First draft about streaming

docs/source/advancedDevelopment/advancedDevelopment.rst

@@ -13,3 +13,5 @@ Chapter :ref:`gettingStarted` provides the basics of Frank development. This cha
    dockerDevel/dockerDevel
    narayana/narayana
    deployment/maven
+   streaming/streaming
+

docs/source/advancedDevelopment/streaming/streaming.rst

@@ -0,0 +1,17 @@
+*Work in progress*
+
+Streaming
+=========
+
+This section is about an implementation detail of the Frank!Framework. The Frank!Framework handles large messages differently from small messages to save memory usage - large messages are *streamed*. Usually Frank developers do not have to worry about streaming. However, some errors you may encounter have to do with streaming and hence you need some knowledge about it.
+
+First, large messages are written to a temporary file before they are streamed. Under Linux, they are typically stored in the `/tmp` folder. If this folder is on a small disk partition, you may experience OutOfMemory exceptions. There is a property to configure the threshold for this behavior, `xxx`. If a message is larger than `xxx` bytes, it is written to a temporary file and then streamed.
+
+TODO: Find the right property name for `xxx` and the right comparison criterion.
+
+Second, messages that are already in a file are not copied to a temp file before they are streamed. This typically applies to the contents of a file handled by a `LocalFileSystemPipe`. This behavior is more general: the Frank!Framework has intelligence to determine whether a file has been stored already - it is only written to a temp file if it is not available otherwise.
+
+Ladybug helps you to debug issues with streaming. Checkpoints about streamed messages are clearly shown to be so.
+
+TODO: Add screenshot here.
+TODO: What is different before 7.9 from the behavior from 7.9 and later?