Updated intro text on Client Library.

Signed-off-by: Ralph Lange <ralph.lange@de.bosch.com>

Author: ralph-lange

_docs/concepts/client_library/introduction/index.md

@@ -6,26 +6,21 @@ redirect_from:
   - /docs/concepts/client_library/
 ---
 
-The client library provides the micro-ROS API for the user code, i.e. for application-level micro-ROS nodes. The overall goal is to provide all relevant, major ROS 2 concepts in suitable implementation for microcontrollers. Where possible, API compatibility with ROS 2 shall be achieved for ease of portability.
+The Client Library provides the micro-ROS API for the user code, i.e., for application-level micro-ROS nodes. The goal is to provide all relevant, major ROS 2 concepts in an optimized implementation for microcontrollers. At the same time, we strive to make the API as compatible as possible to standard ROS 2, to facilitate porting of code.
 
-In this undertaking, to minimize the long-term maintenance cost, we strive to use existing data structures and algorithms from the ROS 2 stack or to bring necessary changes in the mainline stack. This raises a lot of question regarding the applicability of existing ROS 2 layers on microcontrollers in terms of runtime efficiency, portability to different RTOS, dynamic memory management, etc.
+To minimize the long-term maintenance cost, we use existing data structures and algorithms from the ROS 2 stack and to bring necessary changes in the mainline stack as far as possible. That's why the micro-ROS client library is built up from standard [ROS 2 Client Support Library (rcl)](https://github.com/ros2/rcl/) and a new [ROS 2 Client Library package (rclc)](https://github.com/ros2/rclc/). Together, as depicted below, rcl + rclc form a feature-complete client library in the C programming language.
 
-## Two-layered API
+<img src="/img/micro-ROS_architecture.png" style="display:block; width:50%; float:right;"/>
 
-While C is still the dominating programming language for microcontrollers, there is a clear trend towards the use of higher-level languages, in particular C++. This trend is also driven by modern microcontrollers featuring several hundred kilobytes or even few megabytes of RAM.
+Important features and properties:
 
-Therefore, we plan to offer/support two APIs:
+* Use of rcl data structures where possible to avoid runtime overhead by wrappers.
+* Convenience functions for common tasks (e.g., creation of a publisher, finalization of a subscription) provided by rclc.
+* Dedicated Executor for fine-grained control over triggering and processing order of callbacks.
+* Specialized implementations for graphs, lifecycle nodes, diagnostics, etc.
 
-1. A **C API** based on the [ROS 2 Support Client Library (rcl)](https://github.com/ros2/rcl/), enriched with modular packages for execution management, diagnostics, parameters, ...
-2. A **C++ API** based on the [ROS 2 rclcpp](https://github.com/ros2/rclcpp/), which at first requires analyzing the fitness of rclcpp for use on microcontrollers, in particular regarding memory and CPU consumption as well as dynamic memory management.
+Check out our subpages for more information.
 
-<img src="micro-ROS_stack.png" style="display:block; margin-left: auto; margin-right: auto;"/>
+<br style="clear:both;" />
 
-The basis of discussion for this decision is documented in a dedicated [decision paper](decision_paper/).
-
-## Advanced Concepts
-
-Advanced concepts developed in the context of the client library are documented separately. In general, these concepts are developed for the standard rclcpp first, before implementing a tailored C version. These are:
-
-* [Real-Time Executor](real-time_executor/)
-* [System Modes](system_modes/)
+For the interested reader: The rationales for the decision to use a combination of rcl + rclc are explained in our [decision paper](decision_paper/) from 2019.