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Bridge of MQTT
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The concept of Bridge means that EMQ X supports forwarding messages of one of its own topics to another MQTT Broker in some way.

Bridge differs from Cluster in that the bridge does not replicate the topic trie and routing tables and only forwards MQTT messages based on bridging rules.

At present, the bridging methods supported by EMQ X are as follows:

  • RPC bridge: RPC Bridge only supports message forwarding and does not support subscribing to the topic of remote nodes to synchronize data;
  • MQTT Bridge: MQTT Bridge supports both forwarding and data synchronization through subscription topic.

These concepts are shown below:


In addition, the EMQ X message broker supports multi-node bridge mode interconnection

              ---------                     ---------                     ---------
Publisher --> | Node1 | --Bridge Forward--> | Node2 | --Bridge Forward--> | Node3 | --> Subscriber
              ---------                     ---------                     ---------

In EMQ X, bridge is configured by modifying etc/emqx.conf. EMQ X distinguishes between different bridges based on different names. E.g

## Bridge address: node name for local bridge, host:port for remote. =

This configuration declares a bridge named aws and specifies that it is bridged to the MQTT broker of by MQTT mode.

In case of creating multiple bridges, it is convenient to replicate all configuration items of the first bridge, and modify the bridge name and other configuration items if necessary (such as bridge.$name.address, where $name refers to the name of bridge)

The next two sections describe how to create a bridge in RPC and MQTT mode respectively and create a forwarding rule that forwards the messages from sensors. Assuming that two EMQ X nodes are running on two hosts:

Name Node MQTT Port
emqx1 emqx1@ 1883
emqx2 emqx2@ 1883

EMQ X RPC Bridge Configuration

The following is the basic configuration of RPC bridging. A simplest RPC bridging only requires the following three items

## Bridge Address: Use node name (nodename@host) for rpc bridging, and host:port for mqtt connection
bridge.mqtt.emqx2.address = emqx2@

## Forwarding topics of the message
bridge.mqtt.emqx2.forwards = sensor1/#,sensor2/#

## bridged mountpoint
bridge.mqtt.emqx2.mountpoint = bridge/emqx2/${node}/

If the messages received by the local node emqx1 matches the topic sersor1/# or sensor2/#, these messages will be forwarded to the sensor1/# or sensor2/# topic of the remote node emqx2.

forwards is used to specify topics. Messages of the in forwards specified topics on local node are forwarded to the remote node.

mountpoint is used to add a topic prefix when forwarding a message. To use mountpoint, the forwards directive must be set. In the above example, a message with the topic sensor1/hello received by the local node will be forwarded to the remote node with the topic bridge/emqx2/emqx1@

Limitations of RPC bridging:

  1. The RPC bridge of emqx can only forward local messages to the remote node, and cannot synchronize the messages of the remote node to the local node;

  2. RPC bridge can only bridge two EMQ X broker together and cannot bridge EMQ X broker to other MQTT brokers.

EMQ X MQTT Bridge Configuration

EMQ X 3.0 officially introduced MQTT bridge, so that EMQ X can bridge any MQTT broker. Because of the characteristics of the MQTT protocol, EMQ X can subscribe to the remote mqtt broker's topic through MQTT bridge, and then synchronize the remote MQTT broker's message to the local.

EMQ X MQTT bridging principle: Create an MQTT client on the EMQ X broker, and connect this MQTT client to the remote MQTT broker. Therefore, in the MQTT bridge configuration, following fields may be set for the EMQ X to connect to the remote broker as an mqtt client

## Bridge Address: Use node name for rpc bridging, use host:port for mqtt connection
bridge.mqtt.emqx2.address =

## Bridged Protocol Version
## Enumeration value: mqttv3 | mqttv4 | mqttv5
bridge.mqtt.emqx2.proto_ver = mqttv4

## mqtt client's clientid
bridge.mqtt.emqx2.clientid = bridge_emq

## mqtt client's clean_start field
## Note: Some MQTT Brokers need to set the clean_start value as `true`
bridge.mqtt.emqx2.clean_start = true

##  mqtt client's username field
bridge.mqtt.emqx2.username = user

## mqtt client's password field
bridge.mqtt.emqx2.password = passwd

## Whether the mqtt client uses ssl to connect to a remote serve or not
bridge.mqtt.emqx2.ssl = off

## CA Certificate of Client SSL Connection (PEM format)
bridge.mqtt.emqx2.cacertfile = etc/certs/cacert.pem

## SSL certificate of Client SSL connection 
bridge.mqtt.emqx2.certfile = etc/certs/client-cert.pem

## Key file of Client SSL connection 
bridge.mqtt.emqx2.keyfile = etc/certs/client-key.pem

## SSL encryption
bridge.mqtt.emqx2.ciphers = ECDHE-ECDSA-AES256-GCM-SHA384,ECDHE-RSA-AES256-GCM-SHA384

## TTLS PSK password
## Note 'listener.ssl.external.ciphers' and 'listener.ssl.external.psk_ciphers' cannot be configured at the same time
## See ''.
## bridge.mqtt.emqx2.psk_ciphers = PSK-AES128-CBC-SHA,PSK-AES256-CBC-SHA,PSK-3DES-EDE-CBC-SHA,PSK-RC4-SHA

## Client's heartbeat interval
bridge.mqtt.emqx2.keepalive = 60s

## Supported TLS version
bridge.mqtt.emqx2.tls_versions = tlsv1.2,tlsv1.1,tlsv1

## Forwarding topics of the message
bridge.mqtt.emqx2.forwards = sensor1/#,sensor2/#

## Bridged mountpoint
bridge.mqtt.emqx2.mountpoint = bridge/emqx2/${node}/

## Subscription topic for bridging
bridge.mqtt.emqx2.subscription.1.topic = cmd/topic1

## Subscription qos for bridging
bridge.mqtt.emqx2.subscription.1.qos = 1

## Subscription topic for bridging
bridge.mqtt.emqx2.subscription.2.topic = cmd/topic2

## Subscription qos for bridging
bridge.mqtt.emqx2.subscription.2.qos = 1

## Bridging reconnection interval
## Default: 30s
bridge.mqtt.emqx2.reconnect_interval = 30s

## QoS1 message retransmission interval
bridge.mqtt.emqx2.retry_interval = 20s

## Inflight Size.
bridge.mqtt.emqx2.max_inflight_batches = 32

Bridge Cache Configuration

The bridge of EMQ X has a message caching mechanism. The caching mechanism is applicable to both RPC bridging and MQTT bridging. When the bridge is disconnected (such as when the network connection is unstable), the messages with a topic specified in forwards can be cached to the local message queue. Until the bridge is restored, these messages are re-forwarded to the remote node. The configuration of the cache queue is as follows

## emqx_bridge internal number of messages used for batch
bridge.mqtt.emqx2.queue.batch_count_limit = 32

##  emqx_bridge internal number of message bytes used for batch
bridge.mqtt.emqx2.queue.batch_bytes_limit = 1000MB

## The path for placing replayq queue. If it is not specified, then replayq will run in `mem-only` mode and messages will not be cached on disk.
bridge.mqtt.emqx2.queue.replayq_dir = data/emqx_emqx2_bridge/
## Replayq data segment size
bridge.mqtt.emqx2.queue.replayq_seg_bytes = 10MB

bridge.mqtt.emqx2.queue.replayq_dir is a configuration parameter for specifying the path of the bridge storage queue.

bridge.mqtt.emqx2.queue.replayq_seg_bytes is used to specify the size of the largest single file of the message queue that is cached on disk. If the message queue size exceeds the specified value, a new file is created to store the message queue.

CLI for EMQ X Bridge MQTT

CLI for EMQ X Bridge MQTT:

$ cd emqx1/ && ./bin/emqx_ctl bridges
bridges list                                  # List bridges
bridges start <Name>                          # Start a bridge
bridges stop <Name>                           # Stop a bridge
bridges forwards <Name>                       # Show a bridge forward topic
bridges add-forward <Name> <Topic>            # Add bridge forward topic
bridges del-forward <Name> <Topic>            # Delete bridge forward topic
bridges subscriptions <Name>                  # Show a bridge subscriptions topic
bridges add-subscription <Name> <Topic> <Qos> # Add bridge subscriptions topic

List all bridge states

$ ./bin/emqx_ctl bridges list
name: emqx     status: Stopped    $ ./bin/emqx_ctl bridges list
name: emqx     status: Stopped

Start the specified bridge

$ ./bin/emqx_ctl bridges start emqx
Start bridge successfully.

Stop the specified bridge

$ ./bin/emqx_ctl bridges stop emqx
Stop bridge successfully.

List the forwarding topics for the specified bridge

$ ./bin/emqx_ctl bridges forwards emqx
topic:   topic1/#
topic:   topic2/#

Add a forwarding topic for the specified bridge

$ ./bin/emqx_ctl bridges add-forwards emqx topic3/#
Add-forward topic successfully.

Delete the forwarding topic for the specified bridge

$ ./bin/emqx_ctl bridges del-forwards emqx topic3/#
Del-forward topic successfully.

List subscriptions for the specified bridge

$ ./bin/emqx_ctl bridges subscriptions emqx
topic: cmd/topic1, qos: 1
topic: cmd/topic2, qos: 1

Add a subscription topic for the specified bridge

$ ./bin/emqx_ctl bridges add-subscription emqx cmd/topic3 1
Add-subscription topic successfully.

Delete the subscription topic for the specified bridge

$ ./bin/emqx_ctl bridges del-subscription emqx cmd/topic3
Del-subscription topic successfully.

Note: In case of creating multiple bridges, it is convenient to replicate all configuration items of the first bridge, and modify the bridge name and other configuration items if necessary.

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