A method for a message broker computer system to process a message, the method comprising: a master broker computer system receiving a message from a client producer; the master broker computer system, responsive to receiving the message from the client producer, generating a message event, the message event uniquely identifying the message with an epoch value associated with the master broker computer system and a sequence number associated with the message; the master broker computer system distributing the message event to a plurality of event stores (130A-130C) and a plurality of slave broker computer systems (142B-142C), each event store comprising an associated persistent storage system and each slave broker computer system comprising both an associated persistent storage system and an associated memory, wherein each event store stores the message event in the associated persistent storage system, and wherein each slave broker computer system holds the message event in the associated memory; the master broker computer system, responsive to receiving persistence notifications from a quorum of event stores of the plurality of event stores, determining that the message event is stabilized, wherein each persistence notification indicates that the message event has been stored in the associated persistence storage system of the respective event store and wherein the quorum comprises at least two event stores; the master broker computer system, responsive to determining that the message event is stabilized, storing the message event in an associated persistent storage and sending a notification to the client producer; the master broker computer system generating a second message event indicating that the message event has been stabilized; and the master broker computer system distributing the second message event to the plurality of event stores and the plurality of slave broker computer systems, wherein the second message event causes each slave broker computer system to transfer the message event from its respective associated memory to its respective persistent storage system. a master broker computer system receiving a message from a client producer; the master broker computer system, responsive to receiving the message from the client producer, generating a message event, the message event uniquely identifying the message with an epoch value associated with the master broker computer system and a sequence number associated with the message; the master broker computer system distributing the message event to a plurality of event stores (130A-130C) and a plurality of slave broker computer systems (142B-142C), each event store comprising an associated persistent storage system and each slave broker computer system comprising both an associated persistent storage system and an associated memory, wherein each event store stores the message event in the associated persistent storage system, and wherein each slave broker computer system holds the message event in the associated memory; the master broker computer system, responsive to receiving persistence notifications from a quorum of event stores of the plurality of event stores, determining that the message event is stabilized, wherein each persistence notification indicates that the message event has been stored in the associated persistence storage system of the respective event store and wherein the quorum comprises at least two event stores; the master broker computer system, responsive to determining that the message event is stabilized, storing the message event in an associated persistent storage and sending a notification to the client producer; the master broker computer system generating a second message event indicating that the message event has been stabilized; and the master broker computer system distributing the second message event to the plurality of event stores and the plurality of slave broker computer systems, wherein the second message event causes each slave broker computer system to transfer the message event from its respective associated memory to its respective persistent storage system.