System Introspection for System Analysis on Mobile Devices

Figure 5.2: Message exchange between probe and client - On theleft side is the component thread publishing new data <strong>for</strong>connected clients. The data is send to the communicationinterface that handles all the communication withthe communication server. The server itself <strong>for</strong>wards themessage to the client. The communication interface onthe client side notifies all observers. Each observer thenchecks the message and processes it if needed.Using Publish / SubscribeThe mechanism behind exchanging the data is using the publish/subscribeparadigm (Eugster et al., 2003). One entity subscribes to a publisher. Thepublisher collects data and <strong>for</strong>wards it to all subscribers. The probe offersservices and publishes their data on request. The client can subscribe to sucha service. In contrast to a standard implementation of the publish/subscribeparadigm the publisher also knows the subscriber and vise versa. A componentthread is not only acting as a publisher. Each component thread can alsoreceive commands which can change how the data is published. These commandsmay also change the state of a component thread. An example couldbe a component that has several parameters <strong>for</strong> changing the process of filteringthe data presented. The client knows the publisher and can directly sendcommands. Also this can be used to minimize the need of control overheadand decrease the delay between sending and receiving a command. Specificcommands are <strong>for</strong>warded to a component thread which can directly use thecommands <strong>for</strong> further processing.To notify entities of incoming messages, the communication interface is usingthe observer pattern (Freeman et al. (2004), pages 37-78). For the exchange ofdata between the probe and the client, each message created contains the TAGof the sending element and the TAG of the component that receives the data.A TAG is unique to the system and allows to identify its elements. Whenevera new message is received, the interface notifies its observers. Each elementobserving this interface checks if the message contains its TAG and then reads22