "mean": 7500.5,
            "std": 4330.1,
            "q25": 3750.75,
            "q50": 7500.5,
            "q75": 11250.25,
            "count": 15000
        }
    ],
    "row_count": 15000
}
```

**Error Responses**:

| Status | Condition |
|--------|-----------|
| 400 | Cannot undo (at the beginning of history) |

---

### POST /api/history/redo

Redo the next operation, advancing to the subsequent state.

**Request**: No body.

**Response**: `200 OK`

Same schema as undo response.

**Error Responses**:

| Status | Condition |
|--------|-----------|
| 400 | Cannot redo (at the end of history) |

---

### POST /api/history/branch

Branch from a specific historical state, resetting the current pointer to that state.

**Request Body**:

```json
{
    "state_id": "a3b2c1d4"
}
```

Note: The `state_id` is passed as a query parameter in the current implementation.

**Response**: `200 OK`

```json
{
    "state_id": "a3b2c1d4",
    "operation": "upload"
}
```

**Error Responses**:

| Status | Condition |
|--------|-----------|
| 404 | State ID not found in history |

---

### GET /api/history

Retrieve the full history tree for the current session.

**Request**: No parameters.

**Response**: `200 OK`

```json
{
    "nodes": [
        {
            "id": "a3b2c1d4",
            "operation": "upload",
            "params": {"filename": "orders.csv"},
            "parent_id": null,
            "children": ["e5f6a7b8"],
            "is_current": false
        },
        {
            "id": "e5f6a7b8",
            "operation": "filter",
            "params": {"column": "amount", "operator": "gt", "value": 100},
            "parent_id": "a3b2c1d4",
            "children": [],
            "is_current": true
        }
    ],
    "current_id": "e5f6a7b8",
    "order": ["a3b2c1d4", "e5f6a7b8"]
}
```

---

### POST /api/viz/chart

Compute a chart specification for the current dataset.

**Request Body**:

```json
{
    "chart_type": "histogram",
    "params": {
        "column": "amount",
        "bins": 20
    }
}
```

**Chart Type Parameters**:

| Chart Type | Required Params | Optional Params |
|-------------|----------------|-----------------|
| `histogram` | `column` | `bins` (default: 20) |
| `scatter` | `x_col`, `y_col` | `color_col` |
| `bar_topn` | `category_col` | `value_col`, `n` (default: 15), `sort_desc` (default: true) |
| `timeline` | `date_col`, `value_col` | `agg` (default: "sum") |
| `heatmap` | `x_col`, `y_col`, `value_col` | `agg` (default: "mean") |
| `map` | `lat_col`, `lon_col` | `label_col` |

**Response**: `200 OK`

For ECharts-based charts (histogram, scatter, bar_topn, timeline, heatmap):

```json
{
    "chart_type": "histogram",
    "spec": {
        "animation": true,
        "animationDuration": 600,
        "grid": {"left": 60, "right": 30, "top": 60, "bottom": 50, "containLabel": true},
        "title": {"text": "Distribution of amount", "left": "center"},
        "tooltip": {"trigger": "item", "backgroundColor": "rgba(255,255,255,0.95)"},
        "xAxis": {"type": "category", "data": ["0-50", "50-100", "100-150"]},
        "yAxis": {"type": "value", "name": "Count"},
        "series": [{"type": "bar", "data": [3200, 5100, 4500]}]
    }
}
```

For map charts:

```json
{
    "chart_type": "map",
    "spec": {
        "type": "map",
        "points": [{"lat": 40.7, "lon": -74.0, "label": "NYC"}],
        "bounds": {"minLat": 25.0, "maxLat": 48.0, "minLon": -125.0, "maxLon": -67.0},
        "center": {"lat": 36.5, "lon": -96.0},
        "pointCount": 1500
    }
}
```

**Error Responses**:

| Status | Condition |
|--------|-----------|
| 400 | No dataset loaded, or chart requires DuckDB backend |
| 422 | Missing required parameter |
| 500 | Chart computation failed |

---

### GET /api/export/csv

Export the current dataset as a CSV file download.

**Request**: No parameters.

**Response**: `200 OK` with `Content-Type: text/csv`

The response is a file download with the table name as the filename.

**Error Responses**:

| Status | Condition |
|--------|-----------|
| 400 | No dataset loaded |

---

### POST /api/checkpoint

Save a checkpoint of the current session state, including the history tree and data.

**Request**: No body.

**Response**: `200 OK`

```json
{
    "path": "checkpoints/a1b2c3d4e5f6/checkpoint_20240101_120000.json",
    "timestamp": "20240101_120000",
    "state_id": "e5f6a7b8"
}
```

---

### GET /api/checkpoints

List all saved checkpoints for the current session.

**Request**: No parameters.

**Response**: `200 OK`

```json
[
    {
        "path": "checkpoints/a1b2c3d4e5f6/checkpoint_20240101_120000.json",
        "timestamp": "20240101_120000",
        "session_id": "a1b2c3d4e5f6"
    },
    {
        "path": "checkpoints/a1b2c3d4e5f6/checkpoint_20240101_090000.json",
        "timestamp": "20240101_090000",
        "session_id": "a1b2c3d4e5f6"
    }
]
```

---

### GET /health

Health check endpoint for infrastructure monitoring.

**Request**: No parameters.

**Response**: `200 OK`

```json
{
    "status": "ok",
    "version": "0.1.0"
}
```

---

## Status Code Summary

| Code | Meaning | When Used |
|------|---------|-----------|
| 200 | Success | All successful responses |
| 400 | Bad Request | No dataset loaded, invalid state transition |
| 404 | Not Found | Unknown operation key or history state |
| 413 | Payload Too Large | File exceeds MAX_UPLOAD_SIZE |
| 422 | Unprocessable Entity | Parameter validation failure |
| 500 | Internal Server Error | Operation or chart computation failure |