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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