Can I create Stream Layer on ArcGIS Online, or I must have ArcGIS for Server with geoevent processor?
I can confirm that you need to have ArcGIS Server with the GeoEvent extension to create stream services (which can then be consumed by client apps as stream layers).
Currently, ArcGIS Online only supports hosted tiled and feature services.
Hope this helps,
Your best bet is to explore the ...ArcGIS Online Help | ArcGIS
and search by topic... ie. Search Result | ArcGIS
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