Architecture overview

Most of the computing resources needed by the LHC HEP experiments as well as for some other communities are provided by Computing Grids. The Grids provide a uniform access to the computing and storage resources which simplifies a lot their usage. The Grid middleware stack offers also the means to manage the workload and data for the users. However, the variety of requirements of different Grid User Communities is very large and it is difficult to meet everybody’s needs with just one set of the middleware components. Therefore, many of the Grid User Communities, and most notably the LHC experiments, have started to develop their own sets of tools which are evolving towards complete Grid middleware solutions. Examples are numerous, ranging from subsystem solutions (PANDA workload management system or PHEDEX data management system) or close to complete Grid solutions (AliEn system). DIRAC project is providing a complete Grid solution for both workload and data management tasks on the Grid.

Although developed for the LHCb experiment, it is designed to be a generic system with LHCb specific features well isolated as plugin modules. It allows to construct medium sized grids of up to several tens of thousands processors by uniting PC farms with most widely used cluster software systems as well as individual PCs within its integrated Workload Management System. DIRAC also provides means for managing tasks on Grid resources taking over the workload management functions. The DIRAC Data Management components provide access to standard grid storage systems based on the SRM standard interface or ordinary (S)FTP, HTTP file servers. The File Catalog options include the LCG File Catalog (LFC) as well as a native DIRAC File Catalog. The modular organization of the DIRAC components allows selecting a subset of the functionality suitable for particular applications or easily adding the missing functionality. All these features provide a Grid solution for a medium size community of users.

The DIRAC architecture consists of numerous cooperating Distributed Services and Light Agents built within the same DISET framework following the Grid security standards.

DIRAC introduced the now widely used concept of Pilot Agents. This allows efficient Workload Management Systems (WMS) to be built. The workload of the community is optimized in the central Task Queue. The WMS is carefully designed to be resilient to failures in the ever changing Grid environment.

The DIRAC project includes a versatile Data Management System (DMS) which is optimized for reliable data transfers. The DMS automates the routine data distribution tasks.

The DIRAC Transformation Management System is built on top of the Workload and Data Management services. This provides automated data driven submission of processing jobs with workflows of arbitrary complexity

The DIRAC Project has all the necessary components to build Workload and Data management systems of varying complexity. It offers a complete and powerful Grid solution for other user grid communities.

DIRAC design principles

  • DIRAC is conceived as a light grid system.
  • Following the paradigm of a Services Oriented Architecture (SOA), DIRAC is lightweight, robust and scalable. This was inspired by the OGSA/OGSI “grid services” concept and the LCG/ARDA RTAG architecture blueprint
  • It should support a rapid development cycle to accommodate ever-evolving grid opportunities.
  • It should be easy to deploy on various platforms and updates in order to bring in bug fixes and new functionalities should be transparent or even automatic.
  • It is designed to be highly adaptable to the use of heterogeneous computing resources available to the LHCb Collaboration.
  • It must be simple to install, configure and operation of various services. This makes the threshold low for new sites to be incorporated into the system.
  • The system should automate most of the tasks, which allows all the DIRAC resources to be easily managed by a single Production Manager.
  • Redundancy
  • The information which is vital to the successful system operation is duplicated at several services to ensure that at least one copy will be available to client request.This is done for the DIRAC Configuration Service and for the File Catalog each of which has several mirrors kept synchronized with the master instance.
  • All the important operations for which success is mandatory for the functioning of the system without losses are executed in a failover recovery framework which allows retrying them in case of failures. All the information necessary for the operation execution is encapsulated in an XML object called request which is stored in one of the geographically distributed request databases.
  • For the data management operations, for example for initial data file uploading to some grid storage, in case of failure the files are stored temporarily in some spare storage element with a failover request to move the data to the final destination when it becomes available.
  • System state information
    • Keeping the static and dynamic information separately reduces the risk of compromising the static information due to system overloading.
    • In DIRAC the static configuration data is made available to all the clients via the Configuration Service (CS) which has multiple reservations. Moreover, this information can be cached on the client side for relatively short periods without risk of client misbehaviour.
    • The dynamic information is in most cases looked for at its source. This is why, for example, the DIRAC Workload Management System is following the “pull” paradigm where the computing resources availability is examined by a network of agents running in close connection to the sites.
  • Requirements to sites
    • The main responsibility of the sites is to provide resources for the common use in a grid. The resources are controlled by the site managers and made available through middleware services (Computing and Storage Elements).
    • DIRAC puts very low requirements on the sites asking for no special support for the LHCb VO. The data production activity requires no special support from the site managers apart from ensuring availability of the standard services. There is also no special requirement on VO job optimization and accounting.
    • All this allows for the exploitation of numerous sites providing resources to the LHCb VO by a small central team of production managers.

DIRAC Architecture

DIRAC follows the paradigm of a Services Oriented Architecture (SOA).

The DIRAC components can be grouped in the following 4 categories:
  • Resources
  • Services
  • Agents
  • Interfaces

Resources

DIRAC covers all the possible resources available to the LHCb experiment, if necessary, new types of the computing resources can be easily added:

  • Individual PCs
  • Computing farms with various batch systems: PBS/Torque,LSF, Sun Grid Engine, Condor, BQS and Microsoft Compute Cluster.
  • Computing Elements in the EGEE grid which are based on the GRAM interface.

DIRAC does not provide a complex Storage Element service capable of managing multiple disk pools or tertiary storage systems. Storage Element can be:

  • Disk storage managed by a POSIX compliant file system.
  • Storage Elements wit the SRM standard interface: gridftp, (s)ftp, http, and some others.

Sometimes the same physical storage is available through several different protocols. This can be expressed in the storage configuration description and the DIRAC data access tools will be able to use any of the possible protocols in an optimal way. This also adds redundancy ensuring higher storage availability in case of intermittent failures.

Services

  • The DIRAC system is built around a set of loosely coupled services which keep the system state and help to carry out workload and data management tasks. The services are passive components which are only reacting to the requests of their clients possibly soliciting other services in order to accomplish the requests.
  • All services and their clients are built in the DISET framework which provides secure access and flexible authorization rules. Each service has typically a MySQL database backend to store the state information. The services as permanent processes are deployed centrally at CERN and on a number of hosts (VO-boxes) at several sites.
  • The number of sites where services are installed is limited to those with well-controlled environment where an adequate support can be guaranteed. The services are deployed using system start-up scripts and watchdog processes which ensure automatic service restart at boot time and in case of service interruptions or crashes. Standard host certificates typically issued by national Grid Certification Authorities are used for the service/client authentication.
  • The services accept incoming connections from various clients. These can be user interfaces, agents or running jobs. But since services are passive components, they have to be complemented by special applications to animate the system.

Agents

Agents are light and easy to deploy software components which run as independent processes to fulfill one or several system functions.

  • All the agents are built in the same framework which organizes the main execution loop and provides a uniform way for deployment, configuration, control and logging of the agent activity.
  • Agents run in different environments. Those that are part of the DIRAC subsystems, for example Workload Management or Data Distribution, are usually deployed close to the corresponding services. They watch for changes in the service states and react accordingly by initiating actions like job submission or result retrieval.
  • Agents can run on a gatekeeper node of a site controlled by the DIRAC Workload Management System. In this case, they are part of the DIRAC WMS ensuring the pull job scheduling paradigm. Agents can also run as part of a job executed on a Worker Node as so called “Pilot Agents”.

Interfaces

  • The DIRAC main programming language is Python and programming interfaces (APIs) are provided in this language.
  • For the users of the DIRAC system the functionality is available through a command line interface.
  • DIRAC also provides Web interfaces for users and system managers to monitor the system behaviour and to control the ongoing tasks. The Web interfaces are based on the DIRAC Web Portal framework which ensures secure access to the system service using X509 certificates loaded into the user browsers.

DIRAC Framework

The Dirac framework for building secure SOA based systems provides generic components not specific to LHCb which can be applied in the contexts of other VOs as well. The framework is written in the Python language and includes the following components:

  • DISET (DIRAC Secure Transport) secure communication protocol
  • Web Portal framework
  • Configuration System
  • Logging System
  • Monitoring System

Web portal framework

The Web portal framework allows the building of Web interfaces to DIRAC services. It provides Authentication based on user grid credentials and user groups which can be selected during the interactive session. The framework uses the DISET portal functionality to redirect client requests to corresponding services and to collect responses. It provides the means to organize the contents of the DIRAC Web sites using the Pylons contents management system.

All the monitoring and control tools of a DIRAC system are exported through the Web portal which makes them uniform for users working in different environment and on different platforms.

Configuration Service

The Configuration Service is built in the DISET framework to provide static configuration parameters to all the distributed DIRAC components. This is the backbone of the whole system and necessitates excellent reliability. Therefore, it is organized as a single master service where all the parameter updates are done and multiple read-only slave services which are distributed geographically, on VO-boxes at Tier-1 LCG sites in the case of LHCb. All the servers are queried by clients in a load balancing way. This arrangement ensures configuration data consistency together with very good scalability properties.

Logging and Monitoring Services

  • All the DIRAC components use the same logging facility which can be configured with one or more back-ends including standard output, log files or external service.
  • The amount of the logging information is determined by a configurable level specification.
  • Use of the logger permit report to the Logging Service where all the distributed components are encountering system failures.
  • This service accumulates information for the analysis of the behaviour of the whole distributed system including third party services provided by the sites and central grid services.
  • The quick error report analysis allows spotting and even fixing the problems before they hit the user.
  • The Monitoring Service collects activity reports from all the DIRAC services and some agents. It presents the monitoring data in a variety of ways, e.g. historical plots, summary reports, etc. Together with the Logging Service, it provides a complete view of the health of the system for the managers.