Integration / Apache NiFi Interview Questions
What is the ExecuteScript processor and what scripting languages does it support?
ExecuteScript is NiFi's escape hatch for custom logic that cannot be expressed with built-in processors. It allows you to write arbitrary script code that executes within the NiFi processor lifecycle — accessing incoming FlowFiles, creating new FlowFiles, modifying attributes, reading and writing content, and routing FlowFiles to relationships.
Supported scripting languages (via the Java Scripting Engine API):
- Groovy (most popular in NiFi community — expressive, JVM-native)
- Python (via Jython — Python 2.7 dialect; C extensions unavailable)
- ECMAScript / JavaScript (via Nashorn in Java 8, deprecated in Java 11+)
- Ruby (via JRuby)
- Lua
In Groovy, a typical ExecuteScript pattern looks like:
def flowFile = session.get() if (!flowFile) return flowFile = session.write(flowFile, { inputStream, outputStream -> def text = inputStream.getText('UTF-8') outputStream.write(text.toUpperCase().bytes) } as StreamCallback) session.transfer(flowFile, REL_SUCCESS)
ExecuteScript has access to: session (ProcessSession), context (ProcessContext), log (ComponentLogger), and predefined relationship variables (REL_SUCCESS, REL_FAILURE). The script can use any Java library available on the NiFi classpath.
Why can Jython (Python in ExecuteScript) not use libraries like NumPy or Pandas?
Which NiFi object gives ExecuteScript access to read and write FlowFile content and transfer FlowFiles to relationships?
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