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Introduction to parameter server communication and ros common tool commands

0. Parameter server concept
1. The parameter server adds/modifies parameters:
2. The parameter server looks for parameters
3. The parameter server deletes the parameter:
4. Common commands
So far, the introduction of parameter server communication and ros common tool instructions is over

0. Parameter server concept

Compared with the previous communication models, the parameter server is the simplest. In the previous models, ROSMASTER acted as a bridge to connect the two together.
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In the parameter server: ROSMASTER has become a container for storing messages , allowing users to complete additions, deletions, checks, and modifications .

Its design is only to store simple non-binary data. If high-performance operations are required, other communication methods are recommended.

Parameter types that can be stored in the container :

32-bit integers
booleans
strings
doubles
iso8601 dates
lists
base64-encoded binary data
dictionary

1. The parameter server adds/modifies parameters:

initialize node
Use the set_param function to add/modify

set_param is somewhat similar to hash: if there is no such value in the parameter server, it will be increased, and if it is, it will be modified to the latest value

import rospy

rospy.init_node("param")

#设置整形
rospy.set_param("p_int",10)

#设置浮点型
rospy.set_param("p_double",3.14)

#设置bool值
rospy.set_param("p_bool",True)

#设置string
rospy.set_param("p_string","hello python")

#设置List
rospy.set_param("p_list",["hello","haha","xixi"])

#设置字典
rospy.set_param("p_dict",{
    
    "name":"hulu","age":8})

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2. The parameter server looks for parameters

Commonly used are the following functions:

get_param(key,value)
returns the corresponding value when the key exists, and returns the default value if
it
does not exist The role of fast tables in system memory
```
radius2=rospy.get_param_cached("radius_p",0.5)

radius=rospy.get_param("radius_p",0.5)
```
get_param_names()
gets all the parameter names in the current parameter list and returns
```
names=rospy.get_param_names()

for name in names:

 print(name)
```

has_param(key)
determines whether there is a parameter in the parameter list and
returns a Bool value

flag1=rospy.has_param("radius_p")

 if flag1:

     print("存在")

 else :

     print("不存在")

search_param(key)
to find if there is this message, if not, return None, if there is, return this key
```
key=rospy.search_param("radius_p")

print(key)
```

3. The parameter server deletes the parameter:

delete_param(key)
delete the specified key

It is relatively simple and will not demonstrate the usage, and there will be a practical project involving

4. Common commands

The topic communication and service communication used in the previous configuration will be used.

Use these commands to dynamically view the relationship between nodes and message carriers when the robot is running :

rosnode : node
rostopic : topic
rosservice : service
rosmsg : msg message
rossrv : srv message
rosparam : Operational parameter server

4.1 rosnode

Let's first start the sub and pub nodes of the previously configured custom msg . For specific configurations, please refer to the topic communication case of this article.

rosrun lesson2 demo02_pub.py
rosrun lesson2 demo02_sub.py

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Use rosnode directly to view the parameter list
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rosnode ping tests the connection status to the node
```
rosnode ping /person_pub
```
Here /pub_person is the node that sends the message, just a demonstration
rosnode list lists active nodes
```
rosnode list
```
Use this command to view the currently active nodes
rosnode info print node information
```
ronode info /person_pub
ronode info /person_sub
```
The information of the sending and receiving nodes is printed here, including the following :
1. node name
2. use topics
3. receiver/sender
Reasonable use can make it easier to clarify the relationship between nodes

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rosnode machine lists the nodes on the specified device
rosnode kill kills a node
```
rosnode kill /sub_person
```
Killed the subscription node
rosnode cleanup clears unconnectable nodes.
Sometimes after ctrl+c, some nodes will be unavailable, but they are still in the list. At this time, you can use this command to refresh them

4.2 rostopic

First start the two nodes used above
and directly enter rostopic in the terminal to view all available commands
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rostopic list shows all topics currently in use
```
rostopic list
```
For example, the topic used here is /che
rostopic echo acts as the receiver and can print topic information on the screen
```
rostopic echo topic
```
For example, here you can verify whether the information of the publisher is correct
The two functions of rostopic info/type are the same, displaying the message type of topic communication, where info contains the type type
```
rostopic info topic
```
For example, the Person msg message type is used here
rostopic find finds topics using a message type
```
rostopic find lesson2/Person
```
For example, what is the topic of finding the message type of lesson2/Person here?
rostopic pub publishes a message to a topic
```
rostopic pub topic
```
For example, publish information to the subscription node here

4.3 rosmsg

rosmsg is a command-line tool used to display information about ROS message types. It is
also the same as above. You can directly enter rosmsg and the available commands will be displayed.

rosmsg list lists all message types
rosmsg show / info lists the specific types contained in the current message type
```
rosmsg show msg
```
rosmsg package lists all messages under a package
```
rosmsg package pack
```
List all message types in turtlesim
rosmsg packages lists the packages that contain a message
```
rosmsg package pack
```
List all packages containing the turtlesim/Pose message type

4.4 rosservice

To query related service information,
we first start the previously configured custom srv server and client nodes. For specific configuration, please refer to this article

rosrun lesson3_srv demo01_client.py
rosrun lesson3_srv demo01_server.py

You can also see the parameter list by typing rosservice in the terminal
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rosservice list lists the list of parameter services
rosservice call calls the service
here to call /sum2 tab to complete and pass in two parameters
```
rosservice call /sum2
```
rosservice args to view the specific parameters of the srv message
```
rosservice args /sum2
```