ALPS is a middleware solution that offers several location-based publish/subscribe services. With ALPS, messages have contents and topics like in regular publish/subscribe service. In addition, messages have an assigned location, which can either be static, representing for example the location of the publisher at publication time, or dynamic, representing the current location of the publisher. Users can subscribe to messages based on their content like they would in regular publish/subscribe services but in addition they can filter messages based on their proximity. For example, a subscriber could decide to only receive messages located within a 50 meter radius. In addition to subscriber-side filtering, ALPS also allows publishers to restrict their message to a certain proximity. For example, a publisher could explicitly state that one of its message can only be delivered to a subscriber within 100 meters of its current location.


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