How to use the GraalVM plugin

Execute inline JavaScript, Python, or Ruby scripts directly within Kestra flows using GraalVM's polyglot runtime — no Docker container or external runtime needed.

Every task also accepts options (advanced, a map of GraalVM context option key/value pairs, e.g. python.WarnOptions or js.ecmascript-version). Keys that would weaken the sandbox already enforced by these tasks are rejected at execution time: python.PosixModuleBackend, any key related to host access or class loading, and any key prefixed with engine. or sandbox..

File access

Scripts can only access files inside the task working directory. File access through the language itself (for example Python's open, os or tempfile, JavaScript's load(), or Ruby's load) goes through a file system that rejects any path that resolves outside the working directory, including through .. or a symbolic link. The bundled standard library (Python and Ruby) is read-only for scripts. Scripts cannot create symbolic links, and temporary files are created inside the working directory.

Java classes that open files by name or path (java.io.File, FileInputStream, FileOutputStream, java.nio.file.* and similar) cannot be used from scripts, and Path or File objects returned by Kestra APIs such as runContext.workingDir() can only be passed back to Kestra APIs. To store a file in Kestra's internal storage, pass its content as a stream: runContext.storage().putFile(new java.io.ByteArrayInputStream(bytes), 'name.txt').

Python native modules are the exception, see the security note on the Python tasks below.

Tasks

JavaScript

js.Eval executes a JavaScript script — set script (required). Optionally declare outputs (a list of variable names to capture from the script's scope and expose as task outputs).

js.FileTransform applies a JavaScript transformation to each row of an ION file — set script (required) and from (required, a kestra:// URI). Optionally set concurrent (minimum 2) to process rows in parallel.

Python

python.Eval executes a Python script — set script (required). Optionally declare outputs and modules (a map of module names to install or configure before execution).

python.FileTransform applies a Python transformation to each row of an ION file — set script (required) and from (required, a kestra:// URI). Optionally set concurrent (minimum 2).

Security note: both Python tasks enable GraalVM's engine-wide native access so that C-extension-backed stdlib modules (ssl, sqlite3, lzma) work. Standard OS process APIs (os.system, subprocess) stay blocked regardless. However, GraalVM has no way to grant native access for its own C-extension loader while denying it to guest code that reaches native functions directly (e.g. Python's ctypes) — a script can therefore still execute arbitrary OS commands, read or write any file the worker can access, or manipulate process memory this way. Native modules also open files themselves, outside the working directory restriction: sqlite3.connect() can read or create a database file anywhere the worker can. Unlike Kestra's Script/Shell tasks, which typically run in an isolated container or process via a TaskRunner, this code runs inline in the worker JVM process itself, giving it direct access to the worker's own memory, file descriptors, and any secrets or other task state resident in that JVM — a materially larger blast radius than before native access was added. Only run Python scripts from users already trusted with the flow's credentials and infrastructure.

Ruby

ruby.Eval executes a Ruby script — set script (required). Optionally declare outputs.

ruby.FileTransform applies a Ruby transformation to each row of an ION file — set script (required) and from (required, a kestra:// URI). Optionally set concurrent (minimum 2).