# 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()`, a process-specific `rclcpp::Context` object is fetched and CLI arguments are parsed. Much of the work is actually done by `rcl` through a call to `rcl_init()`. This call processes the `rcl_context_t` handle, which is wrapped by the `Context` object. Also, inside this call, `rcl` calls `rmw_init()` to process the `rmw` context (`rmw_context_t`) as well. This `rmw` handle is itself part of the `rcl_context_t` handle.

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 Context
    participant rcl
    participant rmw
    participant tracetools

    Note over rmw: (implementation)

    process->>rclcpp: rclcpp::init(argc, argv)
    Note over rclcpp: fetches process-specific Context object
    rclcpp->>Context: init(argc, argv)
    Note over Context: allocates rcl_context_t handle
    Context->>rcl: rcl_init(out rcl_context_t)
    Note over rcl: validates & processes rcl_context_t handle
    rcl->>rmw: rmw_init(out rmw_context_t)
    Note over rmw: validates & processes rmw_context_t handle

    rcl-->>tracetools: TP(rcl_init, rcl_context_t *)
```

### Node/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(node_name, namespace)
    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(node_name, local_namespace) : 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, depth)

    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*()`, which calls `rmw_take_with_info()`. If that is successful, the `Executor` 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 rmw: (implementation)

    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(...)
        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 rmw: (implementation)

    Note over Component: creates a msg
    Component->>Publisher: publish(msg)
    Note over Publisher: ...
    Publisher->>rcl: rcl_publish(rcl_publisher_t, msg)
    rcl->>rmw: rmw_publish(rmw_publisher_t, msg)
    rmw-->>tracetools: TP(?)
```

### Service creation

Service server creation is similar to subscription creation. The `Component` calls `create_service()` which ends up creating a `rclcpp::Service`. In its constructor, it allocates a `rcl_service_t` handle, then calls `rcl_service_init()`. This processes the handle and validates the service name. It calls `rmw_create_service()` to get the corresponding `rmw_service_t` handle.

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

    Note over rmw: (implementation)

    Component->>rclcpp: create_service(service_name, callback)
    Note over rclcpp: (...)
    rclcpp->>Service: Service(rcl_node_t, service_name, callback, options)
    Note over Service: allocates a rcl_service_t handle
    Service->>rcl: rcl_service_init(out rcl_service_t, rcl_node_t, service_name, options)
    Note over rcl: validates & processes service handle
    rcl->>rmw: rmw_create_service(rmw_node_t, service_name, qos_options) : rmw_service_t
    Note over rmw: creates rmw_service_t handle
    rcl-->>tracetools: TP(rcl_service_init, rcl_node_t *, rmw_node_t *, rcl_service_t *, service_name)
    Service-->>tracetools: TP(rclcpp_service_callback_added, rcl_service_t *, &any_callback)
```

### Service callbacks

Service callbacks are similar to subscription callbacks. In `execute_service()`, the `Executor` allocates request header and request objects. It then calls `rcl_take_request()` and passes them along with the service handle.

`rcl` calls `rmw_take_request()`. If those are successful, then the `Executor` calls `handle_request()` on the `Service`. This casts the request to its actual type, allocates a response object, and calls `dispatch()` on its `AnyServiceCallback` object, which calls the actual `std::function` with the right signature.

For the service response, `Service` calls `rcl_send_response()` which calls `rmw_send_response()`.

```mermaid
sequenceDiagram
    participant Executor
    participant Service
    participant AnyServiceCallback
    participant rcl
    participant rmw
    participant tracetools

    Note over rmw: (implementation)

    Note over Executor: execute_service()
    Note over Executor: allocates request header and request
    Executor->>rcl: rcl_take_request(rcl_service, out request_header, out request) : ret
    rcl->>rmw: rmw_take_request(rmw_service_t, out request_header, out request, out taken)
    opt RCL_RET_OK == ret
        Executor->>Service: handle_request(request_header, request)
        Note over Service: casts request to its actual type
        Note over Service: allocates a response object
        Service->>AnyServiceCallback: dispatch(request_header, typed_request, response)
        AnyServiceCallback-->>tracetools: TP(rclcpp_service_callback_start, this)
        Note over AnyServiceCallback: std::function(...)
        AnyServiceCallback-->>tracetools: TP(rclcpp_service_callback_end, this)
        Service->>rcl: rcl_send_response(rcl_service_t, request_header, response)
        rcl->>rmw: rmw_send_response(rmw_service_t, request_header, response)
    end
```

### Client creation

Client creation is similar to publisher creation. The `Component` calls `create_client()` which ends up creating a `rclcpp::Client`. In its constructor, it allocates a `rcl_client_t` handle, then calls `rcl_client_init()`. This validates and processes the handle. It also calls `rmw_create_client()` which creates the `rmw_client_t` handle.

```mermaid
sequenceDiagram
    participant Component
    participant Node
    participant Client
    participant rcl
    participant rmw
    participant tracetools

    Component->>Node: create_client(service_name, options)
    Node->>Client: Client(service_name, options)
    Note over Client: allocates a rcl_client_t handle
    Client->>rcl: rcl_client_init(out rcl_client_t, rcl_node_t, service_name, options)
    Note over rcl: validates and processes rcl_client_t handle
    rcl->>rmw: rmw_create_client(rmw_node_t, service_name, qos_options) : rmw_client_t
    Note over rmw: creates rmw_client_t handle
    rcl-->>tracetools: TP(rcl_client_init, rcl_node_t *, rcl_client_t *, rmw_client_t *, service_name)
```

### Client request

### Timer creation

Timer creation is similar to subscription creation. The `Component` calls `create_service()` which ends up creating a `rclcpp::WallTimer`. In its constructor, it creates a `rclcpp::Clock` object, which (for a `WallTimer`) is simply a nanosecond clock. It then allocates a `rcl_timer_t` handle, then calls `rcl_timer_init()`. This processes the handle and validates the period.

Note that `rcl_timer_init()` can take a callback as a parameter, but right now that feature is not used anywhere (`nullptr` is given), and callbacks are instead handled in the `rclcpp` layer.

```mermaid
sequenceDiagram
    participant Component
    participant Node
    participant WallTimer
    participant rcl
    participant tracetools

    Component->>Node: create_wall_timer(period, callback)
    Node->>WallTimer: WallTimer(period, callback, Context)
    Note over WallTimer: creates a Clock object
    Note over WallTimer: allocates a rcl_timer_t handle
    WallTimer->>rcl: rcl_timer_init(out rcl_timer_t, Clock, rcl_context_t, period)
    Note over rcl: validates and processes rcl_timer_t handle
    rcl-->>tracetools: TP(rcl_timer_init, rcl_timer_t *, period)
    WallTimer-->>tracetools: TP(rclcpp_timer_callback_added, rcl_timer_t *, &callback)
```

### Timer callbacks

Timer callbacks are similar to susbcription callbacks. In `execute_timer()`, the `Executor` calls `execute_callback()` on the `WallTimer`. The timer then calls `rcl_timer_call()` with its `rcl_timer_t` handle and checks if the callback should be called.

If it that is the case, then the timer will call the actual `std::function`. Depending on the `std::function` that was given when creating the timer, it will either call the callback without any parameters or it will pass a reference of itself.

```mermaid
sequenceDiagram
    participant Executor
    participant WallTimer
    participant rcl
    participant tracetools

    Note over Executor: execute_timer()
    Executor->>WallTimer: execute_callback()
    WallTimer->>rcl: rcl_timer_call(rcl_timer_t) : ret
    Note over rcl: validates and updates timer
    opt RCL_RET_TIMER_CANCELED != ret && RCL_RET_OK == ret
        WallTimer-->>tracetools: TP(rclcpp_timer_callback_start, this)
        Note over WallTimer: std::function(...)
        WallTimer-->>tracetools: TP(rclcpp_timer_callback_end, this)
    end
```