Cluster

Distributed Erlang

Erlang / OTP was originally a programming language platform designed by Ericsson for the development of telecommunication equipment systems. Telecommunication equipment (routers, access gateways) is typically a distributed system that connects the main control board and multiple business boards through the backplane.

Nodes and distributed Erlang

The distributed programs of the Erlang / OTP language platform are composed of distributed interconnected Erlang runtime systems. Each Erlang runtime system is called a node. Nodes are interconnected by TCP to form a network structure.

Erlang nodes are identified by a unique node name, which consists of two parts separated by @:

<name>@<ip-address>

Communication between nodes is addressed by node name. For example, start four shell terminals locally, and then use the -name parameter to start four Erlang nodes respectively:

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

node (). can be used to view the name of this node, and nodes (). can be used to view other nodes that have established a connection with the current node. We now go to the console of ‘node1@127.0.0.1’ and check the current node name and connected nodes:

(node1@127.0.0.1) 4> node().
'node1@127.0.0.1'
(node1@127.0.0.1) 4> nodes().
[]

Then we let node1 initiate connections with other 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

Now we can check other nodes that are already connected to node1:

(node1@127.0.0.1) 4> nodes().
['node2@127.0.0.1','node3@127.0.0.1','node4@127.0.0.1']

We can see that node2, node3, and node4 have established a distributed connection with node1, and these four nodes form a cluster. Note that whenever a new node joins the cluster, it will establish a TCP connection with all the nodes in the cluster. At this point, the four nodes have completed the mesh structure shown in the following figure:

Security

Cookies are used for interconnection authentication between Erlang nodes. A cookie is a string, and only two nodes with the same cookie can establish a connection. In the Previous section, We used the -setcookie my_nodes parameter to set the same cookie of my_nodes to four nodes.

See http://erlang.org/doc/reference_manual/distributed.html (opens new window) for details.

EMQ X Broker Cluster protocol settings

Each node in the Erlang cluster can be connected through TCPv4, TCPv6 or TLS, and the connection method can be configured inetc/emqx.conf:

EMQ X Broker Distributed cluster design

The basic function of EMQ X Broker distribution is to forward and publish messages to subscribers on each node, as shown in the following figure:

To achieve this, EMQ X Broker maintains several data structures related to it: subscription tables, routing tables, and topic trees.

Subscription Table: Topics-Subscribers

When an MQTT client subscribes to a topic, EMQ X Broker maintains a Subscription Table for the Topic-> Subscriber mapping. The subscription table only exists on the EMQ X Broker node where the subscriber is located, for example:

node1:
    topic1 -> client1, client2
    topic2 -> client3
node2:
    topic1 -> client4

Route Table: Topic-Node

All nodes in the same cluster will copy a topic-to-> node mapping table, for example:

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

Topic tree: topic matching with wildcards

In addition to the routing table, each node in the EMQ X Broker cluster also maintains a backup of the Topic Trie.

The following topic-subscription relationship is an example:

When all subscriptions are completed, EMQ X Broker maintains the following Topic Trie and Route Table:

Message Distribution Process

When an MQTT client publishes a message, the node where it is located retrieves the route table and forwards the message to the relevant node according to the message topic, and then the relevant node retrieves the local subscription table and sends the message to the relevant subscriber.

For example, when client1 publishes a message to the topic t/a. The routing and distribution of the message between nodes are as follows:

1. client1 publishes a message with the topic t/a to the node1
2. By querying the topic tree, node1 learns that t/a can match the two existing topics of t/a and t/#.
3. By querying the route table, node1 learns that topic t/a has subscribers only on node3, and topict/#has subscribers only on node2. So node1 forwards the message to node2 and node3.
4. After node2 receives the forwarded t/a message, it queries the local subscription table to obtain the subscribers who have subscribed to t/# on this node and distributes the message to them.
5. After node3 receives the forwarded t/a message, it queries the local subscription table to obtain the subscribers who have subscribed to t/a on this node and distributes the message to them.
6. Message forwarding and distribution are finished.

Data partition and sharing

EMQ X Broker’s subscription table is partitioned in the cluster, while the topic tree and routing table are replicated.

Node discovery and automatic clustering

EMQ X Broker supports Autocluster based on Ekka library. Ekka is a cluster management library developed for Erlang / OTP applications. It supports Service Discovery, Autocluster, Network Partition Autoheal, and Autoclean of Erlang node.

EMQ X supports multiple node discovery strategies:

Creating a cluster manually

The default configuration is to manually create a cluster. Nodes should be added via the command of ./bin/emqx_ctl join \ <Node >:

cluster.discovery = manual

Autocluster based on static node list

Configure a fixed node list to automatically discover and create clusters:

cluster.discovery = static
cluster.static.seeds = emqx1@127.0.0.1,emqx2@127.0.0.1

Autocluster based on mcast

Automatically discover and create clusters based on UDP multicast:

cluster.discovery = mcast
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 based on DNS A records

Automatically discover and create clusters based on DNS A records:

cluster.discovery = dns
cluster.dns.name = localhost
cluster.dns.app  = ekka

Autocluster based on etcd

Automatically discover and create clusters based on etcd (opens new window):

cluster.discovery = etcd
cluster.etcd.server = http://127.0.0.1:2379
cluster.etcd.prefix = emqcl
cluster.etcd.node_ttl = 1m

Autocluster based on kubernetes

Automatically discover and create clusters based on Kubernetes (opens new window):

cluster.discovery = k8s
cluster.k8s.apiserver = http://10.110.111.204:8080
cluster.k8s.service_name = ekka
cluster.k8s.address_type = ip
cluster.k8s.app_name = ekka

Introduction to manual cluster management

Deploy EMQ X Broker cluster on two servers of s1.emqx.io, s2.emqx.io:

Tip

The format of node name isName@Host, and Host must be an IP address or FQDN (server name. Domain name)

Configure emqx@s1.emqx.io node

emqx/etc/emqx.conf:

node.name = emqx@s1.emqx.io
# or
node.name = emqx@192.168.0.10

Configure through environment variables:

export EMQX_NODE_NAME=emqx@s1.emqx.io && ./bin/emqx start

Tip

After a node starts to join the cluster, the node name cannot be changed.

Configure emqx@s2.emqx.io node

emqx/etc/emqx.conf:

node.name = emqx@s2.emqx.io
# or
node.name = emqx@192.168.0.20

Node joins the cluster

After starting two nodes, the join can be executed on 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 executed on 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 on any node:

$ ./bin/emqx_ctl cluster status
Cluster status: [{running_nodes,['emqx@s1.emqx.io','emqx@s2.emqx.io']}]

Exit the cluster

There are two ways for a node to exit the cluster:

1. leave: Leave this node exit the cluster
2. force-leave: Remove other nodes from cluster

Let emqx@s2.emqx.io actively exit the cluster:

$ ./bin/emqx_ctl cluster leave

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

$ ./bin/emqx_ctl cluster force-leave emqx@s2.emqx.io

Network Partition Autoheal

EMQ X supports Network Partition Autoheal, which can be configure in etc/emqx.conf:

cluster.autoheal = on

Network Partition Autoheal Process:

1. The node performs Network Partition confirmation 3 seconds after receiving the inconsistent_database event from Mnesia;
2. After the node confirms that the Network Partition has occurred, it reports the message to the Leader node (the earliest start node in the cluster);
3. After the Leader node delays for a period of time, it create a SplitView when all nodes are online;
4. The Leader node selects the self-healing Coordinator node in the majority partition;
5. The Coordinator node restarts the minority partition node to restore the cluster.

Autoclean of Cluster nodes

EMQ X supports Autoclean frol cluster , which can be configured in etc/emqx.conf :

cluster.autoclean = 5m

Firewall settings

The Node Discovery Ports

If the environment variable WITH_EPMD=1 is set in advance, the epmd (listening port 4369) will be enabled for node discovery when emqx is started, which is called epmd mode.

If the environment variable WITH_EPMD is not set, epmd is not enabled when emqx is started, and emqx ekka is used for node discovery, which is also the default method of node discovery since version 4.0. This is called ekka mode.

epmd mode：

If there is a firewall between cluster nodes, the firewall needs to open TCP port 4369 for each node, to allow peers query each other’s listening port. The firewall should also allow nodes connecting to port in configurable range from node.dist_listen_min to node.dist_listen_max (inclusive, default is 6369 for both)

ekka mode（Default mode since version 4.0）：

In ekka mode, the port mapping is conventional, but not dynamic as in epmd mode. The configurations node.dist_listen_min and node.dist_listen_max take no effect in this case.

If there is a firewall between the cluster nodes, the conventional listening port should be allowed for nodes to connect each other. See below for port mapping rule in ekka mode.

Erlang distribution port mapping rule in ekka mode: ListeningPort = BasePort + Offset, where BasePort is 4370 (which is not made configurable), and Offset is the numeric suffix of the node’s name. If the node name does not have a numeric suffix, Offsset is 0.

For example, having node.name = emqx@192.168.0.12 in emqx.conf should make the node listen on port 4370, and port 4371 for emqx1 (or emqx-1), and so on.

The Cluster RPC Port

Each emqx node also listens on a (conventional) port for the RPC channels, which should also be allowed by the firewall. The port mapping rule is similar to the node discovery ports in ekka mode, but with the BasePort = 5370. That is, having node.name = emqx@192.168.0.12 in emqx.conf should make the node listen on port 5370, and port 5371 for emqx1 (or emqx-1), and so on.