Github user arunmahadevan commented on a diff in the pull request:
https://github.com/apache/storm/pull/1914#discussion_r99744089
--- Diff: docs/Joins.md ---
@@ -0,0 +1,102 @@
+---
+title: Joining Streams in Storm Core
+layout: documentation
+documentation: true
+---
+
+Storm core supports joining multiple data streams into one with the help
of `JoinBolt`.
+`JoinBolt` is a Windowed bolt, i.e. it waits for the configured window
duration to match up the
+tuples among the streams being joined. This helps align the streams within
the Window boundary.
+
+Each of `JoinBolt`'s incoming data streams must be Fields Grouped on a
single field. A stream
--- End diff --
based on the next comment, checks are not done and applying the right
grouping is up to the user.
IMO, adding as many validations as possible and automatically handling the
groupings would make it less error prone. But this may require changes in the
topology builder, so for now may be just add some relevant code that shows the
fields grouping (`builder.setBolt(joinBolt).fieldsGrouping(..)`) in the docs.
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