Github user arunmahadevan commented on a diff in the pull request:
https://github.com/apache/storm/pull/1914#discussion_r99983167
--- 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
+ .join ("stream2", "userId", "stream1") //
join stream2 on stream2.userId = stream1.key1
--- End diff --
@roshannaik may be the earlier comment was not clear. Since the join key is
also the group by field in the api, choosing a subset of those keys in the join
condition is not possible.
E.g.
```java
new JoinBolt(JoinBolt.Selector.STREAM, "stream1", new Fields("key1",
"key2"))
// here its not possible to join on stream1.key1 or stream1.key2
separately
.join("stream2", new Fields("key3", "key4"), "stream1");
```
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