Template

layout: true
<div class="header"><img src="https://rosin-project.eu/wp-content/uploads/rosin_ack_logo_wide.png" style="background-color:transparent"/></div>
<div class="footer"><img src="https://www.ipa.fraunhofer.de/content/dam/ipa/ipa.svg" /><p>© Fraunhofer IPA</p></div>
<div class="triangle"></div>

---
class: center, middle

<h1>
  <center>
    ROS 2 Foundation
  </center>
</h1>
<h3>
  <center>
    An introduction to core concepts
  </center>
</h3>

---

## Overview
- #### ROS computation graph
- #### What is a node?
- #### Node composition
- #### ROS 2 topics and messages
- #### ROS 2 Executor
- #### ROS 2 Services
- #### ROS 2 Actions
- #### ROS 2 Params
- #### ROS 2 Launch files

---

## Computation graph
### Displays the peer to peer network of ROS processes that process data together

.cols[
  .fifty[
  <img src="../static/computation_graph.png" width="500px">
  ]

.fifty[

- Reflects the communication in ROS system

- Participants of the graph are called *Nodes*

- Nodes can typically
  - publish (provide) data OR
  - subscribe (read) data
    ]
]

---

## What is a Node?
- An element that serves a single, modular purpose in a robotics system
  - Camera driver
  - Controlling wheel motors
  - Perception unit

- Used to create publishers, subscribers, services, etc

- Most common ROS client libraries
  - `rclcpp`: Binaries of C++ files
  - `rclpy`:  Python scripts

- Command to run a ROS node
```bash
  ros2 run <package_name> <executable_name> [arguments]
```

- No master required to discover other nodes!

---

## Automatic discovery
### ROS 1 Node communication
<div>
  <center>
    <img src="../static/singlepointfailure.png" width="800px">
  </center>
</div>

---

## ROS 1 vs ROS 2
### Node structure

---

## Automatic discovery
### ROS 2 Node communication

.cols[
  .fifty[ <img src="../static/ros2_discovery.png" width="500px">  ]

.fifty[ <img src="../static/ros2_discovery_2.png" width="500px"> ]
]
<h4>
  <center>
    No central point of failure!
  </center>
</h4>

---

## General ROS Structure
<div>
  <center>
    <img src="../static/ros2_structure.png" width="800px">
  </center>
</div>

---

## ROS 2-Unified API
### ROS 2 Client libraries

.cols[
.fifty[<div>
  <center>
    <img src="../static/unified_api.png" width="400px">
  </center>
</div>]
.fifty[
- APIs are built on top of a common core client library
- Ensures that any changes made to functionalities are reflected in all derived client libraries

]
]

---

## Node Composition
### Unified API

- Aggregation of nodes in a single process

- Simplifies code reuse

#### Rationale
Defers process layout decisions to deploy-time
.cols[
.fifty[
- Nodes in different processes
  - Allow for fault isolation
  - Are easier to debug
]
.fifty[
- Nodes in the same process
  - Low overhead
  - More efficient communication
]
]

---

## Node Composition
### Writing Node as a component
- Inherits from the Node class
  - Allows to be imported and used by other scripts
  - Able to create publishers, subscribers, servers, and clients within the node

.cols[
.fifty[
- Subclass of `rclpy.Node`

<center>
  <img src="../static/python_class.png" height="100px" width="500px" >
</center>
]
.fifty[
- Subclass of `rclcpp.Node`

<center>
  <img src="../static/cpp_class.png" height="80px" width="300px">
</center>
]
]
---

## Node Composition
### Deploying a component

- Use ROS 2 Service APIs:  Load components via
  - service calls
  - command line tools
  - launch

- Alternatively:
  - Manual composition
  - Run standalone executables

---

## Communications Infrastructure
### The ROS middleware provides these facilities:
- Anonymous publish/subscribe message passing
- Recording and playback of messages
- Request/response remote procedure calls
- Pre-emptable request/response remote procedure calls
- Distributed parameter system
---

## ROS 2 Topics
- Asynchronous communication
- Multiple concurrent publishers and subscribers for one topic
- A single node can publish and subscribe to multiple topics
- Works like a “chat room”
  - Not to a specific recipient
  - E.g. laser scan data, images, robot position, ...
- Every topic has a dedicated message type

### Communication is possible ONLY when
  - Receiving messages with matching message type
  - Compatible Quality of Service settings

---

## ROS 2 Messages

- data structure to exchange information between components
- `*.msg` are simple text files with field type and name
- defined in programming language agnostic way

---

## ROS Messages
### Supported field types
- 32 Standard messages supported

<center>
  <table style="border-collapse:collapse;border-spacing:0" class="tg"><thead><tr><th style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;font-weight:normal;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal"><span style="font-weight:normal">Bool</span></th><th style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;font-weight:normal;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal"><span style="font-weight:normal">ColorRGBA</span></th><th style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;font-weight:normal;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal"><span style="font-weight:normal">Float32MultiArray</span></th><th style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;font-weight:normal;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal"><span style="font-weight:normal">Int16</span></th><th style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;font-weight:normal;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal"><span style="font-weight:normal">Int64</span></th><th style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;font-weight:normal;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal"><span style="font-weight:normal">MultiArrayDimension</span></th><th style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;font-weight:normal;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal"><span style="font-weight:normal">UInt16</span></th><th style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;font-weight:normal;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal"><span style="font-weight:normal">UInt64</span></th></tr></thead><tbody><tr><td style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal">Byte</td><td style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal">Duration</td><td style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal">Float64</td><td style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal">Int16MultiArray</td><td style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal">Int64MultiArray</td><td style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal">MultiArrayLayout</td><td style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal">UInt16MultiArray</td><td style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal">UInt64MultiArray</td></tr><tr><td style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal">ByteMultiArray</td><td style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal">Empty</td><td style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal">Float64MultiArray</td><td style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal">Int32</td><td style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal">Int8</td><td style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal">String</td><td style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal">UInt32</td><td style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal">UInt8</td></tr><tr><td style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal">Char</td><td style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal">Float32</td><td style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal">Header</td><td style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal">Int32MultiArray</td><td style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal">Int8MultiArray</td><td style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal">Time</td><td style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal">UInt32MultiArray</td><td style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal">UInt8MultiArray</td></tr></tbody></table>
</center>

- Constructed from primitive data types or other ROS msgs
<center><table style="border-collapse:collapse;border-spacing:0" class="tg"><thead><tr><th style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;font-weight:bold;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal"><span style="font-weight:bold">geometry_msgs/Point.msg</span></th><th style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;font-weight:bold;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal"><span style="font-weight:bold">geometry_msgs/Quaternion.msg</span></th><th style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;font-weight:bold;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal"><span style="font-weight:bold">geometry_msgs/Pose.msg</span></th></tr></thead><tbody><tr><td style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal">float64 x<br>float64 y<br>float64 z <br></td><td style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal">float64 x<br>float64 y<br>float64 z <br>float64 w</td><td style="border-color:inherit;border-style:solid;border-width:1px;font-family:Arial, Helvetica, sans-serif !important;;font-size:12px;overflow:hidden;padding:10px 5px;text-align:left;vertical-align:top;word-break:normal">geometry_msgs/Point  position<br>geometry_msgs/Quaternion orientation</td></tr></tbody></table></center>

---

## ROS 2 Executor

### Coordinates order and timing to execute available communication tasks

.cols[
.seventy[
- Provides spin functions (spin(), spin_once()…) for nodes that invokes callbacks

- Coordinates nodes and callback groups by looking for available work, and dispatch it to one or more threads
  - subscription callback
  - timer callback
  - Service callback
  - Client responses

- Coordination based on threading/concurrency scheme in implementation
]
.thirty[
<img src="../static/exec_spin.png" width="400px" >
]
]

---
## ROS 2 Executor

**Default executor**
<pre><code class="language-ruby">
  rclpy.init(args=args)
  talker = Talker()
  rclpy.spin(talker)
</code></pre>

**Built-in executors**
- `SingleThreadedExecutor - Runs callbacks in the thread that calls executor.spin()`
- `MultiThreadedExecutor - Runs callbacks in a pool of threads`

<pre><code class="language-ruby">
  executor = SingleThreadedExecutor()
  talker = Talker()
  listener = Listener()

executor.add_node(talker)
  executor.add_node(listener)
  executor.spin()

</code></pre>

---

## ROS 2 Services
-   Services are based on a call and response model
-   Similar to remote procedural calls that terminates quickly
    -   Example: spawning a robot into an environment

---

## Service file structure

-   Data structure for services
-   Defined by a pair of messages: Request and Reply.
-   \*.srv is a file with field type and name
-   Request and Response are separated by `---`
-   Use `ros2 interface show <type_name>.srv` to know the service file structure

<pre><code class="language-ruby">
  #request
  int8 a
  int8 b
  ---
  #response
  int8 sum
</code></pre>

---

## Service Server and Clients

- There can be only one service server for a service.
- Multiple server clients can use the same service server.
- Server decides how goals from multiple clients will be handled simultaneously.

---

## ROS 2 Actions

-   Used for tasks that
    - <b>Longer execution time</b>
    - <b>Requires preemption</b>
    - <b>Requires regular feedback</b>
        -   Example. Path planning in navigation
-   Actions are implemented as a combination of services and topics
-   Action interface has
    -   <b>Goal</b>: What action to achieve and how to.
    -   <b>Feedback</b>: Progress towards completing the action
    -   <b>Preempt</b>: To preempt the action that is being executed
    -   <b>Result</b>: Outcome of an action
<div>
  <center>
    <img src="../static/action.png" width="350px">
  </center>
</div>

---

## Actions Client and Server
-   Action clients send goal and result requests to action servers
-   Action servers execute goal and provide feedback and results to action clients
-   Feedback uses topics whereas Goal and Result uses services.
<div>
  <center>
    <img src="../static/action_detailed.png" width="900px">
  </center>
</div>

---

## ROS 2 Actions
### Data structure for Actions

-   Defined by a set of messages: Goal, Result and Feedback.
-   ``\*.action`  is a file with type and name with fields separated by `---`

<pre><code class="language-ruby">
  # Define a goal of washing all dishes
  bool heavy_duty  # Spend extra time cleaning
  ---
  # Define the result that will be published after the action execution ends.
  uint32 total_dishes_cleaned
  ---
  # Define a feedback message that will be published during action execution.
  float32 percent_complete
  uint32 number_dishes_cleaned

</code></pre>

---

class: center
## Actions vs Services
<style type="text/css">
.tg  {border-collapse:collapse;border-color:#aabcfe;border-spacing:0;margin:0px auto;}
.tg td{background-color:#e8edff;border-color:#aabcfe;border-style:solid;border-width:1px;color:#669;
  ;font-size:14px;overflow:hidden;padding:10px 5px;word-break:normal;}
.tg th{background-color:#b9c9fe;border-color:#aabcfe;border-style:solid;border-width:1px;color:#039;
  ;font-size:14px;font-weight:normal;overflow:hidden;padding:10px 5px;word-break:normal;}
.tg .tg-z9lx{background-color:#cbcefb;border-color:inherit;color:#010066;font-size:16px;text-align:left;vertical-align:top}
.tg .tg-2m3o{background-color:#9698ed;border-color:inherit;color:#010066;font-size:16px;font-weight:bold;text-align:center;
  vertical-align:top}
</style>
<table class="tg">
<thead>
  <tr>
    <th class="tg-2m3o">Actions</th>
    <th class="tg-2m3o">Services</th>
  </tr>
</thead>
<tbody>
  <tr>
    <td class="tg-z9lx">Intended for longer running tasks</td>
    <td class="tg-z9lx">For tasks that terminated quickly</td>
  </tr>
  <tr>
    <td class="tg-z9lx">Preemptable</td>
    <td class="tg-z9lx">Not preemptable</td>
  </tr>
  <tr>
    <td class="tg-z9lx">Consists of goal, result and feedback</td>
    <td class="tg-z9lx">Consists of request and response</td>
  </tr>
  <tr>
    <td class="tg-z9lx">Provides steady feedback</td>
    <td class="tg-z9lx">Provides a single response</td>
  </tr>
</tbody>
</table>

---

## ROS 2 Params

---

## ROS 2 Params - 1

- Used to externally configure the nodes at run-time and during run-time, without having to recompile the code for the node.
- These parameters can be dynamically reconfigured using ROS 2 services.
- **Note**: ROS 2 uses a decentralized parameter system.

<pre><code class="language-ruby">
def __init__(self):
  super().__init__('test_params_rclpy')
    self.declare_parameter('my_str')
    self.declare_parameter('my_int')

param_str = self.get_parameter('my_str')
    param_int = self.get_parameter('my_int')
    self.get_logger().info("str: %s, int: %s" % (str(param_str.value), str(param_int.value)))
</code></pre>

#### Setting the ROS 2 params in command line.
<pre><code class="language-sh">
  ros2 run ros2_tutorials test_params_rclpy --ros-args -p my_str:="Hello world" -p my_int:=5
</code>
</pre>

---

## ROS 2 Params - 2
### YAML for ROS 2 params

- Managing parameters when number of params increases becomes complex.
- Adding them in command line is not an option
- **YAML** is more human readable and Great for loading a configuration file at run time
- Each parameter consists of a key and a value, where the key is a string

#### Package architecture
<div>
  <center>
    <img src="../static/params_sample.png" width="1000px">
  </center>
</div>

---

## ROS 2 Params - 3
### Loading YAML config to Node

<pre><code class="language-yaml">
  def __init__(self):
    super().__init__('your_amazing_node')
    self.declare_parameters(
        namespace='',
        parameters=[
            ('bool_value', None),
            ('float_number', None),
            ('str_text', None)
        ])
  </code>
  </pre>

####  From command line
<pre><code class="language-yaml">
ros2 run pkg_name node_name --ros-args --params-file .~/locate_pkg/config/params.yaml
</code>
</pre>

---

## ROS 2 Params - 4
###  From launch file
<pre><code class="language-python">
  def generate_launch_description():
  ld = LaunchDescription()
  config = os.path.join(
      get_package_share_directory('ros2_tutorials'),
      'config',
      'params.yaml'
      )

node=Node(
      package = 'ros2_tutorials',
      name = 'Node_A',
      executable = 'test_yaml_params',
      parameters = [config]
  )
  ld.add_action(node)
  return ld
</code>
</pre>
---

## ROS 2 Params - 5
###  For a Python package, in “setup.py”:
<pre><code class="language-python">
  ...
  data_files=[
      ('share/ament_index/resource_index/packages',
          ['resource/' + package_name]),
      ('share/' + package_name, ['package.xml']),
      (os.path.join('share', package_name, 'launch'), glob('launch/*.launch.py')),
      (os.path.join('share', package_name, 'config'), glob('config/*.yaml'))
  ],
...
</code>
</pre>
[References for ROS 2 Params](https://docs.ros.org/en/jazzy/Concepts/Basic/About-Parameters.html)

---

### Why we need Launch files?
<div>
  <center>
    <img src="../static/launch_motiv.png" width="900px">
  </center>
</div>

---

## Launch files

#### Launch files are used for:

-  Describing the configuration of the system and execute as described
-  Configuration of the system includes
    -   Which program to run
    -   Where to run them
    -   Which arguments to pass to them
-  Loading of parameters
-  Remapping topic names, parameters, etc.

---

## Creating Launch files
### Location of launch file

-   ROS 2 Launch files are coded in python
-   They are executed by ROS 2 CLI tool, `ros2 launch`
-   Create a `launch` directory inside your package as given below
-   Create a launch file with `<launch_filename>.py`inside `launch` directory
<div>
  <center>
    <img src="../static/launch_struct.png" width="400px">
  </center>
</div>

---

## Creating Launch files
### Understanding launch files
<div>
  <center>
    <img src="../static/launch_file.png" width="1000px">
  </center>
</div>

---

## Referencing other launch files
<div>
  <center>
    <img src="../static/ref_launch_files.png" width="1000px">
  </center>
</div>

---
## Launch files
### Using launch files

- Use `colcon build` and ``source workspace`` and then use ros2 CLI.
<pre><code class="language-python">
  ros2 launch package_name launch_file_name
</code>
</pre>

---

class: center, middle
# End of Presentation
###  Questions ?

---