This section describes how to use an open-source Kafka client to access a Kafka instance in plaintext. Two scenarios: private network within a VPC and public network. To access a Kafka instance across VPCs, see Accessing Kafka Using a VPC Endpoint Across VPCs. To access a Kafka instance using DNAT, see Accessing Kafka in a Public Network Using DNAT. To access an instance in your service code, see the Distributed Message Service Developer Guide.

For details on how to use a Kafka client in different languages, visit https://cwiki.apache.org/confluence/display/KAFKA/Clients.

Each Kafka broker allows a maximum of 1000 connections from each IP address. Excess connections will be rejected. You can change the limit by referring to Modifying Kafka Instance Configuration Parameters, that is, to modify parameter max.connections.per.ip.

Prerequisites

Security group rules have been properly configured.
To access a Kafka instance with ciphertext access disabled, configure proper security group rules. For details about security group configuration requirements, see Table 2.
The Kafka instance address has been obtained.
- To access a Kafka instance in a private network within a VPC, use port 9092. Obtain the instance connection address in the Connection section of the Basic Information page, shown as follows.
  Figure 1 Kafka instance addresses for private access within a VPC (in plaintext)
- To access a Kafka instance in a public network, use port 9094. Obtain the instance connection address in the Connection section of the Basic Information page, shown as follows.
  Figure 2 Kafka instance addresses for public access (in plaintext)
If automatic topic creation is not enabled for the Kafka instance, create a topic before connecting to the instance.
Kafka CLI v1.1.0, v2.3.0, v2.7.2, or v3.4.0 is available. Ensure that the Kafka instance and the CLI use the same version.
An ECS that has an EIP has been created. For intra-VPC access, ensure that its VPC, subnet, and security group configurations are the same as those of the Kafka instance. JDK v1.8.111 or later has been installed on the ECS, and the JAVA_HOME and PATH environment variables have been configured as follows:
Add the following lines to the .bash_profile file in the home directory as an authorized user. In this command, /opt/java/jdk1.8.0_151 is the JDK installation path. Change it to the path where you install JDK.
```
export JAVA_HOME=/opt/java/jdk1.8.0_151 
export PATH=$JAVA_HOME/bin:$PATH
```
Run the source .bash_profile command for the modification to take effect.

Accessing the Instance Using CLI

The following uses Linux as an example.

Decompress the Kafka CLI package.
Access the directory where the CLI package is stored and run the following command to decompress the package:

tar -zxf [kafka_tar]

In the preceding command, [kafka_tar] indicates the name of the CLI package.

For example:

tar -zxf kafka_2.12-2.7.2.tgz
Access the /bin directory of the Kafka CLI.
In Windows, you need to access the /bin/windows directory.
Run the following command to create messages:
```
./kafka-console-producer.sh --broker-list ${connection-address} --topic ${topic-name}
```
Parameter description:
- {connection-address}: the address obtained in Prerequisites. For public access, use Address (Public Network, Plaintext). For intra-VPC access, use Address (Private Network, Plaintext).
- {topic-name}: the name of the topic created for the Kafka instance. If automatic topic creation has enabled for the Kafka instance, set this parameter to the name of a created topic or a topic that has not been created.
The following example uses connection addresses 10.xx.xx.45:9094,10.xx.xx.127:9094,10.xx.xx.103:9094. After running the preceding command, you can send a message to the Kafka instance by writing it and pressing Enter. Each line of content is sent as a message.
```
[root@ecs-kafka bin]# ./kafka-console-producer.sh --broker-list 10.xx.xx.45:9094,10.xx.xx.127:9094,10.xx.xx.103:9094  --topic topic-demo
>Hello
>DMS
>Kafka!
>^C[root@ecs-kafka bin]# 
```
To stop creating messages, press Ctrl+C to exit.
Run the following command to retrieve messages:
```
./kafka-console-consumer.sh --bootstrap-server ${connection-address} --topic ${topic-name} --group ${consumer-group-name} --from-beginning
```
Parameter description:
- {connection-address}: the address obtained in Prerequisites. For public access, use Address (Public Network, Plaintext). For intra-VPC access, use Address (Private Network, Plaintext).
- {topic-name}: the name of the topic created for the Kafka instance
- {consumer-group-name}: the consumer group name set based on your service requirements. If a consumer group name has been specified in the configuration file, ensure that you use the same name in the command line. Otherwise, consumption may fail. If a consumer group name starts with a special character, such as an underscore (_) or a number sign (#), the monitoring data cannot be displayed.
Example:
```
[root@ecs-kafka bin]#  ./kafka-console-consumer.sh --bootstrap-server 10.xx.xx.45:9094,10.xx.xx.127:9094,10.xx.xx.103:9094 --topic topic-demo --group order-test --from-beginning
Kafka!
DMS
Hello
^CProcessed a total of 3 messages
[root@ecs-kafka bin]# 
```
To stop retrieving messages, press Ctrl+C to exit.

Connecting to Kafka Using the Client (Plaintext Access)

Prerequisites

Accessing the Instance Using CLI