.. _dirac-setup-structure:

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DIRAC Setup Structure
==========================================

The basic DIRAC components are *Services*, *APIs*, *Agents*, and *Executors*.

  *Services*
    are passive components listening to incoming client requests and reacting accordingly by
    serving requested information from the *Database* backend or inserting requests on the
    *Database* backend. *Services* themselves can be clients of other *Services* from the same
    DIRAC *System* or from other *Systems*.

  *Agents*
    are active components, similar to cron jobs, which execution is invoked periodically.
    Agents are animating the whole system by executing actions, sending requests
    to the DIRAC or third party services.

  *Executors*
    are also active components, similar to consumers of a message queue system, which execution is invoked at request.
    Executors are used within the DIRAC Workload Management System.

  *APIs*
    APIs are implemention of `REST <http://en.wikipedia.org/wiki/Representational_state_transfer>`_ interfaces. Their usage is, for the moment, limited to a specific case.

These components are combined together to form *Systems*.
a *System* is delivering a complex functionality to the rest of DIRAC, providing a solution for a given class of tasks.
Examples of *Systems* are Workload Management System or Configuration System or Data Management System.

And then there are databases, which keep the persistent state of a *System*.
They are accessed by Services and Agents as a kind of shared memory.

To achieve a functional DIRAC installation, cooperation of different *Systems* is required.
A set of *Systems* provide a complete functionality to the end user.
Each *System* instance has a distinct name.

.. image:: ../../_static/setup_structure.png
   :alt: DIRAC setup structure illustration (source https://github.com/TaykYoku/DIRACIMGS/raw/main/DIRAC_Setup_structure.ai)
   :align: center