7. A system for managing internal cluster communications in a cluster of application server instances, the system comprising: server nodes operating in a cluster of application server instances; a plurality of work processes executing in processors of the server nodes operating in the cluster of application server instances; a message server providing internal cluster communication, the internal cluster communication including inter-application server instance communication in accordance with a connection-based message passing protocol for the internal cluster communication between the message server and the server nodes operating in the cluster of application server instances in which the message server receives and relays a notification message for each event occurring in the cluster and sends acknowledgement messages responsive to receiving messages being exchanged between work processes executing in processors of the server nodes operating in the application server instances, wherein each work process executing in a processor of a server node operating in the cluster of application server instances is communicatively coupled to the message server and each other via internal connections for the internal cluster communication, the internal connections having been established between the message server and each of the plurality of work processes executing in processors of the server nodes operating in the application server instances, the internal connections having been established separate from connections for external cluster communication, and wherein the internal cluster communication comprises internal communications relating to at least one of events occurring in the cluster and an exchange of messages between work processes executing in processors of the server nodes operating in the application server instances; a cluster manager executing in processors of the server nodes operating in the cluster of application server instances, the cluster manager to manage internal cluster communications in accordance with the connection-based message passing protocol, the internal cluster communication sent over the internal connections established for internal cluster communication between the message server and each of the plurality of work processes executing in processors of the server nodes operating in the application server instances wherein, on behalf of each application server instance of the cluster of application server instances, the cluster manager operates: an event handler to communicate an event in the cluster of application server instances from the application server instance to the message server, the event handler sending a notification message of the event to the message server; a notification handler to relay the notification message of the event in the cluster from the message server to one or more of the application server instances , the notification handler to cause a function to be performed in one or more of the application server instances responsive to receiving the relayed notification message; and a message server communication handler to communicate messages to be exchanged between the work processes executing in an application server instance and other application server instances in the cluster via the message server, the message server communication handler: acknowledging receipt of messages received from the work processes; and relaying messages received from an originating work process to a destination work process, including replicating a multicast message received from the originating work process for broadcast to all other work processes executing in the application server instances.
7. A system for managing internal cluster communications in a cluster of application server instances, the system comprising: server nodes operating in a cluster of application server instances; a plurality of work processes executing in processors of the server nodes operating in the cluster of application server instances; a message server providing internal cluster communication, the internal cluster communication including inter-application server instance communication in accordance with a connection-based message passing protocol for the internal cluster communication between the message server and the server nodes operating in the cluster of application server instances in which the message server receives and relays a notification message for each event occurring in the cluster and sends acknowledgement messages responsive to receiving messages being exchanged between work processes executing in processors of the server nodes operating in the application server instances, wherein each work process executing in a processor of a server node operating in the cluster of application server instances is communicatively coupled to the message server and each other via internal connections for the internal cluster communication, the internal connections having been established between the message server and each of the plurality of work processes executing in processors of the server nodes operating in the application server instances, the internal connections having been established separate from connections for external cluster communication, and wherein the internal cluster communication comprises internal communications relating to at least one of events occurring in the cluster and an exchange of messages between work processes executing in processors of the server nodes operating in the application server instances; a cluster manager executing in processors of the server nodes operating in the cluster of application server instances, the cluster manager to manage internal cluster communications in accordance with the connection-based message passing protocol, the internal cluster communication sent over the internal connections established for internal cluster communication between the message server and each of the plurality of work processes executing in processors of the server nodes operating in the application server instances wherein, on behalf of each application server instance of the cluster of application server instances, the cluster manager operates: an event handler to communicate an event in the cluster of application server instances from the application server instance to the message server, the event handler sending a notification message of the event to the message server; a notification handler to relay the notification message of the event in the cluster from the message server to one or more of the application server instances , the notification handler to cause a function to be performed in one or more of the application server instances responsive to receiving the relayed notification message; and a message server communication handler to communicate messages to be exchanged between the work processes executing in an application server instance and other application server instances in the cluster via the message server, the message server communication handler: acknowledging receipt of messages received from the work processes; and relaying messages received from an originating work process to a destination work process, including replicating a multicast message received from the originating work process for broadcast to all other work processes executing in the application server instances. 8. The system of claim 7 , wherein the cluster manager executing in processors of the server nodes operating in the cluster of application server instances is implemented as a software library.