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.

Node¶

An erlang runtime system called 'node' is identified by a unique name like email address. 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}]}

Distribution Protocol¶

Erlang nodes can be connected via different distributed protocols including TCPv4, TCPv6 and TLS.

## Specify the erlang distributed protocol.
##
## Value: Enum
##  - inet_tcp: the default; handles TCP streams with IPv4 addressing.
##  - inet6_tcp: handles TCP with IPv6 addressing.
##  - inet_tls: using TLS for Erlang Distribution.
##
## vm.args: -proto_dist inet_tcp
node.proto_dist = inet_tcp

## Specify SSL Options in the file if using SSL for Erlang Distribution.
##
## Value: File
##
## vm.args: -ssl_dist_optfile <File>
## node.ssl_dist_optfile = {{ platform_etc_dir }}/ssl_dist.conf

Cluster Design¶

The cluster architecture of EMQ X broker is based on distributed Erlang/OTP and Mnesia database.

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

When a MQTT client SUBSCRIBE a Topic on a node, the node will tell all the other nodes in the cluster: I have a client subscribed to a Topic.
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 to the Topic.

Finally there will be a global route table(Topic -> Node) that is 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 following subscriptions:

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

The topic trie and route table in the cluster will be:

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]

Cluster Setup¶

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

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

注解

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

emqx@s1.emqx.io config¶

etc/emqx.conf:

node.name = emqx@s1.emqx.io

or

node.name = emqx@192.168.0.10

警告

The name cannot be changed after node joined the cluster.

emqx@s2.emqx.io config¶

etc/emqx.conf:

node.name = emqx@s2.emqx.io

or

node.name = emqx@192.168.0.20

Join the cluster¶

Start the two broker nodes, and 'cluster join ' on emqx@s2.emqx.io:

$ ./bin/emqx_ctl cluster join emqx@s1.emqx.io

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

Or 'cluster join' on emqx@s1.emqx.io:

$ ./bin/emqx_ctl cluster join emqx@s2.emqx.io

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

Query the cluster status:

$ ./bin/emqx_ctl cluster status

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

Leave the cluster¶

Two ways to leave the cluster:

leave: this node leaves the cluster
remove: remove other nodes from the cluster

emqx@s2.emqx.io node tries to leave the cluster:

$ ./bin/emqx_ctl cluster leave

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

$ ./bin/emqx_ctl cluster remove emqx@s2.emqx.io

Node Discovery and Autocluster¶

EMQ X 3.0 supports node discovery and autocluster with various strategies:

Strategy	Description
static	Autocluster by static node list
mcast	Autocluster by UDP Multicast
dns	Autocluster by DNS A Record
etcd	Autocluster using etcd
k8s	Autocluster on Kubernetes

Autocluster by static node list¶

cluster.discovery = static

##--------------------------------------------------------------------
## Cluster with static node list

cluster.static.seeds = emq1@127.0.0.1,ekka2@127.0.0.1

Autocluster by IP Multicast¶

cluster.discovery = mcast

##--------------------------------------------------------------------
## Cluster with multicast

cluster.mcast.addr = 239.192.0.1

cluster.mcast.ports = 4369,4370

cluster.mcast.iface = 0.0.0.0

cluster.mcast.ttl = 255

cluster.mcast.loop = on

Autocluster by DNS A Record¶

cluster.discovery = dns

##--------------------------------------------------------------------
## Cluster with DNS

cluster.dns.name = localhost

cluster.dns.app  = ekka

Autocluster using etcd¶

cluster.discovery = etcd

##--------------------------------------------------------------------
## Cluster with Etcd

cluster.etcd.server = http://127.0.0.1:2379

cluster.etcd.prefix = emqcl

cluster.etcd.node_ttl = 1m

Autocluster on Kubernetes¶

cluster.discovery = k8s

##--------------------------------------------------------------------
## Cluster with k8s

cluster.k8s.apiserver = http://10.110.111.204:8080

cluster.k8s.service_name = ekka

## Address Type: ip | dns
cluster.k8s.address_type = ip

## The Erlang application name
cluster.k8s.app_name = ekka

Network Partition and Autoheal¶

Enable autoheal of Network Partition:

cluster.autoheal = on

When network partition occurs, the following steps are performed to heal the cluster if autoheal is enabled:

Node reports the partitions to a leader node which has the oldest guid.
Leader node create a global netsplit view and choose one node in the majority as coordinator.
Leader node requests the coordinator to autoheal the network partition.
Coordinator node reboots all the nodes in the minority side.

Node down and Autoclean¶

A down node will be removed from the cluster if autoclean is enabled:

cluster.autoclean = 5m

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:

_static/images/10_3.png

The Firewall¶

If the nodes need to go through a Firewall, TCP port 4369 must be allowed for epmd, as well as a sequential range of TCP ports for communication between the distributed nodes.

That range of ports for erlang distribution is configured in etc/emqx.conf, defaults to 6369-7369:

## Distributed node port range
node.dist_listen_min = 6369
node.dist_listen_max = 7369
...

So by default, make sure TCP ports 4369 and 6369-7369 are allowed by your Firewall roles.

Consistent Hash and DHT¶

Consistent Hash and DHT are popular in the design of NoSQL databases. Cluster of EMQ X 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.