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This paper presents a dynamic composer for Web services. The services are enriched with semantic descriptions in OWL-S, based on which the composer automatically creates a combination of services reaching a specified goal. As an example, a trip planning use case is chosen where the goal ranges from booking of a single flight to planning of an entire trip including flight, hotel, transport, etc. The composition is achieved using local and global algorithms satisfying specific quality of service (QoS) constraints and requirements such as the execution time or cost of the invoked Web services. At the same time a more extended HTN planning algorithm is discussed, matching not only service outputs to inputs but also satisfying service preconditions through effects. In addition to the automatic composition, the paper also proposes a recovery mechanism in case of unavailable services. When executing the composition of flight services, unavailable services are dynamically replaced by equivalent services or a new composition achieving the needed result. The presented platform and planning algorithms are put through extensive performance and scalability tests for typical trip booking scenarios, in which basic services are composed to a complex trip planning service.