Building appliances out of components using Pebble

ABSTRACT

Appliances are special purpose systems that offer high processing speed, ease of configuration, safety, fault isolation, and minimal need for administration by human experts. Traditional approaches to building an appliance operating system have been either building it from scratch [CacheOS] or stripping down a monolithic kernel to its basic components [Jaeger99]. The former approach is costly and the resulting product is likely to be highly specialized and not easily extensible. The latter approach is not easy, as the OS code and data structures are often shared and closely intertwined. The resulting OS is also likely to be coarse-grained and not easily customizable. Most appliances are network-centric (e.g. HTTP caches, proxies, file servers, routers) in the sense that they require high-performance network connections. Such performance is often achieved with application-specific specialization of system I/O [Cao95] requiring a modification of a portion of the operating system, such as the protocol stack or the file system.Safety is a major concern for appliances, since new functions are constantly added, and there is never enough time to debug all possible interactions, especially when third party software is running on the appliance. The problem is more severe with legacy software written in C or other unsafe languages (in contrast with type-safe languages such as Java). Extensible routers are an example of network appliances where custom software processing has to be quickly added and safety is paramount. Diagnostic code and custom (e.g. multimedia) schedulers are also examples of extensions that appliances will need to support.Resource management is especially important for appliances. New operating system abstractions such as paths [Mosberger96], resource containers [Banga99], reservation domains [Bruno98] and activities [Jones95] have been proposed for resource management and control. We believe that an appliance operating system should be flexible enough to support such abstractions. It should also be able to overlay resource management in modular operating systems without such support.Pebble [Gabber99] is a component-based operating system that combines efficient IPC with strong modularity and safety features. Pebble provides the necessary infrastructure for building fine-grained, modular operating systems for appliances out of reusable components. We found that a typical network appliance application (e.g. a Web server) built on Pebble using components has comparable performance to a traditional monolithic kernel. In this way, there is no performance reason not to use a safer, more modular component structure for such appliances. Moreover, we claim that Pebble is a general purpose operating system framework that allows us to implement diverse OS structures and mechanisms, alleviating the need for writing specialized operating systems from scratch.

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