Commit a939b8a0 authored by Matthieu Muffato's avatar Matthieu Muffato
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Added a description of dataflows, incl. conditional dataflows

parent 39f45536
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Events in eHive
eHive is an *event-driven* system whereby agents trigger events that
are immediately reacted upon. The main event is called **Dataflow** and
consists of sending some data somewhere. The destination of a Dataflow
event must be defined in the pipeline graph itself, and is then referred to
by a *branch number* (see :doc:`dataflows`).
Within a Runnable, Dataflow events are performed via the ``$self->dataflow_output_id($data,
$branch_number)`` method.
The payload ``$data`` muts be of one of these types:
- Hash-reference that maps parameter names (strings) to their values.
- Array-reference of hash-references of the above type
- ``undef`` to propagate the job's input_id
The branch number defaults to 1 and can be skipped. Generally speaking, it
has to be an integer.
:doc:`dataflows` explains further how to configure branch numbers within a
pipeline. :doc:`dataflow_targets` lists the possible target types.
Conditional dataflows
eHive provides a mechanism to filter Dataflow events. It allows mapping a
given branch number to some targets on certain conditions.
The filtering happens based on the values of the parameters. It uses a
`WHEN-ELSE` syntax. It is similar to traditional `IF-THEN` conditions but
with some important differences:
#. `WHEN` happens when a condition is true.
#. There can be multiple `WHEN` cases, and more than one `WHEN` can flow
(as long asa they are true).
#. `ELSE` is the catch-all if none of the `WHEN` cases are true
The following examples show how single and multiple `WHEN` cases are handled,
together with their `ELSE` clause.
WHEN('#a# > 3' => ['analysis_b'],
'#a# > 5' => ['analysis_c'],
ELSE ['analysis_d'],
| Value of ``a`` | Active targets |
| 2 | analysis_d |
| 4 | analysis_b |
| 6 | analysis_b, analysis_c |
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