NIFI-7534 Consume MQTT Processor. Support Expression Language for Topic Filter Property

[kpsngh1998]

NIFI-7534 Consume MQTT Processor. Support Expression Language for Topic Filter Property

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Description of PR

Added variable registry expression language support for Topic Filter property.
Note: Flowfile attribute expression lanuguage support is not added since evaluation is done in OnScheduled and no flowfile is available there.

Enables X functionality; fixes bug NIFI-7534.

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[pvillard31]

Any reason for not using parameters?

[kpsngh1998]

@pvillard31 Could you elaborate which parameters? Are you asking about parameters of evaluateAttributeExpressions() method? If yes, since possible parameters are related to Flowfiles and ConsumeMQTT don't have any incoming flowfile, so didn't use any parameter.

[pvillard31]

On your process group, you can associate a parameter context where you can define your parameters. Parameters are referenced by using #{myParameter} in the property and parameters are supported everywhere (including sensitive properties and properties where expression language is not enabled). More details here: http://nifi.apache.org/docs/nifi-docs/html/user-guide.html#Parameters

[pvillard31]

I'd also add that with the addition of this concept of parameters (as a far better replacement of the variables), expression language should only be set where it makes sense to call some EL functions or to reference attributes of incoming flow files. But I'm not sure this is required for this specific use case since it does not accept incoming connections. Having said that, your change is valid from a code point of view, so I can merge it if there is use case that requires it.

[kpsngh1998]

Thanks for sharing info on parameters. Regarding EL usecase, I couldn't think of any atleast for MQTT source except only when you have topicName present in some registry file and you want to access it using EL. This jira was already raised so i picked it up as my first PR.

[pvillard31]

That's fine, it's not causing any issue and could be useful for people not willing to move to parameters right away. But going forward we really want to have people using Parameters instead of Variables.

[pvillard31]

Thanks for the contribution @kpsngh1998