【正文】 led environment。 on the other side there are users, interacting with the controlled system with the support of a client user interface. Acting as a broker, the server has to deal with a variety of challenging tasks like: (i) managing and coordinating the data flow between the involved actors, possibly performing adhoc data manipulation and aggregation。 (ii) guaranteeing the synchronization of the status information at the different peers。 and (iii) offering a secure and reliable service by ensuring failsafe execution of user mands.The server’s internal organization has been conceived to enhance modularity, extensibility, ponent reuse, and performance. In Figure 3 and Figure 4, we can identify three macro ponents of the server internal architecture:1. the Field Interface Management (FIM)。2. the Control Interface Management (CIM)。3. the User Interface Management (UIM).The FIM prises all the server subponents responsible for managing the munication with the field devices, while providing the abstraction and modularity required from other ponents to ignore the physical features, topologies, and protocols of the devices.Interaction with the field is acplished through a standard OPC client/server module, thus adding another level of abstraction (and modularity) to the system.The CIM handles all the features related to usermand management, content personalization, and adaptivity. Since multiple users are allowed to interact with the system and user interface’s contents are directly related to user devices and authorizations, there is a need for adhoc data structures and operations able to ply with the performance, scalability, concurrency and reliability requirements. In order to respond to such demands, the internal organization of the CIM relies on orthogonal modules responsible for managing the munication with the FIM, each one dedicated to a single aspect. Every time a client connects to the system, a dedicated munication buffer is assigned to it and is initially filled with all the values needed to build up the current system state view for the client. Finally, the UIM is the ponent delegated to orchestrate and synchronize the interaction with clients。 since MyHMI relies on a Web architecture, the UIM is organized according to the Model View Controller (MVC) design pattern [4], in its Webpatible version known as Model 2 (MVC2) [5].. Clientserver interaction designAn important aspect to consider when dealing with Web architectures is the asymmetric nature of the munication protocol used by clients to municate with the server (HTTP). Only clients are able to perform requests to the server and not vice versa. This constraint hampers the optimization of the interactions because clients cannot be notified by the server of new events (new variable values,instructions, alarms), but need to periodically invoke the server to retrieve the updated information.Modern Web applications start to discover the usefulness of bidirectional munication mechanism and leverage on push technologies to enable Web server proactivity. According to the decision described in Section , we adopted a simulated callback approach. Such approaches usually rely on HTTP/ persistent connection (see [17]). Persistent connections exploit the XMLHttpRequest concept [18] and similar mechanisms to retrieve information from the server without necessarily updating the whole page. Thanks to these technologies, clients are allowed to establish a single connection always available for servers to send data independently from the user interaction. Unfortunately, persistent connections are expensive to manage for servers, making such approach unsuitable for lowputationalpower devices like the ones used by MyHMI.The munication buffer mechanism implemented in the CIM helps at overing the drawbacks ing from the polling process.Figure 5 schematizes a simplified “l(fā)ifecycle” of a value update of a field variable. When the client performs its polling routine, the UIM first checks if, from the previous client request, the CIM notified for new values in the buffer. If no notifications took place, the UIM sets a timeout and waits for new data to be stored into the buffer: if the missing data bees available before the timeout expires, the whole buffer is dispatched to the client in a bulk manner。 otherwise, the UIM closes the polling cycle and returns an empty response.Thanks to buffering and bulk data transfers, the number of client requests decreases dramatically. Moreover, data changes are municated only when needed by some client and thus the congestion of the munication channel is reduced, enhancing scalability and, thus, the overall system performances.. Clientside designThe main role of the client layer in the MyHMI architecture is to manage the data presentation and user interaction. To avoid dependency from a specific technology, we produced a highlevel design that can be implemented in different rendering environments. The designed internal ponent distribution is depicted in Figure 6: the client application incorporates an application shell, executed within the browser environment. The shell (written in a client side scripting language) is separate from the graphic engine of the browser and exploits a MVC organization. The Model contains the business objects of the interface (., data variables, trend monitors) while the View prises the widgets and presentation properties managed by the rendition technology (., the widget to be used to display a data variable, a trend, or an input control). The shell also manages an internal clock, used to automatically trigger requests to the server for the system state refresh。 upon reception of server responses, the shell updates the internal data variables, which automatically refresh the registered business objects and associated widgets. This datacentric approach allows to redraw only the affected widgets, minimizing the putational effort and enhan