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I want to know if is it possible to use flowfile size as an argument in routeOnAttribute i want to make expression like that :

${filename.fileSize>500}
but  it  tells me  that  expression  ought to  return true and my expression returns  string  what should   i do to be able to make new  connection in routeOnAttribute(p.s filename is my flowfile name)
EDIT 05/17/2019 Correct answer is Andy's. That is the best guidance from us (Apache NiFi team) on the subject. The answer marked "correct" only applies in a few cases where Content-Length is set by an upstream processor. Andy's solution, fileSize is universally applicable.
                I want to check if the flowfile size is less than 500( it means  response is  empty) and  i can  make connection  with emty reponse and then log it. should  i use conetn size  instead of  size  ?
– Sagitarius
                Aug 24, 2017 at 6:53
                ok  thank you  I have made it like this ${Content-Length:toNumber():divide(500):equals(1)}
– Sagitarius
                Aug 24, 2017 at 7:31
Your expression is logically correct but does not use valid Apache NiFi syntax. It should use the gt function as below:
${fileSize:gt(500)}
This will return a boolean result.  
Two ways to check empty response present in flowfile.
1.Using Content-Length attribute-->${Content-Length}
2.Using Extract Text processor to extracts entire content in attribute.
flow_content--(.*)
then check ${flow_content:isEmpty():not()}
It may be helpful for your case.
                It is far preferable to use ${Content-Length} because extracting the content to an attribute when the content is large will have a very detrimental effect on the performance of the application. Attributes are held in memory and written to the flowfile repository, which is designed for fast read/write and lightweight data. It is highly discouraged to extract the entire flowfile contents to an attribute just to check the size.
– Andy
                Aug 24, 2017 at 18:55
        
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