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Integration of heterogeneous information sources has been one of important data engineering research issues. Various type of information sources are available today. They include dissemination-based information sources, which actively and autonomously deliver information from server sites to users. We have been developing a mediator/wrapper-based information integration system, in which we employ ECA (event-condition-action) rules to enable users to define new information delivery services integrating multiple existing dissemination-based information sources. However, it is not easy for users to directly specify ECA rules and to verify them. In this paper, we propose a scheme to specify new information delivery services using the framework of the relational algebra. We discuss some important properties of the specification and show how we can derive ECA rules to implement the delivery services.