Github user roshannaik commented on a diff in the pull request:
https://github.com/apache/storm/pull/1914#discussion_r99711705
--- 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
+should only be joined with the other streams using the field on which it
has been FieldsGrouped.
+Knowing this will help understand the join syntax described below.
+
+## Performing Joins
+Consider the following SQL join involving 4 tables called: stream1,
stream2, stream3 & stream4:
+
+```sql
+select userId, key4, key2, key3
+from stream1
+join stream2 on stream2.userId = stream1.key1
+join stream3 on stream3.key3 = stream2.userId
+left join stream4 on stream4.key4 = stream3.key3
+```
+
+This could be expressed using `JoinBolt` over 4 similarly named streams
as:
+
+```java
+new JoinBolt(JoinBolt.Selector.STREAM, "stream1", "key1") //
from stream1
--- End diff --
Absence of the definition of a specific stream upstream is equivalent to
not receiving any data from that stream from the Bolt. Conversely, if a
upstream component sends it data on a stream that the bolt is not aware of, it
will ignore that stream. Bolt doesn't examine the entire topology definition
scanning for incorrect wiring.
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