• abstraction from the hardware: XtreemOS gives to its users the illusion of using a traditional computer while removing the burden of complex resource management issues of a typical Grid environment. When a user runs an application on XtreemOS, the operating system automatically finds all resources necessary for the execution, configures user’s credentials on the selected resources and starts the application. A set of system services, extending those found in the traditional Linux, provides users with all the Grid capabilities associated with current Grid middleware, but fully integrated into the OS;

• secure resource sharing between different users: resources and users are managed through Virtual Organizations (VOs)  which are sets of users that pool resources in order to achieve common goals. Each VO includes some rules governing the sharing of the resources of the given VO.

• scalable: XtreemOS supports hundreds of thousands of nodes and millions of users dynamically joining and leaving the Grid;
• transparent: XtreemOS hides the complexity of the Grid by distributed operating system services allowing to run new and legacy applications seamlessly;
• interoperable: XtreemOS complies with all major standards such as POSIX and SAGA;
• dependable: XtreemOS provides reliability and high availability through checkpointing and replication;
• secure: XtreemOS ensures trust and integrity according to customizable policies.

 XtreemOS is running on a wide range of hardwares ranging from smartphones to PCs and Linux clusters.

User Interface

Easy resources and users administration through Virtual Organizations:

XtreemOS brings the Grid to standard Linux users

• Feeling to work with a Linux machine;

• Standard way of launching applications (for example, use the ps command to check status of own jobs);

• No limit on the kind of applications supported: non interactive and interactive applications.

Transparency for application developers:

• Hierarchy of jobs in the same way as Unix process hierarchy:

• Same system calls: wait for a job, send signals to a job, etc.;

• Processes in a job treated as threads in a Unix process;

• Files stored in the XtreemFS Grid file system which provides a POSIX interface and semantics to access files regardless of their location;

• Transparent fault tolerance to applications;

• A cluster of resources can appear as one resource that adds all the power and memory of the resources (Single System Image of a cluster).

Architecture

 XtreemOS architecture is divided in two main layers:

• the XtreemOS-F layer is specific to the considered resource;
•  the XtreemOS-G layer integrates all XtreemOS-enabled resources as a single computing platform.

The XtreemOS-G layer

• Virtual Organization Management (VOM): building and operating dynamic virtual organizations. The VOLifeCycle service provides the tools to create and manage VOs. The Resource Certification Authority (RCA) server provides the authentication and authorization services. The VO policies are enforced by the Virtual Organization Policy Server (VOPS) during the execution of applications.
• Application Execution Management (AEM): providing scalable resource discovery, job scheduling for distributed interactive applications;
• Data management (XtreemFS): accessing and storing data in a POSIX-like file system spanning the Grid.
  

  XtreemFS is an object-based distributed file system that is designed for federated IT infrastructures that are connected by wide-area networks. It can be mounted from anywhere in the Internet. It supports replication and partitioning of metadata servers, and features striping and replication at file/object level. The design of these mechanisms specifically addresses the challenges posed by high latencies and complex failure cases of a cross-site cross-organization environment. The file system monitors itself and autonomously optimizes file distribution, layout and access. It stores extended metadata for files, enforces POSIX ACLs for fine-grained authorization and integrates into virtual organizations.
  The Object Sharing Service (OSS) aims at providing a transparent and consistent data sharing service. One the one hand it extends XtreemFS by providing memory-mapped files and on the other hand implementing a transactional memory for volatile memory objects. 

• Resource Selection Service (RSS): evaluation of queries in JSDL syntax, returning a list of machine satisfying them;
• Application Directory Service (ADS
• Scalable Resource Discovery Service (SRDS)
• DIXI
• XOSD

The XtreemOS-F layer

• Standard flavour
• Cluster flavour also called LinuxSSI: the cluster favour of XtreemOS is designed to execute the standard XtreemOS-G high level services on top of the customized cluster version of XtreemOS-F foundation layer. The cluster flavour of XtreemOS-F is based on the Single System Image concept. A Single System Image (SSI) cluster operating system gives the illusion that a cluster is a single multiprocessor machine. With LinuxSSI, which implements the SSI concept in the XtreemOS-F layer, a cluster appears as a single powerful Grid node providing the POSIX interface. LinuxSSI leverages the existing Linux-based Kerrighed SSI technology, resulting from research activities conducted at INRIA in collaboration with EDF R&D and now being developed by a community in the framework of Kerrighed open source project.
• Mobile flavour aka XtreemOS-MD: the XtreemOS mobile flavour provides a set of open source modules, easily integrable in any mobile Linux OS, to enable mobile device for grid operation in VOs, in an efficient and transparent way. For example, PDAs, smartphones and other mobile devices can be connected to the Grid to start, monitor and get the result of applications.