# 19 · QoS (服务质量) 完全指南

> **目标**: 理解 ROS2 QoS 的 5 个维度、兼容规则、9 种常用组合、能独立为节点选 QoS。

---

## 目录

- [1. 是什么](#1-是什么)
- [2. 5 大维度](#2-5-大维度)
- [3. 兼容规则](#3-兼容规则)
- [4. 9 种常用组合](#4-9-种常用组合)
- [5. Python API](#5-python-api)
- [6. C++ API](#6-c-api)
- [7. rclpy QoSProfile 详解](#7-rclpy-qosprofile-详解)
- [8. 调试 QoS 不兼容](#8-调试-qos-不兼容)
- [9. 推荐阅读](#9-推荐阅读)

---

## 1. 是什么

**QoS (Quality of Service)** 控制 DDS 通信的可靠性、实时性、持久性等。

ROS2 默认 QoS = `RELIABLE + VOLATILE + KEEP_LAST(10)`,适合大多数场景。
但**高频传感器流** / **控制指令** / **参数发布**等场景需要**自定义 QoS**。

## 2. 5 大维度

| 维度 | 取值 | 默认 | 含义 |
|---|---|---|---|
| **Reliability** | RELIABLE / BEST_EFFORT | RELIABLE | 必须投递 / 丢一帧无所谓 |
| **History** | KEEP_LAST(N) / KEEP_ALL | KEEP_LAST(10) | 队列策略 |
| **Durability** | VOLATILE / TRANSIENT_LOCAL | VOLATILE | 晚订阅者是否收到旧数据 |
| **Deadline** | Duration | ∞ | 最长多久发一次 |
| **Lifespan** | Duration | ∞ | 多旧的消息失效 |

## 3. 兼容规则

**关键规则** — RELIABLE ↔ BEST_EFFORT 单向兼容:

| Publisher ↓ \ Subscriber → | RELIABLE | BEST_EFFORT |
|---|---|---|
| RELIABLE | ✅ | ❌ |
| BEST_EFFORT | ✅ | ✅ |

**为什么 RELIABLE → BEST_EFFORT 不兼容**:
- RELIABLE pub 等 ACK(确保投递)
- BEST_EFFORT sub 不发 ACK
- pub 等不到 ACK → 报 "QoS incompatible"

**报错样例**:
```
[WARN] ... New subscription discovered on this topic with incompatible QoS ...
```

## 4. 9 种常用组合

| Reliability | Durability | History | 适用场景 |
|---|---|---|---|
| RELIABLE | VOLATILE | KEEP_LAST(10) | **默认 / 跨语言互通基线** |
| RELIABLE | VOLATILE | KEEP_LAST(1) | 控制指令(只关心最新) |
| RELIABLE | TRANSIENT_LOCAL | KEEP_LAST(1) | 参数 / 配置(晚订阅者也能拿到) |
| RELIABLE | VOLATILE | KEEP_ALL | 日志(必须投递,不丢) |
| RELIABLE | TRANSIENT_LOCAL | KEEP_ALL | 启动期参数 |
| BEST_EFFORT | VOLATILE | KEEP_LAST(1) | 视频流(丢一帧无所谓) |
| BEST_EFFORT | VOLATILE | KEEP_LAST(10) | Lidar / 雷达 |
| BEST_EFFORT | TRANSIENT_LOCAL | KEEP_LAST(1) | 较老的状态快照 |
| SYSTEM_DEFAULT | VOLATILE | KEEP_LAST(10) | 由 RMW 决定 |

## 5. Python API

```python
from rclpy.qos import (
    QoSProfile,
    ReliabilityPolicy,
    HistoryPolicy,
    DurabilityPolicy,
)

# 视频流:BEST_EFFORT + 深度 1
sensor_qos = QoSProfile(
    reliability=ReliabilityPolicy.BEST_EFFORT,
    history=HistoryPolicy.KEEP_LAST,
    depth=1,
)

# 控制指令:RELIABLE + 深度 1
control_qos = QoSProfile(
    reliability=ReliabilityPolicy.RELIABLE,
    history=HistoryPolicy.KEEP_LAST,
    depth=1,
)

# 参数:TRANSIENT_LOCAL(晚订阅者能拿历史)
param_qos = QoSProfile(
    reliability=ReliabilityPolicy.RELIABLE,
    durability=DurabilityPolicy.TRANSIENT_LOCAL,
    depth=1,
)

# 用
publisher_ = self.create_publisher(String, 'topic', sensor_qos)
```

## 6. C++ API

```cpp
#include "rclcpp/qos.hpp"

using namespace rclcpp;

// 视频流
auto sensor_qos = QoS(1)
  .reliability(RMW_QOS_POLICY_RELIABILITY_BEST_EFFORT)
  .history(RMW_QOS_POLICY_HISTORY_KEEP_LAST);

// 参数
auto param_qos = QoS(1)
  .reliability(RMW_QOS_POLICY_RELIABILITY_RELIABLE)
  .durability(RMW_QOS_POLICY_DURABILITY_TRANSIENT_LOCAL);

// 用
publisher_ = this->create_publisher<String>("topic", sensor_qos);
```

## 7. rclpy QoSProfile 详解

完整参数:

```python
QoSProfile(
    # Reliability
    reliability=ReliabilityPolicy.RELIABLE,  # or BEST_EFFORT, SYSTEM_DEFAULT
    
    # History
    history=HistoryPolicy.KEEP_LAST,  # or KEEP_ALL
    depth=10,  # 仅 KEEP_LAST 时有效
    
    # Durability
    durability=DurabilityPolicy.VOLATILE,  # or TRANSIENT_LOCAL
    
    # Deadline(可省)
    deadline=Duration(seconds=0),  # 0 = 无
    
    # Lifespan(可省)
    lifespan=Duration(seconds=0),  # 0 = 无
    
    # Liveliness(可省)
    liveliness=LivelinessPolicy.SYSTEM_DEFAULT,
    liveliness_lease_duration=Duration(seconds=0),
)
```

## 8. 调试 QoS 不兼容

**症状**: `ros2 topic echo` 没输出,`ros2 topic info -v` 显示 QoS incompatible 警告。

**调试步骤**:

```bash
# 1) 看 pub/sub 各自 QoS
ros2 topic info /topic -v

# 2) 检查 pub/sub 是否在同 domain
echo $ROS_DOMAIN_ID

# 3) 看具体 incompatibility
# 错误信息: PolicyKind=RMW_QOS_POLICY_RELIABILITY
```

**修法**: 改一边 QoS 匹配,或用 SYSTEM_DEFAULT。

## 9. 推荐阅读

- [ROS2 QoS 设计稿](https://design.ros2.org/articles/qos.html)
- [ROS2 Humble QoS 文档](https://docs.ros.org/en/humble/Concepts/About-Quality-of-Service.html)
- [OMG DDS 规范 v1.4](https://www.omg.org/spec/DDS/1.4/) — QoS 源头
- [FastDDS QoS 配置](https://fast-dds.docs.eprosima.com/)
- [`cpp_qos_demo` 包](../src/cpp_qos_demo/README.md)

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