Apache ActiveMQ is an open source message broker written in Java together with a full Java Message Service (JMS) client. It provides "Enterprise Features" which in this case means fostering the communication from more than one client or server.
Supported clients include Java via JMS 1.1 as well as several other "cross language" clients. The communication is managed with features such as computer clustering and ability to use any database as a JMS persistence provider besides virtual memory, cache, and journal persistency.
ActiveMQ employs several modes for high availability, including both file-system and database row-level locking mechanisms, sharing of the persistence store via a shared filesystem.
ActiveMQ is celebrated for its flexibility in configuration, and its support for a relatively large number of transport protocols, including OpenWire, STOMP, MQTT, AMQP, REST, and WebSockets.