Clustering

Distributed Erlang/OTP

Erlang/OTP is a concurrent, fault-tolerant, distributed programming platform. A distributed Erlang/OTP system consists of a number of Erlang runtime systems called ‘node’. Nodes connect to each other with TCP/IP sockets and communicate by Message Passing.

---------         ---------
| Node1 | --------| Node2 |
---------         ---------
    |     \     /    |
    |       \ /      |
    |       / \      |
    |     /     \    |
---------         ---------
| Node3 | --------| Node4 |
---------         ---------

Node

An erlang runtime system called ‘node’ is identified by a unique name like email addreass. Erlang nodes communicate with each other by the name.

Suppose we start four Erlang nodes on localhost:

erl -name node1@127.0.0.1
erl -name node2@127.0.0.1
erl -name node3@127.0.0.1
erl -name node4@127.0.0.1

connect all the nodes:

(node1@127.0.0.1)1> net_kernel:connect_node('node2@127.0.0.1').
true
(node1@127.0.0.1)2> net_kernel:connect_node('node3@127.0.0.1').
true
(node1@127.0.0.1)3> net_kernel:connect_node('node4@127.0.0.1').
true
(node1@127.0.0.1)4> nodes().
['node2@127.0.0.1','node3@127.0.0.1','node4@127.0.0.1']

epmd

epmd(Erlang Port Mapper Daemon) is a daemon service that is responsible for mapping node names to machine addresses(TCP sockets). The daemon is started automatically on every host where an Erlang node started.

(node1@127.0.0.1)6> net_adm:names().
{ok,[{"node1",62740},
     {"node2",62746},
     {"node3",62877},
     {"node4",62895}]}

Cluster Design

The cluster architecture of emqttd broker is based on distrubuted Erlang/OTP and Mnesia database.

The cluster design could be summarized by the following two rules:

  1. When a MQTT client SUBSCRIBE a Topic on a node, the node will tell all the other nodes in the cluster: I subscribed a Topic.
  2. When a MQTT Client PUBLISH a message to a node, the node will lookup the Topic table and forward the message to nodes that subscribed the Topic.

Finally there will be a global route table(Topic -> Node) that replicated to all nodes in the cluster:

topic1 -> node1, node2
topic2 -> node3
topic3 -> node2, node4

Topic Trie and Route Table

Every node in the cluster will store a topic trie and route table in mnesia database.

Suppose that we create subscriptions:

Client Node Topics
client1 node1 t/+/x, t/+/y
client2 node2 t/#
client3 node3 t/+/x, t/a

Finally the topic trie and route table in the cluster:

--------------------------
|          t             |
|         / \            |
|        +   #           |
|      /  \              |
|    x      y            |
--------------------------
| t/+/x -> node1, node3  |
| t/+/y -> node1         |
| t/#   -> node2         |
| t/a   -> node3         |
--------------------------

Message Route and Deliver

The brokers in the cluster route messages by topic trie and route table, deliver messages to MQTT clients by subscriptions. Subscriptions are mapping from topic to subscribers, are stored only in the local node, will not be replicated to other nodes.

Suppose client1 PUBLISH a message to the topic ‘t/a’, the message Route and Deliver process:

title: Message Route and Deliver

client1->node1: Publish[t/a]
node1-->node2: Route[t/#]
node1-->node3: Route[t/a]
node2-->client2: Deliver[t/#]
node3-->client3: Deliver[t/a]
_images/route.png

Cluster Setup

Suppose we deploy two nodes cluster on s1.emqtt.io, s2.emqtt.io:

Node Host(FQDN) IP and Port
emqttd@s1.emqtt.io or emqttd@192.168.0.10 s1.emqtt.io 192.168.0.10:1883
emqttd@s2.emqtt.io or emqttd@192.168.0.20 s2.emqtt.io 192.168.0.20:1883

Warning

The node name is Name@Host, where Host is IP address or the fully qualified host name.

emqttd@s1.emqtt.io setting

emqttd/etc/emq.conf:

node.name = emqttd@s1.emqtt.io

或

node.name = emqttd@192.168.0.10

Warning

The name cannot be changed after node joined the cluster.

emqttd@s2.emqtt.io setting

emqttd/etc/emq.conf:

node.name = emqttd@s2.emqtt.io

或

node.name = emqttd@192.168.0.20

Join the cluster

Start the two broker nodes, and ‘cluster join ‘ on emqttd@s2.emqtt.io:

$ ./bin/emqttd_ctl cluster join emqttd@s1.emqtt.io

Join the cluster successfully.
Cluster status: [{running_nodes,['emqttd@s1.emqtt.io','emqttd@s2.emqtt.io']}]

Or ‘cluster join’ on emqttd@s1.emqtt.io:

$ ./bin/emqttd_ctl cluster join emqttd@s2.emqtt.io

Join the cluster successfully.
Cluster status: [{running_nodes,['emqttd@s1.emqtt.io','emqttd@s2.emqtt.io']}]

Query the cluster status:

$ ./bin/emqttd_ctl cluster status

Cluster status: [{running_nodes,['emqttd@s1.emqtt.io','emqttd@s2.emqtt.io']}]

Leave the cluster

Two ways to leave the cluster:

  1. leave: this node leaves the cluster
  2. remove: remove other nodes from the cluster

emqttd@s2.emqtt.io node tries to leave the cluster:

$ ./bin/emqttd_ctl cluster leave

Or remove emqttd@s2.emqtt.io node from the cluster on emqttd@s1.emqtt.io:

$ ./bin/emqttd_ctl cluster remove emqttd@s2.emqtt.io

Session across Nodes

The persistent MQTT sessions (clean session = false) are across nodes in the cluster.

If a persistent MQTT client connected to node1 first, then disconnected and connects to node2, the MQTT connection and session will be located on different nodes:

                                 node1
                              -----------
                          |-->| session |
                          |   -----------
            node2         |
         --------------   |
client-->| connection |<--|
         --------------

The Firewall

If there is a firewall between clustered nodes, the cluster requires to open 4369 port used by epmd daemon, and a port segment for nodes’ communication.

Configure the port segment in releases/2.0/sys.config, for example:

[{kernel, [
    ...
    {inet_dist_listen_min, 20000},
    {inet_dist_listen_max, 21000}
 ]},
 ...

Network Partitions

The emqttd 1.0 cluster requires reliable network to avoid network partitions. The cluster will not recover from a network partition automatically.

If a network partition occures, there will be critical logs in log/emqttd_error.log:

Mnesia inconsistent_database event: running_partitioned_network, emqttd@host

To recover from a network partition, you have to stop the nodes in a partition, clean the ‘data/mneisa’ of these nodes and reboot to join the cluster again.

Consistent Hash and DHT

Consistent Hash and DHT are popular in the design of NoSQL databases. Cluster of emqttd broker could support 10 million size of global routing table now. We could use the Consistent Hash or DHT to partition the routing table, and evolve the cluster to larger size.