The goal is to document ROS 2's design/architecture 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 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
process->>rclcpp: rclcpp::init()
Note over rclcpp: allocates <div></div> rclcpp::Context object
rclcpp->>rcl: rcl_init()
Note over rcl: validates & processes context object
```
### 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()` the node's `rmw` handle (`rmw_node_t`) to be used later by publishers and subscriptions.
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()`.
Note over rclcpp: allocates rcl_publisher_t handle
rclcpp->>rcl: rcl_publisher_init()
Note over rcl: populates rcl_publisher_t
rcl->>rmw: rmw_create_publisher()
Note over rmw: creates rmw_publisher_t handle
opt is intra process
rclcpp->>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()`.
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()`).
For subscriptions, callbacks are wrapped by an `rclcpp::AnySubscriptionCallback` object, which is registered when creating the `rclcpp::Subscription` object. Subscriptions are handled in the `rclcpp` layer.
In `execute_*subscription()`, the `Executor` allocates a message and calls `rcl_take()`. If that is successful, it then passes that on to the subscription through `rclcpp::SubscriptionBase::handle_message()`. Finally, this calls `dispatch()` on the `rclcpp::AnySubscriptionCallback` object, which calls the actual `std::function` with the right signature.
