车联网架构设计(二)_消息缓存

在上一篇博客车联网架构设计(一)_消息平台的搭建-CSDN博客中，我介绍了车联网平台需要实现的一些功能，并介绍了如何用EMQX+HAPROXY来搭建一个MQTT消息平台。车联网平台的应用需要消费车辆发布的消息，同时也会下发消息给车辆，以实现车辆控制等功能。通常我们会在MQTT消息平台收到车辆消息后对消息进行缓存，以供上层应用使用。我们可以直接把消息保存到数据库，或者引入一个消息队列，这样可以方便对应用和车辆之间进行解耦合。

这里我将介绍一下如何引入一个Kafka消息队列，把车辆以及上层应用之间需要交互的消息缓存到这个消息队列之中。

在EMQX的企业版中，提供了丰富的数据桥接功能，可以支持把MQTT消息桥接到其他外部系统，例如Kafka或数据库中。但是在开源版，只提供了很有限的数据桥接，不支持Kafka。为此我们可以通过给EMQX开发Hook extension的方式，来加载我们的插件，实现把数据桥接到Kafak。

在EMQX官网的介绍中，Hook扩展是通过gRPC的方式来实现的，支持多种编程语言。如下图：

这里我以Python为例子，来定义一个扩展。

搭建Kafka

首先是在K8S上部署一个Kafka集群，这里我选择了Strimizi的Kafka operator来部署

先创建一个namespace

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kubectl create namespace kafka

安装Operator, CRD以及定义RBAC等

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kubectl create -f 'https://strimzi.io/install/latest?namespace=kafka' -n kafka

创建一个只包含一个节点的Kafka

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kubectl apply -f https://strimzi.io/examples/latest/kafka/kafka-persistent-single.yaml -n kafka

打开两个终端，分别运行以下的订阅和发布的指令，测试Kafka是否正常工作

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kubectl -n kafka run kafka-producer -ti --image=quay.io/strimzi/kafka:0.38.0-kafka-3.6.0 --rm=true --restart=Never -- bin/kafka-console-producer.sh --bootstrap-server my-cluster-kafka-bootstrap:9092 --topic my-topic
kubectl -n kafka run kafka-consumer -ti --image=quay.io/strimzi/kafka:0.38.0-kafka-3.6.0 --rm=true --restart=Never -- bin/kafka-console-consumer.sh --bootstrap-server my-cluster-kafka-bootstrap:9092 --topic my-topic --from-beginning

开发ExHook

首先是获取当前EMQX版本定义的gPRC proto。在EMQX服务器的/opt/emqx/lib/emqx_exhook-5.0.14/priv/protos/目录下面有一个exhook.proto文件。

运行以下命令来基于这个proto生成python文件

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python -m grpc_tools.protoc -I./ --python_out=. --pyi_out=. --grpc_python
_out=. ./exhook.proto

运行之后，在当前目录下会新生成三个文件，exhook_pb2_grpc.py，exhook_pb2.py，exhook_pb2.pyi

新建一个exhook_server.py文件，继承exhook_pb2_grpc里面的HookProviderServicer，注册对应事件的处理方法，如以下代码：

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from concurrent import futures
import logging
from multiprocessing.sharedctypes import Value

import grpc

import exhook_pb2
import exhook_pb2_grpc

import pickle
from kafka import KafkaProducer

class HookProvider(exhook_pb2_grpc.HookProviderServicer):
    def __init__(self):
        self.producer = KafkaProducer(bootstrap_servers='my-cluster-kafka-bootstrap.kafka.svc.cluster.local:9092')

    def OnProviderLoaded(self, request, context):
        print("OnProviderLoaded:", request)
        '''
        specs = [exhook_pb2.HookSpec(name="client.connect"),
                 exhook_pb2.HookSpec(name="client.connack"),
                 exhook_pb2.HookSpec(name="client.connected"),
                 exhook_pb2.HookSpec(name="client.disconnected"),
                 exhook_pb2.HookSpec(name="client.authenticate"),
                 exhook_pb2.HookSpec(name="client.authorize"),
                 exhook_pb2.HookSpec(name="client.subscribe"),
                 exhook_pb2.HookSpec(name="client.unsubscribe"),

                 exhook_pb2.HookSpec(name="session.created"),
                 exhook_pb2.HookSpec(name="session.subscribed"),
                 exhook_pb2.HookSpec(name="session.unsubscribed"),
                 exhook_pb2.HookSpec(name="session.resumed"),
                 exhook_pb2.HookSpec(name="session.discarded"),
                 exhook_pb2.HookSpec(name="session.takenover"),
                 exhook_pb2.HookSpec(name="session.terminated"),

                 exhook_pb2.HookSpec(name="message.publish"),
                 exhook_pb2.HookSpec(name="message.delivered"),
                 exhook_pb2.HookSpec(name="message.acked"),
                 exhook_pb2.HookSpec(name="message.dropped")
                ]
        '''
        specs = [exhook_pb2.HookSpec(name="message.publish")]
        return exhook_pb2.LoadedResponse(hooks=specs)

    def OnProviderUnloaded(self, request, context):
        print("OnProviderUnloaded:", request)
        return exhook_pb2.EmptySuccess()

    def OnClientConnect(self, request, context):
        print("OnClientConnect:", request)
        return exhook_pb2.EmptySuccess()

    def OnClientConnack(self, request, context):
        print("OnClientConnack:", request)
        return exhook_pb2.EmptySuccess()

    def OnClientConnected(self, request, context):
        print("OnClientConnected:", request)
        return exhook_pb2.EmptySuccess()

    def OnClientDisconnected(self, request, context):
        print("OnClientDisconnected:", request)
        return exhook_pb2.EmptySuccess()

    def OnClientAuthenticate(self, request, context):
        print("OnClientAuthenticate:", request)
        reply = exhook_pb2.ValuedResponse(type="STOP_AND_RETURN", bool_result=True)
        return reply

    def OnClientAuthorize(self, request, context):
        print("OnClientAuthorize:", request)
        reply = exhook_pb2.ValuedResponse(type="STOP_AND_RETURN", bool_result=True)
        return reply

    def OnClientSubscribe(self, request, context):
        print("OnClientSubscribe:", request)
        return exhook_pb2.EmptySuccess()

    def OnClientUnsubscribe(self, request, context):
        print("OnClientUnsubscribe:", request)
        return exhook_pb2.EmptySuccess()

    def OnSessionCreated(self, request, context):
        print("OnSessionCreated:", request)
        return exhook_pb2.EmptySuccess()

    def OnSessionSubscribed(self, request, context):
        print("OnSessionSubscribed:", request)
        return exhook_pb2.EmptySuccess()

    def OnSessionUnsubscribed(self, request, context):
        print("OnSessionUnsubscribed:", request)
        return exhook_pb2.EmptySuccess()

    def OnSessionResumed(self, request, context):
        print("OnSessionResumed:", request)
        return exhook_pb2.EmptySuccess()

    def OnSessionDiscarded(self, request, context):
        print("OnSessionDiscarded:", request)
        return exhook_pb2.EmptySuccess()

    def OnSessionTakenover(self, request, context):
        print("OnSessionTakenover:", request)
        return exhook_pb2.EmptySuccess()

    def OnSessionTerminated(self, request, context):
        print("OnSessionTerminated:", request)
        return exhook_pb2.EmptySuccess()

    def OnMessagePublish(self, request, context):
        self.producer.send('testtopic', pickle.dumps(nmsg))
        reply = exhook_pb2.ValuedResponse(type="STOP_AND_RETURN", message=nmsg)
        return reply

    ## case2: stop publish the 't/d' messages
    #def OnMessagePublish(self, request, context):
    #    nmsg = request.message
    #    if nmsg.topic == 't/d':
    #        nmsg.payload = b""
    #        nmsg.headers['allow_publish'] = b"false"
    #
    #    reply = exhook_pb2.ValuedResponse(type="STOP_AND_RETURN", message=nmsg)
    #    return reply

    def OnMessageDelivered(self, request, context):
        print("OnMessageDelivered:", request)
        return exhook_pb2.EmptySuccess()

    def OnMessageDropped(self, request, context):
        print("OnMessageDropped:", request)
        return exhook_pb2.EmptySuccess()

    def OnMessageAcked(self, request, context):
        print("OnMessageAcked:", request)
        return exhook_pb2.EmptySuccess()

def serve():
    server = grpc.server(futures.ThreadPoolExecutor(max_workers=10))
    exhook_pb2_grpc.add_HookProviderServicer_to_server(HookProvider(), server)
    server.add_insecure_port('[::]:9000')
    server.start()

    print("Started gRPC server on [::]:9000")

    server.wait_for_termination()


if __name__ == '__main__':
    logging.basicConfig()
    serve()

解释一下代码，在OnProvidedLoader里面是加载各种事件的钩子，这里只加载message.publish事件。在OnMessagePublish是对应事件的处理函数，这里把收到的MQTT消息通过Pickle进行序列化，发送到Kafka的对应topic

部署ExHook

写一个Dockerfile，把代码打包为一个镜像

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FROM python:3.7-slim
WORKDIR /app
COPY requirements.txt ./
RUN pip install -r requirements.txt
COPY . .
CMD ["python", "./exhook_server.py"]

requirements.txt文件内容为

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grpcio==1.59.3
grpcio-tools==1.59.3
kafka-python==2.0.2

运行以下命令来构建镜像

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docker build --network=host -t emqx_plugin_test:v1.0 .

创建一个部署这个镜像的deployment和service，然后部署到K8S

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apiVersion: apps/v1
kind: Deployment
metadata:
  name: emqx-hookserver-deployment
  labels:
    app: hookserver
  namespace: emqx
spec:
  replicas: 1
  selector:
    matchLabels:
      app: hookserver
  template:
    metadata:
      labels:
        app: hookserver
    spec:
      containers:
      - name: hookserver
        image: emqx_plugin_test:v1.0
        imagePullPolicy: Never
        resources:
          requests:
            memory: "250Mi"
            cpu: "100m"
          limits:
            memory: "250Mi"
            cpu: "100m"
        ports:
        - name: rpc
          containerPort: 9000
---
apiVersion: v1
kind: Service
metadata:
  name: hookserver-service
  namespace: emqx
spec:
  selector:
    app: hookserver
  ports:
    - name: rpc
      port: 9000

回到EMQX的控制面板Dashboard，在ExHook里面添加，url填入http://hookserver-service.emqx.svc.cluster.local:9000，然后选择启用即可，可以看到状态为连接成功，并且显示注册了1个钩子。

在minikube上部署，一开始是显示连接中，等了很久仍然无法连接成功，最后查了资料，原来是coredns的问题，运行以下命令重启即可：

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kubectl -n kube-system rollout restart deployment coredns

之后打开订阅Kafka的testtopic，然后通过MQTT连接到EMQX发送消息，可以看到Kafka能成功收到EMQX转发的消息。