Triggers design patterns
========================

Applications of type Triggers (see :ref:`application_types`) are implemented using the following design patterns.

Queue design pattern
+++++++++++++++++++++

The design of a Queue Application implies steps for:

- Data discovery,
- Data selection,
- Parameters definition,
- Data Item(s) creation,
- Data Item(s) submission to the Processing Queue.

An example of control flow is the following:

.. uml::
 
    !define DIAG_NAME Trigger Queue example
    !include includes/skins.iuml
    skinparam backgroundColor #FFFFFF
    skinparam componentStyle uml2

    start
    
    :Discover Data;

    while (Are there Data objects?) is (yes)
      if (Does Data object satisfy processing criteria?) then (yes)
        :Define processing parameters;
        :Create Data Item;
        :Add Data Item to Processing Queue;
      else (no)
      endif
    endwhile (no)

    stop
    
.. tip::
  It's important to note that Queue Applications, when discovering and selecting data, deal with metadata only, leveraging an Opensearch catalog (see :ref:`da-catalog`). Stage-in data is not usually required.

Pipe design pattern
++++++++++++++++++++

Since Pipe Applications are not developed by the Users but provided as a service by the platform, the design pattern is provided as an additional information, that can be used to understand its behaviour.

A Pipe Application performs the following steps:

- Data Items retrieval,
- Data Items analysis,
- Stand-alone Processing Application execution submission,
- Stand-alone Processing Application execution monitor.

An example of control flow is the following:

.. uml::
    
    !define DIAG_NAME Trigger Pipe example
    !include includes/skins.iuml
    skinparam backgroundColor #FFFFFF
    skinparam componentStyle uml2

    start
    
    :Retrieve Data Items;

    while (Are there Data Item objects to process?) is (yes)
      if (Has Data Item a status 'in'?) then (yes)
        :Monitor execution status;
        if (Succedeed?) then (yes)
          :Update Data Item status to 'out';
        else (no)
          :Update Data Item status to 'err';
        endif
      else (no)
        if (Has Data Item a status 'queue'?) then (yes)
          :Check processing capacity;
          if (Is there processing capacity?) then (yes)
            :Extract information from Data Item;
            :Submit exection of a Stand-alone Processing Application;
            :Update Data Item status to 'in';
          else (no)
          endif
        else (no)
        endif
      endif
    endwhile (no)

    stop