- The anatomy of the Grid. - The term ‘the Grid’ was coined in the mid-1990s to denote a proposed distributed com- puting infrastructure for advanced science and engineering [1]. - Is there really a distinct ‘Grid problem’ and hence a need for new ‘Grid technologies’? If so, what is the nature of these technologies, and what is their domain of applicability? While numerous groups have interest in Grid concepts. - For example, enterprise distributed computing systems can use Grid technologies to achieve resource sharing across institutional boundaries. - An industrial consortium formed to develop a feasibility study for a next-generation supersonic aircraft undertakes a highly accurate multidisciplinary simulation of the entire aircraft. - A crisis management team responds to a chemical spill by using local weather and soil models to estimate the spread of the spill, determining the impact based on population location as well as geographic features such as rivers and water supplies, creating a short-term mitigation plan (perhaps based on chemical reaction models), and task- ing emergency response personnel by planning and coordinating evacuation, notifying hospitals, and so forth.. - These four examples differ in many respects: the number and type of participants, the types of activities, the duration and scale of the interaction, and the resources being shared.. - For example, members of a consortium may provide access to specialized software and data and/or pool their computational resources.. - For example, a participant in VO P of Figure 6.1 might allow VO partners to invoke their simulation service only for. - Resource consumers may also place constraints on properties of the resources they are prepared to work with. - For example, a participant in VO Q might accept only pooled computational resources certified as ‘secure.’ The implementation of. - Sharing relationships can vary dynamically over time, in terms of the resources involv- ed, the nature of the access permitted, and the participants to whom access is permitted.. - The dynamic nature of sharing relationships means that we require mechanisms for discovering and characterizing the nature of the relationships that exist at a particular point in time. - For example, a new participant joining VO Q must be able to determine what resources it is able to access, the ‘quality’ of these resources, and the policies that govern access.. - The same resource may be used in different ways, depending on the restrictions placed on the sharing and the goal of the sharing. - Because of the lack of a priori knowledge about how a resource may be used, performance metrics, expectations, and limitations (i.e., quality of service) may be part of the conditions placed on resource sharing or usage.. - Together, this technology and architecture constitute what is often termed middleware (‘the services needed to support a common set of applications in a distributed network environment’ [12. - Why are protocols critical to interoperability? A protocol definition specifies how distributed system elements interact with one another in order to achieve a specified behavior, and the structure of the information exchanged during this interaction. - Since pro- tocols govern the interaction between components, and not the implementation of the components, local control is preserved.. - In specifying the various layers of the Grid architecture, we follow the principles of the ‘hourglass model’ [14]. - The narrow neck of the hourglass defines a small set of core. - Figure 6.2 The layered Grid architecture and its relationship to the Internet protocol architecture.. - In our architecture, the neck of the hourglass consists of Resource and Connectivity protocols, which facilitate the sharing of individual resources. - The Grid Fabric layer provides the resources to which shared access is mediated by Grid protocols: for example, computational resources, storage systems, catalogs, network resources, and sensors. - in such cases, a resource imple- mentation may involve internal protocols (e.g., the NFS storage access protocol or a cluster resource management system’s process management protocol), but these are not the concern of Grid architecture.. - However, as in practice few resources support advance reservation ‘out of the box,’. - Computational resources: Mechanisms are required for starting programs and for mon- itoring and controlling the execution of the resulting processes. - While alternatives certainly exist, we assume here that these protocols are drawn from the TCP/IP protocol stack: specifically, the Internet (IP and ICMP), transport (TCP, UDP), and application (DNS, OSPF, RSVP, etc.) layers of the Internet layered protocol architecture [26]. - With respect to security aspects of the Connectivity layer, we observe that the com- plexity of the security problem makes it important that any solutions be based on existing standards whenever possible. - As with communication, many of the security standards developed within the context of the Internet protocol suite are applicable.. - User-based trust relationships: In order for a user to use resources from multiple providers together, the security system must not require each of the resource providers to cooperate or interact with each other in configuring the security environment. - example, if a user has the right to use sites A and B, the user should be able to use sites A and B together without requiring that A’s and B’s security administrators interact.. - GSI builds on and extends the Transport Layer Security (TLS) protocols [31] to address most of the issues listed above:. - such issues are the concern of the Collective layer discussed next.. - Information protocols are used to obtain information about the structure and state of a resource, for example, its configuration, current load, and usage policy (e.g., cost).. - Management protocols are used to negotiate access to a shared resource, specifying, for example, resource requirements (including advanced reservation and quality of service) and the operation(s) to be performed, such as process creation, or data access. - An extended version of the File Transfer Protocol, GridFTP, is a management protocol for data access. - For example, the Grid Resource Information Service (GRIS) implements server-side LDAP functionality, with callouts allowing for publication of arbitrary resource information [33].. - An important server-side element of the overall toolkit is the ‘gatekeeper,’ which pro- vides what is in essence a GSI-authenticated ‘inetd’ that speaks the GRAM protocol and can be used to dispatch various local operations. - For this reason, we refer to the next layer of the. - Examples include Grid-enabled implementations of the Message Passing Interface [41, 42] and manager-worker frameworks [43, 44].. - Software discovery services discover and select the best software implementation and execution platform based on the parameters of the problem being solved [45]. - For example, Figure 6.3 shows a Collective co-allocation API and SDK (the middle tier) that uses a Resource layer management protocol to manipulate underlying resources. - Collective components may be tailored to the requirements of a specific user commu- nity, VO, or application domain, for example, an SDK that implements an application- specific coherency protocol, or a co-reservation service for a specific set of network resources. - We emphasize that what we label ‘applications’ and show in a single layer in Figure 6.4 may in practice call upon sophisticated frameworks and libraries (e.g., the Common Com- ponent Architecture [54], SCIRun [45], CORBA [55, 56], Cactus [57], workflow sys- tems [58]) and feature much internal structure that would, if captured in our figure, expand it out to many times its current size. - These frameworks may themselves define protocols, services, and/or APIs (e.g., the Simple Workflow Access Protocol [58. - Table 6.1 The Grid services used to construct the two example applications of Figure 6.1 Multidisciplinary Simulation Ray Tracing Collective. - In the case of the ray tracing application, we assume that this is based on a high- throughput computing system [25, 39]. - In order to manage the execution of large numbers of largely independent tasks in a VO environment, this system must keep track of the set of active and pending tasks, locate appropriate resources for each task, stage executables to those resources, detect and respond to various types of failure, and so forth. - In the case of the multidisciplinary simulation application, the problems are quite dif- ferent at the highest level. - 6.6 ‘ON THE GRID’: THE NEED FOR INTERGRID PROTOCOLS. - For example, we can construct both Kerberos-and PKI-based protocols at the Connectivity layer – and access these security mechanisms via the same API, thanks to GSS-API (see Appendix). - For example, today’s Web browsers typically use TLS for authentication but do not support single sign-on or delegation.. - For example, the single sign-on capabilities provided in the GSI extensions to TLS would, if integrated into Web browsers, allow for single sign-on to multiple Web servers. - For example, the use of VPNs means that it is typically impossible for an ASP application to access data located on storage managed by a separate SSP. - For example, standard Grid services and protocols can be used to achieve a decoupling of the hardware and software. - Flexible delegation and access control mechanisms would allow a customer to grant an application running on an ASP computer direct, efficient, and securely access to data on SSP storage – and/or to couple resources from multiple ASPs and SSPs with their own resources, when required for more complex problems. - However, these mechanisms address none of the specific VO requirements listed above. - For example, in the case of CORBA, we could construct an object. - For example, Jini’s protocols and implementation are geared toward a small collection of devices. - Peer-to-peer computing (as implemented, for example, in the Napster, Gnutella, and Freenet [60] file sharing systems) and Internet computing (as implemented, for example, by the SETI@home, Parabon, and Entropia systems) is an example of the more gen- eral (‘beyond client-server’) sharing modalities and computational structures that we referred to in our characterization of VOs. - For example, single sign-on, delegation, and autho- rization technologies become important when computational and data-sharing services must interoperate, and the policies that govern access to individual resources become more complex.. - We summarize here – and critique – some alternative perspectives (given in italics).. - The Grid is a next-generation Internet: ‘The Grid’ is not an alternative to ‘the Internet’:. - We have provided in this article a concise statement of the ‘Grid problem,’ which we define as controlled and coordinated resource sharing and resource use in dynamic, scalable virtual organizations. - This work was supported in part by the Mathematical, Information, and Computational Sciences Division subprogram of the Office of Advanced Scientific Computing Research, United States Department of Energy, under Contract W-31-109-Eng-38. - For example, the FTP protocol definition indicates the format of the messages used to negotiate a file transfer but does not make clear how the receiving entity should manage its files.. - A service is defined in terms of the protocol one uses to interact with it and the behavior expected in response to various protocol message exchanges (i.e., ‘service = protocol + behavior.’).. - For example, in the Globus Toolkit, both the replica catalog [35] and information service [33]. - In the latter case, the API refers to particular a definition of command line arguments to the program, the input and output of the program, and the exit status of the program. - 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