The present invention relates to a data processing system, method and computer program product for improving the operation of, for example, a message broker. By not constructing messages from stored data in advance of a demand, the invention reduces the processing overhead. A further aspect provides for maintaining a data structure for a messaging system having an application capable of producing an output structured message in a prescribed format and capable of adding a received structure message to a data structure contained within a storage medium of the system. The steps followed are: receiving a first structured message in a first format; the first structured message being associated with the output structured message in the prescribed format; adding the first structured message in the first format to the data structure; receiving a second structured message in a second format; the second structured message being associated with the output structured message in the prescribed format; and adding the second structured message in the second format to the data structure such that the first and second structured messages are stored non-contiguously within the data structure.