# Use Guide

# How to use EMQX?

EMQX Broker is free and it can be download at https://www.emqx.com/en/try?product=broker (opens new window).

EMQX Enterprise can be downloaded and evaluated for free. You can download it from https://www.emqx.com/en/try?product=enterprise (opens new window), and then apply trial license at https://www.emqx.com/en/apply-licenses/emqx (opens new window).

Also you can use the EMQX enterprise version through public cloud service.

# How to update EMQX license?

Tags: Update

You need to two steps:

  1. The license file will be sent by email, find the attached zip file and unzip it.

  2. Extract the license file (emqx.lic) from the zip file to a directory readable by the EMQX user.

After the extraction is complete, the license needs to be reloaded from the command line to complete the update:

emqx_ctl license reload [license file path]

The update commands for different installation modes:

## zip packages
./bin/emqx_ctl license reload path/to/emqx.lic

emqx_ctl license reload path/to/emqx.lic

## Docker
docker exec -it emqx-ee emqx_ctl license reload path/to/emqx.lic


On a multi-node cluster, the emqx_ctl license reload command needs to be executed only on one of the nodes, as the license will be replicated and applied to all members. Each one will contain a copy of the new license under the configured data directory for EMQX, as well as a backup of the old license, if any.

Note that this command only takes effect on the local node executing the command for EMQX versions prior to e4.3.10, so this command will require being executed on each node of the cluster for those older versions.

# Can EMQX support customized protocols? How to implement?

Tags: Protocol Extends


# Can I capture device online and offline events? How to use it?

Tags: WebHook System Topic

EMQX supports to capture device online and offline events through below 3 approaches,

  • Web Hook

  • Subscribe related $SYS topics

    • $SYS/brokers/${node}/clients/${clientid}/connected
    • $SYS/brokers/${node}/clients/${clientid}/disconnected
  • Directly save events into database

    The final approach is only supported in enterprise version, and supported database includes Redis, MySQL, PostgreSQL, MongoDB and Cassandra. User can configure database, client.connected and client.disconnected events in the configuration file. When a device is online or offline, the information will be saved into database.

# I want to control topics can be used for specific clients, how to configure it in EMQX?

Tags: ACL Pub/Sub

EMQX can constrain clients used topics to realize device access controls. To use this feature, ACL (Access Control List) should be enabled, disable anonymous access and set acl_nomatch to 'deny' (For the convenience of debugging, the last 2 options are enabled by default, and please close them).

ACL can be configured in configuration file, or backend databases. Below is one of sample line for ACL control file, the meaning is user 'dashboard' can subscribe '$SYS/#' topic. ACL configuration in backend databases is similar, refer to EMQX document for more detailed configurations.

{allow, {user, "dashboard"}, subscribe, ["$SYS/#"]}.

# Can EMQX support traffic control?

Tags: Rate Limit

Yes. Currently EMQX supports to control connection rate and message publish rate. Refer to below for sample configuration.

## Value: Number
listener.tcp.external.max_conn_rate = 1000

## Value: rate,burst
listener.tcp.external.rate_limit = 1024,4096

# How does the EMQX achieve high concurrency and high availability?

Tags: Performance

High concurrency and availability are design goals of EMQX. To achieve these goals, several technologies are applied:

  • Making maximum use of the soft-realtime, high concurrent and fault-tolerant Erlang/OTP platform;
  • Full asynchronous architecture;
  • Layered design of connection, session, route and cluster;
  • Separated messaging and control panel;

With the well design and implementation, a single EMQX cluster can handle million level connections.

EMQX supports clustering. The EMQX performance can be scale-out with the increased number of nodes in cluster, and the MQTT service will not be interrupted when a single node is down.

# Can EMQX store messages to database?

Tags: Persistence

The EMQX Enterprise supports data persistence. Supported databases are:

  • Redis
  • MongoDB
  • MySQL
  • PostgreSQL
  • Cassandra

# Can I disconnect an MQTT connection from EMQX server?

Tags: REAT API Dashboard

Yes. You can do it by invoking REST API provided by EMQX, but the implementation is different in EMQX 2.x and 3.x:

  • EMQX customized protocol in 2.x versions.
  • Follow the process defined in MQTT 5.0 protocol after version 3.0.

Refer to below for API invocation:

<!--Please notice the 2nd section in URL, and use the correct version number according to your EMQX version. -->

Returned response:
    "code": 0,
    "result": []

# Can EMQX forward messages to Kafka?

Tags: Kafka Bridge Persistence

The EMQX Enterprise integrates a Kafka bridge, it can bridge data to Kafka.

# I use Kafka bridge in EMQX enterprise, when will the MQTT Ack packet sent back to client? Is the time when message arriving EMQX or after getting Ack message from Kafka?

Tags: Kafka

It's up to Kafka bridge configuration, the configuration file is at /etc/emqx/plugins/emqx_bridge_kafka.conf

## Pick a partition producer and sync/async.
bridge.kafka.produce = sync
  • Sync: MQTT Ack packet will be sent back to client after receiving Ack from Kafka.
  • Async: MQTT Ack packet will be sent back to client right after EMQX receiving the message, and EMQX will not wait the Ack returned from Kafka.

If the backend Kafka server is not available, then the message will be accumulated in EMQX broker.

  • The message will be cached in memory before EMQX 2.4.3 version, if the memory is exhausted, then the EMQX server will be down.
  • The message will be cached in disk after EMQX 2.4.3 version, message will probably lost if the disk is full.

So we suggest you to closely monitor Kafka server, and recover Kafka service as soon as possible when it has any questions.

# Does EMQX support cluster auto discovery? What clustering methods are supported?

Tags: Cluster

EMQX supports cluster auto discovery. EMQX clustering can be done manually or automatically.

Currently supported clustering methods:

  • Manual clustering
  • Static clustering
  • Auto clustering using IP multi-cast
  • Auto clustering using DNS
  • Auto clustering using ETCD
  • Auto clustering using K8S

# Can I forward MQTT messages EMQX to other MQTT broker, like RabbitMQ?

Tags: RabbitMQ Bridge Persistence

EMQX support forward messages to other MQTT broker. Using MQTT bridge, EMQX can forward messages of interested topics to other broker.

# Can I forward messages from EMQX to MQTT services hosted on public cloud?

Tags: Bridge Cloud

EMQX can forward messages to IoT Hub hosted on public cloud, this is a feature of EMQX bridge.

# Can other MQTT broker (for example Mosquitto) forward messages to EMQX?

Tags: Mosquitto Bridge

EMQX can receive messages from other broker, but it depends also on the implementation of other brokers, Mosquitto can forward messages to EMQX, please refer to TODO (opens new window)

# What should I do if I want trace the subscription and publish of some particular message?

Tags: Trace

EMQX support the tracing of messages from particular client or under particular topic. You can use the command line tool emqx_ctl for tracing. The example below shows how to trace messages under 'topic' and save the result in 'trace_topic.log'. For more details, please refer to EMQX document.

./bin/emqx_ctl trace topic "topic" "trace_topic.log"

Tags: TLS

EMQX Support SSL/TLS. In production, we recommend to terminate the TLS connection by Load Balancer. By this way, the connection between device and server(load balancer) use secured connection, and connection between load balancer and EMQX nodes use general TCP connection.

# How to troubleshoot if EMQX can't start after installation?

Tags: Debug

Execute $ emqx console to view the output.

  • logger command is missing

    $ emqx console
    Exec: /usr/lib/emqx/erts- -boot /usr/lib/emqx/releases/v3.2.1/emqx -mode embedded -boot_var ERTS_LIB_DIR /usr/lib/emqx/erts- -mnesia dir "/var/lib/emqx/mnesia/emqx@" -config /var/lib/emqx/configs/app.2019. -args_file /var/lib/emqx/configs/vm.2019. -vm_args /var/lib/emqx/configs/vm.2019. -- console
    Root: /usr/lib/emqx
    /usr/bin/emqx: line 510: logger: command not found


    • Centos/Redhat

      $ yum install rsyslog
    • Ubuntu/Debian

      $ apt-get install bsdutils
  • Missssl is missing

    $ emqx console
    Exec: /emqx/erts-10.3/bin/erlexec -boot /emqx/releases/v3.2.1/emqx -mode embedded -boot_var ERTS_LIB_DIR /emqx/erts-10.3/../lib -mnesia dir "/emqx/data/mnesia/emqx@" -config /emqx/data/configs/app.2019. -args_file /emqx/data/configs/vm.2019. -vm_args /emqx/data/configs/vm.2019. -- console
    Root: /emqx
    Erlang/OTP 21 [erts-10.3] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:32] [hipe]

    {"Kernel pid terminated",application_controller,"{application_start_failure,kernel,{{shutdown,{failed_to_start_child,kernel_safe_sup,{on_load_function_failed,crypto}}},{kernel,start,[normal,[]]}}}"}
    Kernel pid terminated (application_controller) ({application_start_failure,kernel,{{shutdown,{failed_to_start_child,kernel_safe_sup,{on_load_function_failed,crypto}}},{kernel,start,[normal,[]]}}})

    Crash dump is being written to: log/crash.dump...done

Solution: Install openssl above version 1.1.1

  • License file is missing
  $ emqx console
  Exec: /usr/lib/emqx/erts- -boot /usr/lib/emqx/releases/v3.2.1/emqx -mode embedded -boot_var ERTS_LIB_DIR /usr/lib/emqx/erts- -mnesia dir "/var/lib/emqx/mnesia/emqx@" -config /var/lib/emqx/configs/app.2019. -args_file /var/lib/emqx/configs/vm.2019. -vm_args /var/lib/emqx/configs/vm.2019. -- console
  Root: /usr/lib/emqx
  Erlang/OTP 21 [erts-] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:32] [hipe]

  Starting emqx on node emqx@
  Start http:management listener on 8080 successfully.
  Start http:dashboard listener on 18083 successfully.
  Start mqtt:tcp listener on successfully.
  Start mqtt:tcp listener on successfully.
  Start mqtt:ws listener on successfully.
  Start mqtt:ssl listener on successfully.
  Start mqtt:wss listener on successfully.
  EMQX Broker 3.2.1 is running now!
  "The license certificate is expired!"
  2019-07-23 05:52:51.355 [critical] The license certificate is expired!
  2019-07-23 05:52:51.355 [critical] The license certificate is expired! System shutdown!
  Stop mqtt:tcp listener on successfully.
  Stop mqtt:tcp listener on successfully.
  Stop mqtt:ws listener on successfully.
  Stop mqtt:ssl listener on successfully.
  Stop mqtt:wss listener on successfully.
  [os_mon] memory supervisor port (memsup): Erlang has closed
  [os_mon] cpu supervisor port (cpu_sup): Erlang has closed

Solution: Go to emqx.io (opens new window) to apply for a license or install the open source version of EMQX Broker

# Use of ssl resumption session in EMQX

Tags: Performance

Modify the reuse_sessions = on in the emqx.conf configuration and take effect. If the client and the server are successfully connected through SSL, when the client connection is encountered for the second time, the SSL handshake phase is skipped, the connection is directly established to save the connection time and increase the client connection speed.

# MQTT client disconnect statistics

Tags: Metrics

Execute emqx_ctl listeners to view the shutdown_count statistics under the corresponding port.

Client disconnect link error code list:

  • keepalive_timeout:MQTT keepalive time out

  • closed: TCP client disconnected (the FIN sent by the client did not receive the MQTT DISCONNECT)

  • normal: MQTT client is normally disconnected

  • einval:EMQX wants to send a message to the client, but the Socket has been disconnected

  • function_clause:MQTT packet format error

  • etimedout:TCP Send timeout (no TCP ACK response received)

  • proto_unexpected_c:Repeatedly received an MQTT connection request when there is already an MQTT connection

  • idle_timeout: After the TCP connection is established for 15s, the connect packet has not been received yet.

# Can EMQX guarantee the original order when forwarding messages to subscribers?

Tags: Pub/Sub

EMQX ensures that messages with the same topic from the same client are forwarded in the order they were received, regardless of the QoS level. The message forwarding order remains consistent regardless of message loss or duplication, as per MQTT requirements.

However, EMQX does not guarantee the forwarding order of messages from different topics. These messages can be considered as entering separate channels. For example, if messages from topic A arrive at EMQX before messages from topic B, it is possible that messages from topic B will be forwarded earlier.

Tags: Debug

EMQX's debug logs already capture all the behaviors and phenomena. By viewing the debug logs, we can determine when the client initiated the connection, the parameters specified during the connection, the success of rejection of the connection, and the reasons for rejection, among other details. However, the extensive information logged in debug mode can consume additional resources and make it challenging to analyze individual clients or topics.

To address this, EMQX provides a Log Trace feature. We can specify the clients or topics we want to trace, and EMQX will output all the debug logs related to those clients or topics to the designated log file. This facilitates self-analysis and seeking assistance from the community.

It's important to note that if the client cannot establish a connection with EMQX due to network issues, the log tracing feature will not be useful since EMQX does not receive any messages in such cases. This situation often arises from network configuration problems like firewalls or security groups, resulting in closed server ports. This is particularly common when deploying EMQX on cloud instances. Therefore, in addition to log tracing, troubleshooting network-related issues involves checking port occupation, listening status, and network configurations.

# Why are there client IDs like "CENSYS" or other unfamiliar clients?

Tags: Security

CENSYS is an internet scanning and reconnaissance tool that performs regular scans of the IPv4 address space to identify default ports for various protocols such as HTTP, SSH, MQTT, and etc. Therefore, if you notice MQTT clients with a client ID of "CENSYS" or other unfamiliar clients accessing your MQTT broker, it indicates a relatively lower level of security protection. To address this issue effectively, consider implementing the following measures:

  1. Avoid using default configurations, such as the AppID and AppSecret used for verifying HTTP API access permissions in EMQX.
  2. Enable authentication mechanisms like password-based authentication or JWT authentication to prevent unauthorized access where only knowledge of an IP address is sufficient for login.
  3. Enable TLS mutual authentication to allow access only to clients with valid certificates.
  4. Enable proper authorization mechanisms to restrict access to sensitive data for unauthorized devices.
  5. Configure your firewall to close unnecessary ports as much as possible.

# Can I subscribe to system topics using shared subscriptions?

Tags: Shared Subscription

Yes, it is possible to use shared subscriptions to subscribe to certain system messages, such as client online/offline events, which are published frequently. Shared subscriptions are particularly useful for clients in such cases. For instance, you can subscribe to the following topic using a shared subscription: $share/group1/$SYS/brokers/+/clients/+/connected.

# Why can't I receive retained messages when using shared subscriptions?

Tags: Shared Subscription

According to the MQTT protocol, when a client uses a shared subscription, the server is not allowed to send retained messages to that client.

# Why do messages sometimes get lost when using shared subscriptions?

Tags: Shared Subscription

When a shared subscriber's connection is disconnected but the session remains active, the server continues to deliver messages to the subscriber, which are temporarily stored in the session. As a result, other active shared subscribers may appear as if they have not consumed all the messages. In addition, if the shared subscriber chooses to create a new session when reconnecting, the messages cached in the old session will be permanently lost.

If you have verified that the aforementioned situation does not occur, yet the issue of message loss persists, you can use the client tracking feature of EMQX to conduct further investigation.

# What should I do if EMQX prompts that the port is occupied (eaddrinuse) when starting?

Tags: Failed to Start

By default, EMQX will occupy 7 ports when it starts. They are:

  1. Port 1883, used for MQTT over TCP listener. It can be modified through configuration.
  2. Port 8883, used for MQTT over SSL/TLS listener. It can be modified through configuration.
  3. Port 8083, used for MQTT over WebSocket listener. It can be modified through configuration.
  4. Port 8084, used for MQTT over WSS (WebSocket over SSL) listener. It can be modified through configuration.
  5. Port 18083, the default listening port for the HTTP API service. The dashboard also relies on this port, which can be modified through configuration.
  6. Port 4370, used for remote function calls in the EMQX distributed cluster and Mnesia data synchronization. This port is occupied by default, even if no cluster is formed. The listening port is determined by BasePort (4370) + Offset, where 4370 is fixed and cannot be modified, and Offset is determined by the numeric suffix in the node name (Name@Host). If there is no numeric suffix, it defaults to 0. For example, the Offset for emqx@ is 0, and the Offset for emqx1@ is 1.
  7. Port 5370, the cluster RPC port used for load sharing. It is mainly used for forwarding MQTT messages between nodes. Similar to port 4370, this port is occupied by default, even if no cluster is formed. The actual listening port is BasePort (5370) + Offset, where 5370 is fixed and cannot be modified, and Offset is determined by the Name part of the node name (Name@Host). If there is no numeric suffix, it defaults to 0.

Tags: Log

The complete WARNING log is as follows:

WARNING: Default (insecure) Erlang cookie is in use.
WARNING: Configure node.cookie in /usr/lib/emqx/etc/emqx.conf or override from environment variable EMQX_NODE__COOKIE
WARNING: NOTE: Use the same cookie for all nodes in the cluster.

Only EMQX nodes using the same cookie can form a cluster. While a cookie does not secure cluster communication, it prevents a node from connecting to a cluster it did not intend to communicate with. By default, EMQX nodes uniformly use the cookie value emqxsecretcookie. However, we recommend that users change the cookie value when building a cluster to enhance security.

The second warning log indicates two ways to modify the cookie: by editting node.cookie in the emqx.conf configuration file or by setting the environment variable EMQX_NODE__COOKIE.

# Why does restarting the EMQX Docker container cause data loss, such as configured rules and resources?

Tags: Persistence

The runtime data of EMQX is stored in the /opt/emqx/data directory, including configuration rules, resources, retained messages, etc. To ensure data persistence during container restarts, it's important to mount the /opt/emqx/data directory to a local host directory or a data volume.

However, even if the /opt/emqx/data directory is properly mounted, data loss may still occur after container restarts. This is because the runtime data of EMQX is stored in the /opt/emqx/data/mnesia/${Node Name} directory, and when the container is restarted, the node name of EMQX changes, leading to the creation of a new storage directory.

EMQX node name consists of Name and Host, with the Host derived from the container's IP address by default. Under the default network configurations, the container's IP may change upon restarting, so you need to maintain a fixed IP for the container.

To address this issue, EMQX provides an environment variable, EMQX_HOST, which allows you to set the Host part of the node name. However, it is crucial that this Host value is reachable by other nodes, so it should be used in conjunction with a network alias. Here is an example command for running the EMQX Docker container with the EMQX_HOST environment variable and a network alias:

docker run -d --name emqx -p 18083:18083 -p 1883:1883 -e EMQX_HOST=alias-for-emqx --network example --network-alias alias-for-emqx --mount type=bind,source=/tmp/emqx,target=/opt/emqx/data emqx:5.0.24

# Common Log Reasons in EMQX

Tags: Log

# Keyword: “reason: {shutdown, takeovered}”

In EMQX, each MQTT client connection is maintained by an independent process. When a client connects, EMQX creates a new process. If the client wants to resume communication from an existing session state, EMQX needs to migrate the session state from the old process to the new one. This process is called "takeover," and the old process is closed after the takeover is completed with the reason {shutdown, takeovered}. If the original client is still connected, EMQX sends a DISCONNECT packet with Reason Code 0x8E (Session taken over) before closing the old connection.

# Keyword: “reason: {shutdown, discarded}”

In contrast to session takeover, if a client indicates that it wants to start a new session upon connecting, EMQX needs to discard the old session state. EMQX closes the old process with the reason {shutdown, discarded}. If the original client is still connected, EMQX sends a DISCONNECT packet with Reason Code 0x8E (Session taken over) before closing the old connection.

# Keyword: “reason: {shutdown, kicked}”

Indicates that the client connection was manually kicked out, either by clicking the "Kick Out" button on the Dashboard or by calling the DELETE clients/{clientid} API. If the original client is still connected, EMQX sends a DISCONNECT packet with Reason Code 0x98 (Administrative action) before closing the old connection.

# Keyword: “reason: {shutdown, tcp_closed}”

This log indicates that the client directly closed the network connection without sending a DISCONNECT packet. The corresponding reason would be {shutdown, normal}, which means that the client first sent a DISCONNECT packet with a Reason Code of 0 and then closed the network connection.

If you encounter the tcp_closed keyword in the EMQX logs and this disconnection behavior is not expected, it is recommended to investigate the client's implementation for any potential issues.

# Keyword: “maximum heap size reached”

This log appears when the heap occupied by the current client process exceeds the preset maximum value. The client process will be forcibly terminated to ensure EMQX's availability and to prevent excessive memory usage leading to EMQX being Out of Memory (OOM).

Usually, message accumulation is the main cause of increased heap occupation, often due to the client's consumption capacity not matching the peer's message production capacity. It is recommended to optimize the client processing code or use shared subscriptions to distribute the load.

The related configuration item is force_shutdown_policy, with the format <Maximum Message Queue Length>|<Maximum Heap Size>, for example, 10000|64MB. <Maximum Heap Size> refers to the maximum heap memory that each client process can occupy.

# Keyword: “Parse failed for function_clause”

This log indicates a failure in packet parsing. It may occur when a non-MQTT packet is sent to the MQTT port, or when the packet contains non-UTF-8 characters. Searching for the Frame data keyword in the Parse failed... log can provide the complete message and help analyze the possible reasons for the parsing failure.

# Keyword: “Dropped msg due to mqueue is full”

EMQX can send multiple unconfirmed QoS 1 and QoS 2 messages simultaneously, but limited by the performance of the client, we usually limit the maximum number of messages in flight. After reaching this limit, subsequent incoming messages will be cached by EMQX in the message queue of each client process.

However, the message queus still have a length limit. If the message queue is full when a new message arrives, the most recent message will be enqueued, but the oldest message in the queue will be dequeued and dropped. EMQX generates the log message: "Dropped msg due to mqueue is full."

If this log only appears during traffic peaks, you can increase the maximum length of the message queue by modifying the max_mqueue_len configuration item. However, if it continues to appear, it indicates poor client consumption capability. In such cases, it is recommended to optimize the client's code or use shared subscriptions to distribute the load.

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