From 677c3242761e0be61b3224d5aaf6db5c9673f27a Mon Sep 17 00:00:00 2001
From: Andreas Motl <andreas.motl@panodata.org>
Date: Mon, 13 Jul 2026 00:15:05 +0200
Subject: [PATCH] Documentation: Educate about idempotency of user-defined
 functions

---
 docs/configure/transformation.md | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/docs/configure/transformation.md b/docs/configure/transformation.md
index be6bb663..8bd8816e 100644
--- a/docs/configure/transformation.md
+++ b/docs/configure/transformation.md
@@ -269,6 +269,9 @@ Those [topic options](#topic-options) *do **not** accept* a user-defined functio
 - `topic`
 - `qos`
 
+Functions get invoked once per target. If idempotency of a custom function is
+not guaranteed (function(msg) returns always the same result), the message
+might not get dispatched to every target the same way.
 
 ### Getting started