We introduce activity-based navigation, which uses human activities derived from sensor data to help people navigate, in particular to retrace a "trail" previously taken by that person or another person. Such trails may include step counts, walking up/down stairs or taking elevators, compass directions, and photos taken along a user's path, in addition to absolute positioning (GPS and maps) when available. To explore the user experience of activity-based navigation, we built Greenfield, a mobile device interface for finding a car. We conducted a ten participant user study comparing users' ability to find cars across three different presentations of activity-based information as well as verbal instructions. Our results show that activity-based navigation can be used for car finding and suggest its promise more generally for supporting navigation tasks. We present lessons for future activity-based navigation interfaces, and motivate further work in this space, particularly in the area of robust activity inference.

In most modern operating systems, a process is a hardware-protected abstraction for isolating code and data. This protection, however, is selective. Many common mechanisms---dynamic code loading, run-time code generation, shared memory, and intrusive system APIs---make the barrier between processes very permeable. This paper argues that this traditional open process architecture exacerbates the dependability and security weaknesses of modern systems.

Message-based communication offers the potential benefits of providing stronger specification and cleaner separation between components. Compared with shared-memory interactions, message passing has the potential disadvantages of more expensive data exchange (no direct sharing) and more complicated programming.In this paper we report on the language, verification, and run-time system features that make messages practical as the sole means of communication between processes in the Singularity operating system. We show that using advanced programming language and verification techniques, it is possible to provide and enforce strong system-wide invariants that enable efficient communication and low-overhead software-based process isolation. Furthermore, specifications on communication channels help in detecting programmer mistakes early---namely at compile-time---thereby reducing the difficulty of the message-based programming model.The paper describes our communication invariants, the language and verification features that support them, as well as implementation details of the infrastructure. A number of benchmarks show the competitiveness of this approach.

Run-time conflicts can affect even the most rigorously tested software systems. A reliance on execution-based testing makes it prohibitively costly to test every possible interaction among potentially thousands of programs with complex configurations. In order to reduce configuration problems, detect developer errors, and reduce developer effort, we have created a new first class operating system abstraction, the application abstraction, which enables both online and offline reasoning about programs and their configuration requirements.We have implemented a subset of the application abstraction for device drivers in the Singularity operating system. Programmers use the application abstraction by placing declarative statements about hardware and communication requirements within their code. Our design enables Singularity to learn the input/output and interprocess communication requirements of drivers without executing driver code. By reasoning about this information within the domain of Singularity's strong software isolation architecture, the installer can execute a subset the system's resource management algorithm at install time to verify that a new driver will not conflict with existing software. This abstract representation also allows the system to run the full algorithm at driver start time to ensure that there are never resource conflicts between executing drivers, and that drivers never use undeclared resources.