robertwb commented on a change in pull request #15227:
URL: https://github.com/apache/beam/pull/15227#discussion_r694277635
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File path: website/www/site/content/en/documentation/programming-guide.md
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@@ -448,6 +448,12 @@ individual elements. A Beam Transform might process each
element of a
`PCollection` and generate new pipeline data (as a new `PCollection`), *but it
does not consume or modify the original input collection*.
+> **Note:** Beam SDKs avoid unnecessary copying of elements, so `PCollection`
+> contents are logically immutable, not physically immutable. Changes to input
+> elements may be visible to DoFns executing within the same bundle, and may
Review comment:
may be visible to other DoFns...
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