Introduction

EMQ X 3.0, named "promise of Tomorrow", is a major release.

EMQ X 3.0 is the first release that supports MQTT 5 Protocol standard; meanwhile it is backward compatible with MQTT 3 (3.1 & 3.1.1)

Besides supporting MQTT 5, EMQ X 3.0 comes with more functional features. Performance and stability are also improved significantly after refactoring some core components.

MQTT 5 Protocol Support

The EMQ X 3.0 implements most features of MQTT 5 specification. New MQTT 5 features include:

• New packet type.

  In MQTT 5 there is new packet type AUTH for authentication exchange.

• Session expiry

  Clean session flag in MQTT 3 is now split to Clean Start Flag and a Session Expiry Interval.

• Message expiry

  Allow an expiry interval to be set when a message is published.

• Reason code on all ACKs

  All responding packet includes a reason code. The communication partner can know if a request is successful or failed with what reason.

• Reason string on all ACKs

  An optional reason string to reason code is allowed.

• Server disconnect

  Now server can disconnect a connection.

• Payload format and content type

  Can specify the payload format and a MIME style content type when publishing.

• Request/Response

  Formalized request and response communication pattern.

• Shared subscriptions

  EMQ X 2.x supports shared subscription on single-node as an unstandardized feature. Now in EMQ X 3.0, the shared subscription is cluster-wide.

• Subscription ID

  With a subscription ID the client is able to know the message comes from which subscription.

• Topic alias

  Topic can can have an integer alias, this reduces the communication overhead.

• User properties

  User properties can be added in most packets.

• Maximum packet size

  Broker specified max packet size is implemented in EMQ X 2.x already, When a oversized message is received, it will be dropped, and the client will get disconnected without informed about the reason. Now in EMQ X 3.0, the broker can disconnect the MQTT connection with a reason code.

• Optional Server feature availability

  Define the allowed features of the broker and specify them to the client.

• Subscription options

  Provide subscription options primarily to allow for message bridge applications.

• Will delay

  Allow to specify a delay between end of connection and sending the will message. This allows for brief interruptions of the connection without notification to others.

• Server keep alive

  Server can specify a keepalive value it wishes the client to use.

• Assigned ClientID

  In case where the ClientID is assigned by the server, return the assigned ClientID.

• Server reference

  Allow the server to specify an alternate server to use on CONNACK or DISCONNECT.

Evolved Clustering Architecture

The clustering architecture is evolved. Now a single cluster is able to serve ten-millions of concurrent connections.

----------             ----------
|  EMQX  |<--- MQTT--->|  EMQX  |
|--------|             |--------|
|  Ekka  |<----RPC---->|  Ekka  |
|--------|             |--------|
| Mnesia |<--Cluster-->| Mnesia |
|--------|             |--------|
| Kernel |<----TCP---->| Kernel |
----------             ----------

• Ekka is introduced to auto-cluster EMQX, and to auto-heal the cluster after net-split, following clustering methods are now supported:

  • manual: nodes join a cluster manually;
  • static: auto-clustering from a pre-defined node list;
  • mcast: auto-clustering using IP multicast;
  • dns: auto-clustering using DNS A-records;
  • etcd: auto-clustering using etcd;
  • ks8: auto-clustering using kubernetes.

• A scalable RPC is introduced to mitigate network congestion among nodes to reduce the risk of net-split.

Rate limiting

The rate limiting is introduced to make the broker more resilient, there are 2 kinds of configurations to control:

1. The overall message receiving rate in bytes;
2. The rate of accepting new connections.

Other Feature improvements and Bug Fixes

• Upgraded esockd;
• Switched to cowboy HTTP stack for higher HTTP connection performance;
• Refactored the ACL caching mechanism;
• Added local and remote MQTT bridge;
• Introduced "zone" in to EMQX, different zone can have different configuration;
• Refactored session module, data copy among nodes is reduced, thus higher inter-nodes communication efficiency;
• Improved of OpenLDAP Access Control;
• Added a new plugin for delayed publish;
• Support new statistic and metrics to Prometheus;
• Improved the hooks.