A computing system configured to implement a message queuing telemetry transport, MQTT, broker cell, the computing device comprising a logic subsystem comprising one or more processors; and a storage subsystem comprising one or more storage devices including instructions executable by the logic subsystem to operate two or more back-end brokers arranged in an m x k matrix in the broker cell, the matrix comprising m vertical chains of back-end brokers and k back-end brokers in each vertical chain, where m and k are integers, m is greater than zero, and k is greater than one, each vertical chain comprising at least a head back-end broker and a tail back-end broker, each vertical chain configured to replicate a state update received at the head back-end broker through the vertical chain to the tail back-end broker, each tail back-end broker configured to determine one or more subscribers to a topic; operate n front-end brokers in the broker cell, each front-end broker configured to output a control message to a selected vertical chain of the m vertical chains and to output an application message for publication to subscribers and to one or more other MQTT broker cells, where n is an integer greater than zero; and operate r networking devices configured to communicate application messages to subscribers and control messages to one or more other MQTT broker cells, where r is an integer greater than one. a logic subsystem comprising one or more processors; and a storage subsystem comprising one or more storage devices including instructions executable by the logic subsystem to operate two or more back-end brokers arranged in an m x k matrix in the broker cell, the matrix comprising m vertical chains of back-end brokers and k back-end brokers in each vertical chain, where m and k are integers, m is greater than zero, and k is greater than one, each vertical chain comprising at least a head back-end broker and a tail back-end broker, each vertical chain configured to replicate a state update received at the head back-end broker through the vertical chain to the tail back-end broker, each tail back-end broker configured to determine one or more subscribers to a topic; operate n front-end brokers in the broker cell, each front-end broker configured to output a control message to a selected vertical chain of the m vertical chains and to output an application message for publication to subscribers and to one or more other MQTT broker cells, where n is an integer greater than zero; and operate r networking devices configured to communicate application messages to subscribers and control messages to one or more other MQTT broker cells, where r is an integer greater than one. operate two or more back-end brokers arranged in an m x k matrix in the broker cell, the matrix comprising m vertical chains of back-end brokers and k back-end brokers in each vertical chain, where m and k are integers, m is greater than zero, and k is greater than one, each vertical chain comprising at least a head back-end broker and a tail back-end broker, each vertical chain configured to replicate a state update received at the head back-end broker through the vertical chain to the tail back-end broker, each tail back-end broker configured to determine one or more subscribers to a topic; operate n front-end brokers in the broker cell, each front-end broker configured to output a control message to a selected vertical chain of the m vertical chains and to output an application message for publication to subscribers and to one or more other MQTT broker cells, where n is an integer greater than zero; and operate r networking devices configured to communicate application messages to subscribers and control messages to one or more other MQTT broker cells, where r is an integer greater than one.