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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