# ROS 2 design notes for instrumentation

The goal is to document ROS 2's design/architecture through descriptions of the main execution flows in order to properly design the instrumentation for it.

## Notes on client libraries

ROS 2 has changed the way it deals with client libraries. It offers a base ROS client library (`rcl`) written in C. This client library is the base for any language-specific implementation, such as `rclcpp` and `rclpy`.

However, `rcl` is obviously fairly basic, and still does leave a fair amount of implementation work up to the client libraries. For example, callbacks are not at all handled in `rcl`, and are left to the client library implementations.

This means that some instrumentation work might have to be re-done for every client library that we want to trace. We cannot simply instrument `rcl`, nor can we only instrument the base `rmw` interface if we want to dig into that.

This document will (for now) mainly discuss `rcl` and `rclcpp`, but `rclpy` should eventually be added and supported.

## Flow description

### Process creation

In the call to `rclcpp::init(argc, argv)`, an `rclcpp::Context` object is created and CLI arguments are parsed. Much of the work is actually done by `rcl` through a call to `rcl_init()`.

This has to be done once per process, and usually at the very beginning. The components that are then instanciated share this context.

```mermaid
sequenceDiagram
    participant process
    participant rclcpp
    participant rcl
    participant tracetools

    process->>rclcpp: rclcpp::init()
    Note over rclcpp: allocates <div></div> rclcpp::Context object
    rclcpp->>rcl: rcl_init(out context)
    Note over rcl: validates & processes context object

    rcl->>tracetools: TP(rcl_init, &context)
```

### Note/component creation

In ROS 2, a process can contain multiple nodes. These are sometimes referred to as "components."

These components are instanciated by the containing process. They are usually classes that extend `rclcpp::Node`, so that the node initialization work is done by the parent constructor.

This parent constructor will allocate its own `rcl_node_t` handle and call `rcl_node_init()`, which will validate the node name/namespace. `rcl` will also call `rmw_create_node()` to get the node's `rmw` handle (`rmw_node_t`). This will be used later by publishers and subscriptions.

```mermaid
sequenceDiagram
    participant process
    participant Component
    participant rclcpp
    participant rcl
    participant rmw
    participant tracetools

    Note over rmw: (implementation)

    process->>Component: Component()
    Component->>rclcpp: : Node()
    Note over rclcpp: allocates rcl_node_t handle
    rclcpp->>rcl: rcl_node_init(out rcl_node_t, node_name, namespace)
    Note over rcl: validates node name/namespace
    Note over rcl: populates rcl_note_t
    rcl->>rmw: rmw_create_node() : rmw_node_t
    Note over rmw: creates rmw_node_t handle

    rcl->>tracetools: TP(rcl_node_init, rcl_node_t *, rmw_node_t *, node_name, namespace)
```

### Publisher creation

The component calls `create_publisher()`, a `rclcpp::Node` method for convenience. That ends up creating an `rclcpp::Publisher` object which extends `rclcpp::PublisherBase`. The latter allocates an `rcl_publisher_t` handle, fetches the corresponding `rcl_node_t` handle, and calls `rcl_publisher_init()` in its constructor. `rcl` does topic name expansion/remapping/validation. It creates an `rmw_publisher_t` handle by calling `rmw_create_publisher()` of the given `rmw` implementation and associates with the node's `rmw_node_t` handle and the publisher's `rcl_publisher_t` handle.

If intra-process publishing/subscription is enabled, it will be set up after creating the publisher object, through a call to `PublisherBase::setup_intra_process()`, which calls `rcl_publisher_init()`.

```mermaid
sequenceDiagram
    participant Component
    participant rclcpp
    participant Publisher
    participant rcl
    participant rmw
    participant tracetools

    Note over rmw: (implementation)

    Component->>rclcpp: create_publisher(topic_name, options, use_intra_process)
    Note over rclcpp: (...)
    rclcpp->>Publisher: Publisher(topic_name, options)
    Note over Publisher: allocates rcl_publisher_t handle
    Publisher->>rcl: rcl_publisher_init(out rcl_publisher_t, rcl_node_t, topic_name, options)
    Note over rcl: populates rcl_publisher_t
    rcl->>rmw: rmw_create_publisher(rmw_node_t, topic_name, qos_options) : rmw_publisher_t
    Note over rmw: creates rmw_publisher_t handle

    rcl->>tracetools: TP(rcl_publisher_init, rcl_node_t *, rmw_node_t *, rcl_publisher_t *, topic_name, depth)

    opt use_intra_process
        rclcpp->>Publisher: setup_intra_process()
        Publisher->>rcl: rcl_publisher_init(...)
    end
```

### Subscription creation

Subscription creation is done in a very similar manner.

The componenent calls `create_publisher()`, which ends up creating an `rclcpp::Subscription` object which extends `rclcpp::SubscriptionBase`. The latter allocates an `rcl_subscription_t` handle, fetches its `rcl_node_t` handle, and calls `rcl_subscription_init()` in its constructor. `rcl` does topic name expansion/remapping/validation. It creates an `rmw_subscription_t` handle by calling `rmw_create_subscription()` of the given `rmw` implementation and associates it with the node's `rmw_node_t` handle and the subscription's `rcl_subscription_t` handle.

If intra-process publishing/subscription is enabled, it will be set up after creating the subscription object, through a call to `Subscription::setup_intra_process()`, which calls `rcl_subscription_init()`. This is very similar to a normal (inter-process) subscription, but it sets some flags for later.

```mermaid
sequenceDiagram
    participant Component
    participant rclcpp
    participant Subscription
    participant rcl
    participant rmw
    participant tracetools

    Note over rmw: (implementation)

    Component->>rclcpp: create_subscription(topic_name, callback, options, use_intra_process)
    Note over rclcpp: (...)
    rclcpp->>Subscription: Subscription(topic_name, callback, options)
    Note over Subscription: allocates rcl_subscription_t handle
    Subscription->>rcl: rcl_subscription_init(out rcl_subscription_t, rcl_node_t, topic_name, options)
    Note over rcl: populates rcl_subscription_t
    rcl->>rmw: rmw_create_subscription(rmw_node_t, topic_name, qos_options) : rmw_subscription_t
    Note over rmw: creates rmw_subscription_t handle

    rcl->>tracetools: TP(rcl_subscription_init, rcl_node_t *, rmw_node_t *, rcl_subscription_t *, topic_name)

    opt use_intra_process
        rclcpp->>Subscription: setup_intra_process()
        Subscription->>rcl: rcl_subscription_init(...)
    end

    rclcpp->>tracetools: TP(rclcpp_subscription_callback_added, rcl_subscription_t *, &any_callback)
```

### Executors

An `rclcpp::executor::Executor` object is created for a given process. It can be a `SingleThreadedExecutor` or a `MultiThreadedExecutor`.

Components are instanciated, usually as a `shared_ptr` through `std::make_shared<Component>()`, then added to the executor with `Executor::add_node()`.

After all the components have been added, `Executor::spin()` is called. `SingleThreadedExecutor::spin()` simply loops forever until the process' context isn't valid anymore. It fetches the next `rclcpp::AnyExecutable` (e.g. subscription, timer, service, client), and calls `Executor::execute_any_executable()` with it. This then calls the relevant `execute*()` method (e.g. `execute_timer()`, `execute_subscription()`, `execute_intra_process_subscription()`, `execute_service()`, `execute_client()`).

```mermaid
sequenceDiagram
    participant process
    participant Executor
    participant tracetools

    process->>Executor: Executor()
    Note over process: instanciates components
    process->>Executor: add_node(component)
    process->>Executor: spin()
    loop until shutdown
        Executor->>tracetools: TP(?)

        Note over Executor: get_next_executable()
        Note over Executor: execute_any_executable()
        Note over Executor: execute_*()
    end
```

### Subscription callbacks

Subscriptions are handled in the `rclcpp` layer. Callbacks are wrapped by an `rclcpp::AnySubscriptionCallback` object, which is registered when creating the `rclcpp::Subscription` object.

In `execute_*subscription()`, the `Executor` asks the `Subscription` to allocate a message though `Subscription::create_message()`. It then calls `rcl_take*()`. If that is successful, it then passes that on to the subscription through `rclcpp::SubscriptionBase::handle_message()`. This checks if it's the right type of subscription (i.e. inter vs. intra process), then it calls `dispatch()` on the `rclcpp::AnySubscriptionCallback` object with the message (cast to the actual type). This calls the actual `std::function` with the right signature.

Finally, it returns the message object through `Subscription::return_message()`.

```mermaid
sequenceDiagram
    participant Executor
    participant Subscription
    participant AnySubscriptionCallback
    participant rcl
    participant rmw
    participant tracetools

    Note over Executor: execute_subscription()
    Executor->>Subscription: create_message(): std::shared_ptr<void>
    Executor->>rcl: rcl_take*(rcl_subscription_t, out msg) : ret
    rcl->>rmw: rmw_take_with_info(rmw_subscription_t, out msg, out taken)
    Note over rmw: copies available message to msg if there is one
    opt RCL_RET_OK == ret
        Executor->>Subscription: handle_message(msg)
        Note over Subscription: casts msg to its actual type
        Subscription->>AnySubscriptionCallback: dispatch(typed_msg)
        AnySubscriptionCallback->>tracetools: TP(rclcpp_subscription_callback_start, this, is_intra_process)
        Note over AnySubscriptionCallback: std::function::operator(...)
        AnySubscriptionCallback->>tracetools: TP(rclcpp_subscription_callback_end, this)
    end
    Executor->>Subscription: return_message(msg)
```

### Message publishing

To publish a message, an object is first allocated and then populated by the `Component` (or equivalent). Then, the message is sent to the `Publisher` through `publish()`. This then passes that on to `rcl`, which itself passes it to `rmw`.

TODO add inter- vs. intra-process execution flow
TODO talk about IntraProcessManager stuff?

```mermaid
sequenceDiagram
    participant Component
    participant Publisher
    participant rcl
    participant rmw
    participant tracetools

    Note over Component: creates a msg
    Component->>Publisher: publish(msg)
    Note over Publisher: ...
    Publisher->>rcl: rcl_publish(rcl_publisher_t, msg)
    rcl->>rmw: rmw_publisher(rmw_publisher, msg)
```

### Service creation

### Timer creation

### Service callbacks

### Timer callbacks