We introduce an approach to robust computation in distributed systems. This approach is the foundation for reliability in the Clouds decentralized operating system. It is based on atomic actions operating on instances of abstract data types (objects). We present an event-based model of computation in which scheduling of responses to operation invocations is controlled by objects. We discuss an integrated strategy for synchronization and recovery which uses relationships between the abstract states of objects to track dependencies between actions. Serializability is defined in terms of the semantics of operations. This permits high concurrency to be obtained in non-serializable implementations without deviation from serializable abstract behavior. We define a class of schedulers that allows objects to make autonomous scheduling decisions. We present the use of non-serializable operation semantics. Finally, we discuss implementation of the model, including action synchronization, object operation ordering using action-based counting semaphores, and action recovery.

A file access system, flash, for use in building database systems is described. It supports access from several languages, including pascal, fortran, and interlisp. Flash provides record level access to a file with multiple indexes using symbolic keys. It is portable and written in Pascal with support routines in dec System 20 macro. The file access system is designed to run on computers of various sizes and capabilities, including micros. Concurrent and simultaneous access by several users is supported given that the operating systems provides multiprogramming. Flash is designed to be highly reliable. It assumes the existence of underlying operating system file services that read or write named files directly. Transfer to files occurs in units which are efficient, typically a block.

The programming language Modula is reviewed for its ability to support the wide variety of time usages required in a process control system. It is shown that in many instances concurrent languages supporting device control and multiprogramming may need additional facilities for the support of time.