Hades (Highly Available Distributed Embedded System) is an environment for the design, timing analysis and execution of distributed hard real-time dependable applications.

Hades provides a set of tools, executed off-line, to develop applications, modeled thanks to annotated distributed Directed Acyclic Graphs (DAGs) and verify their timing correctness. More precisely, the off-line tools encompass feasibility tests associated with a panel of scheduling policies, a fault-tolerance tool, and a tool that determines the right amount of memory to be allocated for the application execution.  The fault-tolerance tool changes the structure of application tasks to make them fault tolerant; it takes into account the requirements of the application designer (e.g., tasks or the portions of tasks to be replicated, fault-tolerance strategy to be used such as active, passive or semi-active replication).

Application execution is managed by the Hades run-time support. This run-time support is built as a middleware software layer running on top of off-the-shelf real-time kernels (it has been ported on Chorus and RTEMS and emulated on Solaris). It consists in a set of services mandatory for the execution of a large panel of dependable distributed real-time tasks. Examples of such services are scheduling policies, communication primitives, group membership management, distributed execution control, and clock synchronization.  All these services exhibit timeliness and dependability properties.  The Hades run-time support has been designed to guarantee three fundamental and complementary aspects:

Real-time: support of applications that exhibit strict timing constraints. The achievement of the real-time aspect mainly relies on an accurate estimation of the worst-case execution times (WCETs) for all the activities to be executed in the system (application tasks, run-time support tasks, real-time kernel).

Fault-tolerance: achievement of a high degree of reliability thanks to fault-tolerance mechanisms which are transparent to the application designer. The fault-tolerance aspect of the run-time support relies on the fault-tolerance tool, and on fault-detection and exception handling mechanisms offered by the run-time support.

An important feature of Hades is that reliability and hard real-time aare studied as a whole.

Flexibility: Parts of the the run-time support without having to rewrite it entirely. As a consequence, the ruun-time support can be tailored to the specific constraints of the applications.

In the remainder, we present Hades through the design and the execution of a toy application called bees. This application manages the pursuit of a wasp by several bees. Among the bees, we distinguish those who are critical (thos that must survive a machine crash) from those who are not-critical. The Hades platform for this demonstration is made of four computers. The wasp is moved by computer Hades 0. Critical bees are moved by Hades 1 and Hades 3 in order to treat failure of one of these two computers. Finally, non-critical bees are moved by Hades 2.