distributed, fault-tolerant Jabber/XMPP + MQTT server

 ejabberd is a Jabber/XMPP + MQTT server written in Erlang, featuring:
  * distributed operation with load-balancing across a cluster;
  * fault-tolerant database replication and storage on multiple nodes,
    allowing nodes to be added or replaced "on the fly";
  * virtual hosting (several virtual domains can be served using a single
    ejabberd instance);
  * XMPP compliance;
  * MQTT 5 compliance;
  * web-based administration;
  * SSL/TLS support;
  * conferencing via Multi-User Chat;
  * Jabber Users Directory, based on users' vCards;
  * service discovery;
  * shared roster.