**vernemq:**

VerneMQ is used by Orchestrator to publish the link up/down status of an element, 
SnmpManagers can subscribe to this and each SnmpManager will get a unique message(using MQTT shared subscription).


<code>
sudo yum install vernemq-<VERSION>.centos7.x86_64.rpm
</code>

**setup users and topic access**
    
config file:

<code>
vmq_passwd.password_file = /etc/vernemq/vmq.passwd
vmq_acl.acl_file = /etc/vernemq/vmq.acl
listener.tcp.allowed_protocol_versions = 3,4,5
#10mb buffer
outgoing_clustering_buffer_size = 10000000
</code>

The above variables are necessary, you may increase the numbers but do not decrease them.

an example vmq.acl will look as follows:

<code>
user orchestrator
topic write #
topic read #


user subscriber01
topic read $share/subscriber01_group/subscriber01/linkpoll


user subscriber02
topic read $share/subscriber02_group/subscriber02/linkpoll
</code>


The user `orchestrator` has full access while subscriber01 & 02 only have access to their respective paths.
subscriber01 could be set of EXT SnmpManagers, while subscriber02 could be set of ATC SnmpManagers

Now these users need authentication, this is where vmq.passwd comes in, 
Link is provided below on how to manage users and the file:https://docs.vernemq.com/configuration/file-auth

an example vmq.passwd file looks like the one below:
<code>
orchestrator:$6$dUN1ebgDVV/78XAN$gONxR/TTlCTbJr/iG0Y5rFB7VdcIWONZVk7jJ2VmMsFYgzI3sniKqQGulit3eTGVAVtkcC0J5fgwBT3noxa10w==
subscriber01:$6$QbrGZoA0ynrMG//3$OyipONxTUCatxye6t0jVJ6N95VgGovhHbAMPMB2lYhJOsCLhHrq7v5nGxTrUu0pWF9/uO3wIDEmTj02/fyCnuw==
subscriber02:$6$QbrGZoA0ynrMG//3$OyipONxTUCatxye6t0jVJ6N95VgGovhHbAMPMB2lYhJOsCLhHrq7v5nGxTrUu0pWF9/uO3wIDEmTj02/fyCnuw==
</code>