doc: reduce streaming (numaproj#1689)

Signed-off-by: Yashash H L <yashashhl25@gmail.com>
Signed-off-by: Vigith Maurice <vigith@gmail.com>
Co-authored-by: Vigith Maurice <vigith@gmail.com>

Author: yhl25

docs/user-guide/user-defined-functions/reduce/windowing/fixed.md

@@ -58,10 +58,44 @@ event time characteristic) of `2031-09-29T18:47:00Z` will belong to the window w
 `[2031-09-29T18:47:00Z, 2031-09-29T18:48:00Z)`
 
 It is important to note that because of this property, for a constant throughput, the first window 
-may contain fewer elements than other windows. 
+may contain fewer elements than other windows.
 
+Check the links below to see the UDF examples for different languages.
 
+- [Python](https://github.com/numaproj/numaflow-python/tree/main/examples/reduce)
+- [Golang](https://github.com/numaproj/numaflow-go/tree/main/pkg/reducer/examples)
+- [Java](https://github.com/numaproj/numaflow-java/tree/main/examples/src/main/java/io/numaproj/numaflow/examples/reduce)
 
 
 
+## Streaming Mode
+
+Reduce can be enabled on streaming mode to stream messages or forward partial responses to the next vertex.
+This is useful for custom triggering, where we want to forward responses to the next vertex quickly,
+even before the fixed window closes. The close-of-book and a final triggering will still happen even if
+partial results have been emitted.
+
+
+To enable reduce streaming, set the `streaming` flag to `true` in the fixed window configuration.
+
+```yaml
+vertices:
+  - name: my-udf
+    udf:
+      groupBy:
+        window:
+          fixed:
+            length: duration
+            streaming: true # set streaming to true to enable reduce streamer
+```
+
+Note: UDFs should use the ReduceStreamer functionality in the SDKs to use this feature.
+
+Check the links below to see the UDF examples in streaming mode for different languages.
+
+- [Python](https://github.com/numaproj/numaflow-python/tree/main/examples/reducestream)
+- [Golang](https://github.com/numaproj/numaflow-go/tree/main/pkg/reducestreamer/examples)
+- [Java](https://github.com/numaproj/numaflow-java/tree/main/examples/src/main/java/io/numaproj/numaflow/examples/reducestreamer/sum)
+
+
 

docs/user-guide/user-defined-functions/reduce/windowing/session.md

@@ -68,6 +68,13 @@ later, meaning it's beyond the session gap of the previous window, initiating a
 `Event-4` and `Event-5`, and it closes 30 seconds after `Event-5` at `2031-09-29T18:47:40Z`, if no further events arrive
 for the key until the timeout.
 
+Note: Streaming mode is by default enabled for session windows. 
+
+Check the links below to see the UDF examples for different languages. Currently, we have the SDK support for Golang and Java.
+
+- [Golang](https://github.com/numaproj/numaflow-go/tree/main/pkg/sessionreducer)
+- [Java](https://github.com/numaproj/numaflow-java/tree/main/examples/src/main/java/io/numaproj/numaflow/examples/reducesession/counter)
+
 
 
 

docs/user-guide/user-defined-functions/reduce/windowing/sliding.md

@@ -78,6 +78,43 @@ window `present + sliding`)
 From the point above, it follows then that immediately upon startup, for the first window, fewer elements may get 
 aggregated depending on the current _lateness_ of the data stream.
 
+Check the links below to see the UDF examples for different languages.
+
+- [Python](https://github.com/numaproj/numaflow-python/tree/main/examples/reduce)
+- [Golang](https://github.com/numaproj/numaflow-go/tree/main/pkg/reducer/examples)
+- [Java](https://github.com/numaproj/numaflow-java/tree/main/examples/src/main/java/io/numaproj/numaflow/examples/reduce)
+
+
+## Streaming Mode
+
+Reduce can be enabled on streaming mode to stream messages or forward partial responses to the next vertex.
+This is useful for custom triggering, where we want to forward responses to the next vertex quickly,
+even before the fixed window closes. The close-of-book and a final triggering will still happen even if
+partial results have been emitted.
+
+
+To enable reduce streaming, set the `streaming` flag to `true` in the sliding window configuration.
+
+```yaml
+vertices:
+  - name: my-udf
+    udf:
+      groupBy:
+        window:
+          sliding:
+            length: duration
+            slide: duration
+            streaming: true # set streaming to true to enable reduce streamer
+```
+
+Note: UDFs should use the ReduceStreamer functionality in the SDKs to use this feature.
+
+Check the links below to see the UDF examples in streaming mode for different languages.
+
+- [Python](https://github.com/numaproj/numaflow-python/tree/main/examples/reducestream)
+- [Golang](https://github.com/numaproj/numaflow-go/tree/main/pkg/reducestreamer/examples)
+- [Java](https://github.com/numaproj/numaflow-java/tree/main/examples/src/main/java/io/numaproj/numaflow/examples/reducestreamer/sum)
+
 
 
 

pkg/udf/forward/forward_test.go

@@ -1671,7 +1671,7 @@ func validateMetrics(t *testing.T, batchSize int64) {
 
 	err := testutil.CollectAndCompare(metrics.ReadDataMessagesCount, strings.NewReader(metadata+expected), "forwarder_data_read_total")
 	if err != nil {
-		t.Errorf("unexpected collecting result:\n%s", err)
+		t.Errorf("unexpected collecting result: %v", err)
 	}
 
 	writeMetadata := `