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From justinleet <...@git.apache.org>
Subject [GitHub] metron pull request #718: METRON-1135: Make Stellar field transformations ha...
Date Mon, 28 Aug 2017 12:08:43 GMT
Github user justinleet commented on a diff in the pull request:

    https://github.com/apache/metron/pull/718#discussion_r135507857
  
    --- Diff: metron-platform/metron-parsers/README.md ---
    @@ -212,6 +212,49 @@ into `{ "protocol" : "TCP", "source.type" : "bro", ...}`
     * `STELLAR` : This transformation executes a set of transformations
       expressed as [Stellar Language](../metron-common) statements.
     
    +### Assignment to `null`
    +
    +If, in your field transformation, you assign a field to `null`, the field will be removed.
    +You can use this capability to rename variables.
    +
    +Consider this example:
    +```
    + "fieldTransformations" : [
    +         { "transformation" : "STELLAR"
    +         ,"output" : [ "new_field", "old_field"]
    +         ,"config" : {
    +           "new_field" : "old_field"
    +          ,"old_field" : "null"
    +                     }
    +         }
    + ]
    +```
    +This would set `new_field` to the value of `old_field` and remove `old_field`.
    +
    +### Warning: Transforming the same field twice
    +
    +Currently, the stellar expressions are expressed in the form of a map where the keys
define
    +the fields and the values define the Stellar expressions.  You order the expression evaluation
    +in the `output` field.  A consequence of this choice to store the assignments as a map
is that
    +the same field cannot appear in the map as a key twice.
    +
    +For instance, the following will not function as expected:
    --- End diff --
    
    I think this should be reworded. It's not that it doesn't function as expected; that would
imply a bug or an improvement that needs to happen.


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