前言
桥接是一种连接多个 EMQ X 或者其他 MQTT 消息中间件的方式。不同于集群，工作在桥接模式下的节点之间不会复制主题树和路由表。桥接模式所做的是：
- 按照规则把消息转发至桥接节点；
- 从桥接节点订阅主题，并在收到消息后在本节点/集群中转发该消息。
环境：
emqx1 | v4.1.2 | 192.168.5.207:2883 | 节点node
emqx2 | v3.1.1 | 192.168.5.209:2883 | 远程broker
用Docker在两个服务器上面各自跑了一个emqx的容器，暴露在宿主机的端口分别都是2883；
桥接
前提
使用桥接方式之前，需要了解到的一点是，基于Docker的运行方式，如果需要使用桥接的功能，那么需要修改emqx.conf文件，该文件在容器的具体目录为/opt/emqx/etc/emqx.conf，需要挂载该文件出来，但是EMQX版本需要大于v3.1.1，如果处于并且小于这个版本，那么是无法挂载该文件的，这是emqx的一个bug，也即是在v3.1.2才可以挂载emqx.conf文件，所以我们用了当前最新的v4.1.2版本；
该版本有关桥接部分的配置已被插件化，所以需要挂载另一个配置文件进行配置，容器内的/opt/emqx/etc/plugins/emqx_bridge_mqtt.conf文件需要被挂载；
docker-compose.yml文件配置如下所示：
volumes:
- ./mqtt/volumes/plugins/emqx_bridge_mqtt.conf:/opt/emqx/etc/plugins/emqx_bridge_mqtt.conf
- ./mqtt/volumes/data/loaded_plugins:/opt/emqx/data/loaded_plugins
emqx_bridge_mqtt.conf文件在容器中的具体位置如图所示：
emqx_bridge_mqtt.conf文件内容如下：
##====================================================================
## Configuration for EMQ X MQTT Broker Bridge
##====================================================================
##--------------------------------------------------------------------
## Bridges to aws
##--------------------------------------------------------------------
## Bridge address: node name for local bridge, host:port for remote.
##
## Value: String
## Example: emqx@127.0.0.1, 127.0.0.1:1883
bridge.mqtt.aws.address = 192.168.5.209:2883
## Protocol version of the bridge.
##
## Value: Enum
## - mqttv5
## - mqttv4
## - mqttv3
bridge.mqtt.aws.proto_ver = mqttv4
## Start type of the bridge.
##
## Value: enum
## manual
## auto
bridge.mqtt.aws.start_type = manual
## Whether to enable bridge mode for mqtt bridge
##
## This option is prepared for the mqtt broker which does not
## support bridge_mode such as the mqtt-plugin of the rabbitmq
##
## Value: boolean
bridge.mqtt.aws.bridge_mode = true
## The ClientId of a remote bridge.
##
## Value: String
bridge.mqtt.aws.clientid = bridge_aws
## The Clean start flag of a remote bridge.
##
## Value: boolean
## Default: true
##
## NOTE: Some IoT platforms require clean_start
## must be set to 'true'
bridge.mqtt.aws.clean_start = true
## The username for a remote bridge.
##
## Value: String
bridge.mqtt.aws.username = user
## The password for a remote bridge.
##
## Value: String
bridge.mqtt.aws.password = passwd
## Topics that need to be forward to AWS IoTHUB
##
## Value: String
## Example: topic1/#,topic2/#
bridge.mqtt.aws.forwards = topic1/#,topic2/#
## Forward messages to the mountpoint of an AWS IoTHUB
##
## Value: String
#bridge.mqtt.aws.forward_mountpoint = bridge/aws/${node}/
## Need to subscribe to AWS topics
##
## Value: String
bridge.mqtt.aws.subscription.1.topic = get/#
## Need to subscribe to AWS topics QoS.
##
## Value: Number
bridge.mqtt.aws.subscription.1.qos = 1
## A mountpoint that receives messages from AWS IoTHUB
##
## Value: String
## bridge.mqtt.aws.receive_mountpoint = receive/aws/
## Bribge to remote server via SSL.
##
## Value: on | off
bridge.mqtt.aws.ssl = off
## PEM-encoded CA certificates of the bridge.
##
## Value: File
bridge.mqtt.aws.cacertfile = etc/certs/cacert.pem
## Client SSL Certfile of the bridge.
##
## Value: File
bridge.mqtt.aws.certfile = etc/certs/client-cert.pem
## Client SSL Keyfile of the bridge.
##
## Value: File
bridge.mqtt.aws.keyfile = etc/certs/client-key.pem
## SSL Ciphers used by the bridge.
##
## Value: String
bridge.mqtt.aws.ciphers = ECDHE-ECDSA-AES256-GCM-SHA384,ECDHE-RSA-AES256-GCM-SHA384,ECDHE-ECDSA-AES256-SHA384,ECDHE-RSA-AES256-SHA384,ECDHE-ECDSA-DES-CBC3-SHA,ECDH-ECDSA-AES256-GCM-SHA384,ECDH-RSA-AES256-GCM-SHA384,ECDH-ECDSA-AES256-SHA384,ECDH-RSA-AES256-SHA384,DHE-DSS-AES256-GCM-SHA384,DHE-DSS-AES256-SHA256,AES256-GCM-SHA384,AES256-SHA256,ECDHE-ECDSA-AES128-GCM-SHA256,ECDHE-RSA-AES128-GCM-SHA256,ECDHE-ECDSA-AES128-SHA256,ECDHE-RSA-AES128-SHA256,ECDH-ECDSA-AES128-GCM-SHA256,ECDH-RSA-AES128-GCM-SHA256,ECDH-ECDSA-AES128-SHA256,ECDH-RSA-AES128-SHA256,DHE-DSS-AES128-GCM-SHA256,DHE-DSS-AES128-SHA256,AES128-GCM-SHA256,AES128-SHA256,ECDHE-ECDSA-AES256-SHA,ECDHE-RSA-AES256-SHA,DHE-DSS-AES256-SHA,ECDH-ECDSA-AES256-SHA,ECDH-RSA-AES256-SHA,AES256-SHA,ECDHE-ECDSA-AES128-SHA,ECDHE-RSA-AES128-SHA,DHE-DSS-AES128-SHA,ECDH-ECDSA-AES128-SHA,ECDH-RSA-AES128-SHA,AES128-SHA
## Ciphers for TLS PSK.
## Note that 'bridge.${BridgeName}.ciphers' and 'bridge.${BridgeName}.psk_ciphers' cannot
## be configured at the same time.
## See 'https://tools.ietf.org/html/rfc4279#section-2'.
#bridge.mqtt.aws.psk_ciphers = PSK-AES128-CBC-SHA,PSK-AES256-CBC-SHA,PSK-3DES-EDE-CBC-SHA,PSK-RC4-SHA
## Ping interval of a down bridge.
##
## Value: Duration
## Default: 10 seconds
bridge.mqtt.aws.keepalive = 60s
## TLS versions used by the bridge.
##
## Value: String
bridge.mqtt.aws.tls_versions = tlsv1.2,tlsv1.1,tlsv1
## Bridge reconnect time.
##
## Value: Duration
## Default: 30 seconds
bridge.mqtt.aws.reconnect_interval = 30s
## Retry interval for bridge QoS1 message delivering.
##
## Value: Duration
bridge.mqtt.aws.retry_interval = 20s
## Publish messages in batches, only RPC Bridge supports
##
## Value: Integer
## default: 32
bridge.mqtt.aws.batch_size = 32
## Inflight size.
##
## Value: Integer
bridge.mqtt.aws.max_inflight_size = 32
## Base directory for replayq to store messages on disk
## If this config entry is missing or set to undefined,
## replayq works in a mem-only manner.
##
## Value: String
bridge.mqtt.aws.queue.replayq_dir = data/emqx_aws_bridge/
## Replayq segment size
##
## Value: Bytesize
bridge.mqtt.aws.queue.replayq_seg_bytes = 10MB
## Replayq max total size
##
## Value: Bytesize
bridge.mqtt.aws.queue.max_total_size = 5GB
如果不知道怎么修改的情况下，那么直接把bridge.mqtt.aws.address的地址和mqtt的远程端口修改为你需要转发的地址和端口，如207服务器的mqtt broker是作为转发节点的，209是作为接收端的mqtt broker，那么就填入bridge.mqtt.aws.address = 192.168.5.209:2883（这里的209的mqtt端口我使用的是2883，默认是1883）
## Bridge address: node name for local bridge, host:port for remote.
##
## Value: String
## Example: emqx@127.0.0.1, 127.0.0.1:1883
bridge.mqtt.aws.address = 192.168.5.209:2883
设置需要转发的topic，转发topic1和topic2主题，也就是任何一个向207发布topic1主题和topic2主题的mqtt消息都会转发到209mqtt broker去；
这里需要注意一个点，就是mountpoint配置，该配置在官网介绍是作为一个消息发布的主题前缀信息来配置的，但是这里测试的时候发现如果这句配置不注释，那么转发会不成功；
向209订阅主题，如有人向209的主题发布消息，那么207broker相应的也会收到消息并发布给向207订阅了该主题的对象，这里207向209订阅了test/#主题，qos为1；
## Topics that need to be forward to AWS IoTHUB
##
## Value: String
## Example: topic1/#,topic2/#
bridge.mqtt.aws.forwards = topic1/#,topic2/#
## Forward messages to the mountpoint of an AWS IoTHUB
##
## Value: String
#bridge.mqtt.aws.forward_mountpoint = bridge/aws/${node}/
## Need to subscribe to AWS topics
##
## Value: String
bridge.mqtt.aws.subscription.1.topic = test/#
## Need to subscribe to AWS topics QoS.
##
## Value: Number
bridge.mqtt.aws.subscription.1.qos = 1
## A mountpoint that receives messages from AWS IoTHUB
##
## Value: String
## bridge.mqtt.aws.receive_mountpoint = receive/aws/
配置文件启动插件（loaded_plugins）
说完emqx_bridge_mqtt.conf，我们来看看loaded_plugins文件，默认启动emqx的时候，emqx_bridge_mqtt插件是不启动的，需要在控制面板中的插件那里点击启动才行，如何使其在启动emqx启动的时候，bridge插件随之启动呢，我们在emqx.conf配置文件中找到这段一个配置，有关插件的，这里提示，插件自启动的配置在/opt/emqx/data/loaded_plugins这里，如图：
然后修改loaded_plugins文件，将emqx_bridge_mqtt的自启动设置为true即可
将emqx_bridge_mqtt的自启动设置为true之后，运行容器的时候，该插件就默认启动了，如果没有修改该配置的话，那么就得进入emqx的控制台界面去自己启动该插件，每一次都要人为启动的话，效率不高，所以这里设置了自启动，以下截图为控制台启动该插件的图示：
启用配置好的桥接网络
bridge插件启动之后，还得进入容器对已配置好的桥接网络进行启动，这里就不进入容器内操作了，直接宿主机执行容器内指令，如下操作：
容器外执行容器内指令，可使用docker的exec指令。emqx是可以在配置文件里面配置多个桥接网络的，通过bin目录下面的emqx_ctl可以查询配置了哪些桥接网络，还可以启动配置好的桥接网络，转发的主题和订阅的主题等操作；
# 查询配置了哪些桥接网络
docker exec emqx /bin/sh -c "bin/emqx_ctl bridges list"
# 启动配置好的aws桥接网络
docker exec emqx /bin/sh -c "bin/emqx_ctl bridges start aws"
# 查询已启动的aws桥接网络转发的主题
docker exec emqx /bin/sh -c "bin/emqx_ctl bridges forwards aws"
# 查询已启动的aws桥接网络订阅的主题
docker exec emqx /bin/sh -c "bin/emqx_ctl bridges subscriptions aws"
测试桥接网络
通过图示描述，如下图手绘所示：
- fx1：mqttfx客户端，向207服务器订阅sub主题，推送to1和to2主题到207服务器；
- 207服务器：转发任何客户端push过来的to1、to2主题的消息到209服务器，向209服务器订阅sub主题，接收209服务器推送过来的sub主题，向那些订阅了sub主题的客户端推送sub主题；
- 209服务器：接收207服务器转发过来的to1、to2主题的消息，向所有订阅了sub主题的客户端推送消息，向所有订阅了to1、to2主题的客户端推送消息；
- fx2：mqttfx客户端，向209服务器订阅to1、to2主题，推送sub主题；
如上述过程无异常，即为测试成功；