Front-end system for the VLT's data-flow system

The Observation Handling Subsystem (OHS) of the ESO VLT Data- Flow System was designed to collect, verify, store and distribute observation preparation information. This rather generic definition includes high-level Observing Proposals submitted once per semester to apply for telescope time (typically referred to as 'Phase I' proposals) as well as detailed descriptions of the observations to be performed (which is often called 'Phase II' data); in the Data-Flow System, such descriptions are defined as Observation Blocks (OBs). Observation queues and long- and short-term schedules are also produced, ranging in scope from an observation semester to a few hours. The OHS is a distributed system composed of a collection of loosely coupled software tools. The tools communicate mostly through a set of relational databases, which are distributed between Garching and The Chilean observatories. A number of communication protocols are also used, from the e-mail based Receiver process of the Proposal Handling and Reporting System to the proprietary protocol used to serve the telescope and instrument control systems. Data and commands flow through the OHS, supporting the operational procedures of ESO's Observing Programmes Committee and of the different operation teams in Garching and in Chile. This paper presents the overall architecture of the OHS, and each module's technical features and underlying operational concepts. It also discusses the current implementation choices and development plans.