docs: add pubsub develop example for kafka (#7059)

docs/en/latest/pubsub.md

@@ -58,6 +58,87 @@ Apache APISIX implement an extensible pubsub module, which is responsible for st
 - Implement the required message system instruction processing functions
 - Optional: Create plugins to support advanced configurations of this messaging system
 
-### Example
+### Example of Apache Kafka
 
-TODO, an example will be added later to point out how to support other messaging systems.
+#### Add new commands and response body definitions to `pubsub.proto`
+
+The core of the protocol definition in `pubsub.proto` is the two parts `PubSubReq` and `PubSubResp`.
+
+First, create the `CmdKafkaFetch` command and add the required parameters. Then, register this command in the list of commands for `req` in `PubSubReq`, which is named `cmd_kafka_fetch`.
+
+Then create the corresponding response body `KafkaFetchResp` and register it in the `resp` of `PubSubResp`, named `kafka_fetch_resp`.
+
+The protocol definition [pubsub.proto](https://github.com/apache/apisix/blob/master/apisix/include/apisix/model/pubsub.proto).
+
+#### Add a new option to the `scheme` configuration item in upstream
+
+Add a new option `kafka` to the `scheme` field enumeration in the `upstream` of `apisix/schema_def.lua`.
+
+The schema definition [schema_def.lua](https://github.com/apache/apisix/blob/master/apisix/schema_def.lua).
+
+#### Add a new `scheme` judgment branch to `http_access_phase`
+
+Add a `scheme` judgment branch to the `http_access_phase` function in `apisix/init.lua` to support the processing of `kafka` type upstreams. Because Apache Kafka has its clustering and partition scheme, we do not need to use the Apache APISIX built-in load balancing algorithm, so we intercept and take over the processing flow before selecting the upstream node, using the `kafka_access_phase` function.
+
+The APISIX init file [init.lua](https://github.com/apache/apisix/blob/master/apisix/init.lua).
+
+#### Implement the required message system commands processing functions
+
+First, create an instance of the `pubsub` module, which is provided in the `core` package.
+
+Then, an instance of the Apache Kafka client is created and omitted code here.
+
+Next, add the command registered in the protocol definition above to the `pubsub` instance, which will provide a callback function that provides the parameters parsed from the communication protocol, in which the developer needs to call the kafka client to get the data and return it to the `pubsub` module as the function return value.
+
+:::note Callback function prototype
+
+The `params` is the data in the protocol definition; the first return value is the data, which needs to contain the fields in the response body definition, and returns the `nil` value when there is an error; the second return value is the error, and returns the error string when there is an error
+
+:::
+
+Finally, it enters the loop to wait for client commands, and when an error occurs, it returns the error and stops the processing flow.
+
+The kafka pubsub implementation [kafka.lua](https://github.com/apache/apisix/blob/master/apisix/pubsub/kafka.lua).
+
+#### Optional: Create plugins to support advanced configurations of this messaging system
+
+Add the required fields to the plugin schema definition and write them to the context of the current request in the `access` function.
+
+The `kafka-proxy` plugin [kafka-proxy.lua](https://github.com/apache/apisix/blob/master/apisix/plugins/kafka-proxy.lua).
+
+Add this plugin to the list of plugins in the APISIX configuration file.
+
+The plugins list [config-default.yaml](https://github.com/apache/apisix/blob/master/conf/config-default.yaml).
+
+#### Results
+
+After this is done, create a route like the one below to connect to this messaging system via APISIX using the WebSocket.
+
+```shell
+curl -X PUT 'http://127.0.0.1:9080/apisix/admin/routes/kafka' \
+    -H 'X-API-KEY: ${api-key}' \
+    -H 'Content-Type: application/json' \
+    -d '{
+    "uri": "/kafka",
+    "plugins": {
+        "kafka-proxy": {
+            "sasl": {
+              "username": "user",
+              "password": "pwd"
+            }
+        }
+    },
+    "upstream": {
+        "nodes": {
+            "kafka-server1:9092": 1,
+            "kafka-server2:9092": 1,
+            "kafka-server3:9092": 1
+        },
+        "type": "none",
+        "scheme": "kafka",
+        "tls": {
+            "verify": true
+        }
+    }
+}'
+```