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PubSub

What is PubSub#

Publish-subscribe is a messaging paradigm:

  • Producers send messages to specific brokers rather than directly to consumers.
  • Brokers cache messages sent by producers and then actively push them to subscribed consumers or pull them.

The system architectures use this pattern to decouple or handle high traffic scenarios.

In Apache APISIX, the most common scenario is handling north-south traffic from the server to the client. Combining it with a publish-subscribe system, we can achieve more robust features, such as real-time collaboration on online documents, online games, etc.

Architecture#

Currently, Apache APISIX supports WebSocket communication with the client, which can be any application that supports WebSocket, with Protocol Buffer as the serialization mechanism, see the protocol definition.

Supported messaging systems#

How to support other messaging systems#

Apache APISIX implement an extensible pubsub module, which is responsible for starting the WebSocket server, coding and decoding communication protocols, handling client commands, and adding support for the new messaging system.

Basic Steps#

  • Add new commands and response body definitions to pubsub.proto
  • Add a new option to the scheme configuration item in upstream
  • Add a new scheme judgment branch to http_access_phase
  • Implement the required message system instruction processing functions
  • Optional: Create plugins to support advanced configurations of this messaging system

Example of Apache Kafka#

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.

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.

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.

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.

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.

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.

Add this plugin to the existing list of plugins in the APISIX configuration file config.yaml. For instance:

conf/config.yaml
plugins:         # see `conf/config.yaml.example` for an example
  - ...          # add existing plugins
  - kafka-proxy

Results#

After this is done, create a route like the one below to connect to this messaging system via APISIX using the WebSocket.

curl -X PUT 'http://127.0.0.1:9180/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
        }
    }
}'