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https://issues.apache.org/jira/browse/NIFI-8634?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17356536#comment-17356536
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Jon Kessler commented on NIFI-8634:
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I don't believe they evaluate ExpressionLanguage but the ReaderLookup and
RecordSetWriterLookup controller services introduced by
https://issues.apache.org/jira/browse/NIFI-5829 get you pretty close to this
requirement.
> Create Controller Services that allows user to choose between Record
> Readers/Writers based on Expression Language
> -----------------------------------------------------------------------------------------------------------------
>
> Key: NIFI-8634
> URL: https://issues.apache.org/jira/browse/NIFI-8634
> Project: Apache NiFi
> Issue Type: New Feature
> Components: Extensions
> Reporter: Mark Payne
> Priority: Major
>
> Typically when a user builds a flow, the user must choose a JSON Reader, CSV
> Reader, JSON Writer, CSV Writer, etc. whatever makes sense for their use case.
> The downside to this approach is that it makes it difficult to build a flow
> is more generically distributable.
> We should create a RecordReaderLookup service and a RecordWriterLookup
> service. Each would have a single well-known property for the name of the
> Record Reader/Writer service to use. User-defined properties would then be
> used to define those Record Readers/Writers.
> For example, one might configure RecordReaderLookup as such:
> {code:java}
> Selected Reader: #{DataFOrmat}
> csv: CSVReader
> json: JsonTreeReader
> avro: AvroReader {code}
> In this case, CSVReader, JsonTreeReader, and AvroReader are other Record
> Reader Controller Services. Now, a parameter can be defined with the name
> {{DataFormat}}. If that parameter has a value of {{csv}}, the CSVReader would
> be used. If the parameter has a value of {{json}}, the JsonTreeReader would
> be used, and so forth.
> Same principle would be followed for the Record Writer.
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