[BEAM-11106] documentation for SDF truncation in Go (#17781)

* documentation for sdf truncation

* update import

* update snippet

sdks/go/examples/snippets/04transforms.go

@@ -167,6 +167,23 @@ func (fn *weDoFn) ProcessElement(e *CustomWatermarkEstimator, element string) {
 
 // [END watermarkestimation_customestimator]
 
+// [START sdf_truncate]
+
+// TruncateRestriction is a transform that is triggered when pipeline starts to drain. It helps to finish a
+// pipeline quicker by truncating the restriction.
+func (fn *splittableDoFn) TruncateRestriction(rt *sdf.LockRTracker, element string) offsetrange.Restriction {
+	start := rt.GetRestriction().(offsetrange.Restriction).Start
+	prevEnd := rt.GetRestriction().(offsetrange.Restriction).End
+	// truncate the restriction by half.
+	newEnd := prevEnd / 2
+	return offsetrange.Restriction{
+		Start: start,
+		End:   newEnd,
+	}
+}
+
+// [END sdf_truncate]
+
 // [START cogroupbykey_output_helpers]
 
 func formatCoGBKResults(key string, emailIter, phoneIter func(*string) bool) string {

website/www/site/content/en/documentation/programming-guide.md

@@ -6524,6 +6524,11 @@ unbounded restrictions finish processing at the next SDF-initiated checkpoint or
 You are able to override this default behavior by defining the appropriate method on the restriction
 provider.
 
+{{< paragraph class="language-go" >}}
+Note: Once the pipeline drain starts and truncate restriction transform is triggered, the `sdf.ProcessContinuation`
+will not be rescheduled.
+{{< /paragraph >}}
+
 {{< highlight java >}}
 {{< code_sample "examples/java/src/main/java/org/apache/beam/examples/snippets/Snippets.java" SDF_Truncate >}}
 {{< /highlight >}}
@@ -6533,7 +6538,7 @@ provider.
 {{< /highlight >}}
 
 {{< highlight go >}}
-This is not supported yet, see BEAM-11106.
+{{< code_sample "sdks/go/examples/snippets/04transforms.go" sdf_truncate >}}
 {{< /highlight >}}
 
 ### 12.7. Bundle finalization {#bundle-finalization}