Mattang-Dan opened a new issue, #3185:
URL: https://github.com/apache/hop/issues/3185
### What needs to happen?
Below is the text of a new page, but it is also attached including pictures
in MS Word.
**Fields, parameters, and variables**
Fields, parameters, and variables implicitly are available downstream if in
scope. You can pass them again any number of levels, but the "get
parameters/variables" button only retrieves from one level above.
**Fields**: Fields are columns in data row(s) and are viewable in some
transform field textboxes and columns and you can see the fields in scope and
its value when looking at transform row results after executing a
workflow/pipeline (click the little grid icon on the bottom-right of a
transform to see a preview of the cached results). Field values can be passed
upstream for example if you use a pipeline executor and fill in the Result rows
tab and use it in conjunction with a “copy rows to result” transform in the
child pipeline.
**Parameters**: Think of parameters as function arguments, which turn into
variables with the same name. When adding parameters you are basically creating
MyPipeline(parameter1, parameter2,..). Parameters only need to be created and
declared once in a workflow. Parameters can be passed upstream by using “Set
variables”, but then they are not parameters anymore but variables. If you set
a parameter to a default value in becomes explicit (e.g. when editing a
pipeline in pipeline properties), it will take precedence over the same named
implicit variable (a passed-in variable), but not take precedence over the same
named implicit parameter. So whenever you see a transform with a column title
“Parameters/Variables” that simply means an explicit parameter is sent to a
function that will then set a variable name overriding any previous set
parameter with the same name (useful if wanting to override an explicitly set
param/variable in the child).
To change implicit parameter behavior to explicit, you can also disable
“pass all parameters” on the pipeline action. Parameters are simply variables
which are explicitly defined in a workflow or pipeline to make them
recognizable from outside those objects. They can also have a description and a
default value.
You cannot combine implicit variable inheritance with explicit parameter
definitions. So, if you add parameters to the pipeline definition (or to a
pipeline executor) and you want it set, you must add it to the parameters tab
of the child pipeline action/transform even if the same variable already exists.
Advanced: there are multiple layers in hop where you can set variables that
do or do not get overwritten downstream (Java -> hop environment -> project ->
run configuration -> workflow -> pipeline).
**Variables**: Variables are more global and the scope can be targeted
(entire Java VM, root workflow, etc.) whereas fields are the data flowing
between the transforms. Local variables can be set in a pipeline but should not
be used in the same pipeline as they are not thread-safe and can inherit a
previous value. It is better to send or return variables to another
pipeline/workflow before using them. Variables can be passed upstream if the
variable was set to a larger scope. E.g.: if a variable was set in a parent
with “valid in the current workflow”.
**Think of variables in 2 scopes:** runtime variables and
environment/workflow variables
• **Runtime variables** – Runtime variables depend on pipeline
information to generate, so they cannot be set beforehand, you need to declare
those differently to be able to use them.
• **Environment/workflow variables or parameters** – Environment
variables/parameters are set once and used when needed in any downstream
workflow/pipeline and there is no need to use Get Variables , you can refer to
them directly like ${myVariable} unless you need it in a field/data stream.
o E.g.: Define a parameter only once, even just in the Pipeline
Executor (no need to define in receiving pipeline)
A pipeline needs to start to get new variables. A running or nested pipeline
can't fetch new variable values. A pipeline is considered started when a
pipeline starts for every row in a pipeline executor. An alternative is to use
parameters.
[Fields and Parameters and
Variables.docx](https://github.com/apache/hop/files/12476436/Fields.and.Parameters.and.Variables.docx)
### Issue Priority
Priority: 3
### Issue Component
Component: Documentation
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