Add stream context and window trigger

core/src/main/java/org/opensearch/sql/planner/streaming/StreamContext.java

@@ -0,0 +1,19 @@
+/*
+ * Copyright OpenSearch Contributors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package org.opensearch.sql.planner.streaming;
+
+import lombok.Data;
+
+/**
+ * Stream context required by stream processing components and can be
+ * stored and restored between executions.
+ */
+@Data
+public class StreamContext {
+
+  /** Current watermark timestamp. */
+  private long watermark;
+}

core/src/main/java/org/opensearch/sql/planner/streaming/windowing/trigger/AfterWatermarkWindowTrigger.java

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+/*
+ * Copyright OpenSearch Contributors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package org.opensearch.sql.planner.streaming.windowing.trigger;
+
+import lombok.RequiredArgsConstructor;
+import org.opensearch.sql.planner.streaming.StreamContext;
+import org.opensearch.sql.planner.streaming.windowing.Window;
+
+/**
+ * After watermark window trigger fires window state output once a window is below watermark.
+ * Precisely speaking, after watermark means the window boundary (max timestamp) is equal to
+ * or less than the current watermark timestamp.
+ */
+@RequiredArgsConstructor
+public class AfterWatermarkWindowTrigger implements WindowTrigger {
+
+  /** Stream context that contains the current watermark. */
+  private final StreamContext context;
+
+  @Override
+  public TriggerResult trigger(Window window) {
+    if (window.maxTimestamp() <= context.getWatermark()) {
+      return TriggerResult.FIRE;
+    }
+    return TriggerResult.CONTINUE;
+  }
+}

core/src/main/java/org/opensearch/sql/planner/streaming/windowing/trigger/TriggerResult.java

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+/*
+ * Copyright OpenSearch Contributors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package org.opensearch.sql.planner.streaming.windowing.trigger;
+
+import lombok.Getter;
+import lombok.RequiredArgsConstructor;
+
+/**
+ * Result determined by a trigger for what should happen to the window.
+ */
+@Getter
+@RequiredArgsConstructor
+public enum TriggerResult {
+
+  /** Continue without any operation. */
+  CONTINUE(false, false),
+
+  /** Fire and purge window state by default. */
+  FIRE(true, true);
+
+  /** If window should be fired to output. */
+  private final boolean fire;
+
+  /** If the window state should be discarded. */
+  private final boolean purge;
+}

core/src/main/java/org/opensearch/sql/planner/streaming/windowing/trigger/WindowTrigger.java

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+/*
+ * Copyright OpenSearch Contributors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package org.opensearch.sql.planner.streaming.windowing.trigger;
+
+import org.opensearch.sql.planner.streaming.windowing.Window;
+
+/**
+ * A window trigger determines if the current window state should be evaluated to emit output.
+ * Typically, trigger strategy works with downstream Sink operator together to meet the semantic
+ * requirements. For example, per-event trigger can work with Sink for materialized view semantic.
+ */
+public interface WindowTrigger {
+
+  /**
+   * Return trigger result for a window.
+   *
+   * @param window given window
+   * @return trigger result
+   */
+  TriggerResult trigger(Window window);
+}

core/src/test/java/org/opensearch/sql/planner/streaming/windowing/trigger/AfterWatermarkWindowTriggerTest.java

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+/*
+ * Copyright OpenSearch Contributors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package org.opensearch.sql.planner.streaming.windowing.trigger;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+import org.opensearch.sql.planner.streaming.StreamContext;
+import org.opensearch.sql.planner.streaming.windowing.Window;
+
+class AfterWatermarkWindowTriggerTest {
+
+  private final StreamContext context = new StreamContext();
+
+  private final AfterWatermarkWindowTrigger trigger = new AfterWatermarkWindowTrigger(context);
+
+  @Test
+  void shouldNotFireWindowAboveWatermark() {
+    context.setWatermark(999);
+    assertEquals(TriggerResult.CONTINUE, trigger.trigger(new Window(500, 1500)));
+    assertEquals(TriggerResult.CONTINUE, trigger.trigger(new Window(500, 1001)));
+    assertEquals(TriggerResult.CONTINUE, trigger.trigger(new Window(1000, 1500)));
+  }
+
+  @Test
+  void shouldFireWindowBelowWatermark() {
+    context.setWatermark(999);
+    assertEquals(TriggerResult.FIRE, trigger.trigger(new Window(500, 800)));
+    assertEquals(TriggerResult.FIRE, trigger.trigger(new Window(500, 1000)));
+  }
+}