## 3 Semantic Service Description and Context-Based Orchestration  
### Use Cases  
Manufacturing companies have been under pressure recently due to shorter product life cycles and greater product individualization. In order to stay competitive in this environment, processes and systems need to become more flexible and agile [24, 25]. This new requirement poses a great challenge for today's automation systems because changing a production process for example from one product to a new product generation is usually associated with a high amount of programming and configuration. Three exemplary use cases illustrate different aspects of changing production processes and are used to deduce requirements for a dynamic context-based orchestration and the underlying semantic models with the goal of creating flexible automation systems. The first use case is that of a field device in the production process being replaced with a new possibly better (e.g., resource-efficient) device that can fulfill the same task. This can often be the case in production plants because of component failure or the need to improve some process parameter by installing a better device. With traditional automation systems, this device has to be manually integrated into the programming environment of the PLC (programmable logic controller). This requires the configuration of the communication interface and adaption of the PLC program to the new devices' functional interface (often on signal-level), which depicts a time-consuming process that requires on-line testing to verify the correctness of the new program and thus expensive down-times of machinery. In order to achieve an improvement of this problem, we envision a plug-and-play-like replacement of components that automatically integrates the device into the automation system. The second use case which describes the lack of flexibility in today's production environments and which we want to address with our approach focusses on the need to change the production process because some component is not available on short notice. In some cases this could be countered with rerouting the current product to another production station (e.g., if there are several equivalent welding robot cells) or changing the sequence of production steps. This ad-hoc reaction to changes in component availability can keep the production process running and reduce down-time losses. The problem that needs to be solved in order to achieve this dynamic process rerouting is finding a device or production unit that can fulfill the same task or a new sequence of process steps that result in the same outcome. The third problem that we want to focus on results from the need for greater product individualization because of consumer demand. For each product variant a concrete program needs to be specified for each production station and executed once the product reaches this station. Changes in the product specification result in the need to modify each program at the decentralized stations. Recent research offers a new possibility of dealing with this challenge: a product-mediated communication within a production system [5] allows a product to carry information about the customer order or the status of its production. Extending this idea beyond merely carrying status and order information, a product could also carry an abstract representation of the manufacturing processes needed for its completion. Enabling automation systems to read this representation and automatically choose and execute appropriate concrete process steps for each abstract stage can lead to a greatly improved flexibility in production processes and new possibilities for product individualization. Having discussed the three use cases, the following requirements can be derived that must be met by a semantic discovery and context-based orchestration system to achieve flexible manufacturing control:  
1. An automatic registration of devices in the system must be possible.