Producer- write monotonically increasing delta chain version counter

Author: Sunjeet

hollow/src/main/java/com/netflix/hollow/api/producer/AbstractHollowProducer.java

@@ -17,6 +17,7 @@
 package com.netflix.hollow.api.producer;
 
 import static com.netflix.hollow.api.producer.ProducerListenerSupport.ProducerListeners;
+import static com.netflix.hollow.core.HollowStateEngine.HEADER_TAG_DELTA_CHAIN_VERSION_COUNTER;
 import static com.netflix.hollow.core.HollowStateEngine.HEADER_TAG_TYPE_RESHARDING_INVOKED;
 import static java.lang.System.currentTimeMillis;
 import static java.util.stream.Collectors.toList;
@@ -524,6 +525,20 @@ private void updateHeaderTags(HollowWriteStateEngine writeEngine, long toVersion
         }
         writeEngine.addHeaderTag(HollowStateEngine.HEADER_TAG_PRODUCER_TO_VERSION, String.valueOf(toVersion));
         writeEngine.getHeaderTags().remove(HEADER_TAG_TYPE_RESHARDING_INVOKED);
+
+        long prevDeltaChainVersionCounter = 0l;
+        if (readStates.hasCurrent()) {
+            String str = readStates.current().getStateEngine().getHeaderTag(HEADER_TAG_DELTA_CHAIN_VERSION_COUNTER);
+            if (str != null) {
+                try {
+                    prevDeltaChainVersionCounter = Long.valueOf(str);
+                } catch (NumberFormatException e) {
+                    prevDeltaChainVersionCounter = 0;
+                }
+            }
+        }
+        long deltaChainVersionCounter = prevDeltaChainVersionCounter + 1;
+        writeEngine.addHeaderTag(HEADER_TAG_DELTA_CHAIN_VERSION_COUNTER, String.valueOf(deltaChainVersionCounter));
     }
 
     void populate(

hollow/src/main/java/com/netflix/hollow/core/HollowStateEngine.java

@@ -75,6 +75,11 @@ public interface HollowStateEngine extends HollowDataset {
      */
     String HEADER_TAG_PRODUCER_TO_VERSION = "hollow.blob.to.version";
 
+    /**
+     * A header tag indicating monotonically increasing version in the same delta chain
+     */
+    String HEADER_TAG_DELTA_CHAIN_VERSION_COUNTER = "hollow.delta.chain.version.counter";
+
     @Override
     List<HollowSchema> getSchemas();
 

hollow/src/test/java/com/netflix/hollow/core/util/HollowWriteStateCreatorTest.java

@@ -16,6 +16,7 @@
  */
 package com.netflix.hollow.core.util;
 
+import static com.netflix.hollow.core.HollowStateEngine.HEADER_TAG_DELTA_CHAIN_VERSION_COUNTER;
 import static com.netflix.hollow.core.HollowStateEngine.HEADER_TAG_METRIC_CYCLE_START;
 import static com.netflix.hollow.core.HollowStateEngine.HEADER_TAG_PRODUCER_TO_VERSION;
 import static org.junit.Assert.assertEquals;
@@ -46,6 +47,7 @@ public void recreatesUsingReadEngine() throws IOException {
         }
         writeEngine.addHeaderTag("CopyTag", "copied");
         writeEngine.addHeaderTag(HEADER_TAG_METRIC_CYCLE_START, String.valueOf(System.currentTimeMillis()));
+        writeEngine.addHeaderTag(HEADER_TAG_DELTA_CHAIN_VERSION_COUNTER, "1");
         String toVersion = String.valueOf(System.currentTimeMillis());
         writeEngine.addHeaderTag(HEADER_TAG_PRODUCER_TO_VERSION, toVersion);
 
@@ -55,6 +57,7 @@ public void recreatesUsingReadEngine() throws IOException {
         HollowWriteStateEngine recreatedWriteEngine = HollowWriteStateCreator.recreateAndPopulateUsingReadEngine(readEngine);
         assertEquals(cycleStartTime, recreatedWriteEngine.getPreviousHeaderTags().get(HEADER_TAG_METRIC_CYCLE_START));
         assertEquals(readEngineToVersion, recreatedWriteEngine.getPreviousHeaderTags().get(HEADER_TAG_PRODUCER_TO_VERSION));
+        assertEquals("1", recreatedWriteEngine.getPreviousHeaderTags().get(HEADER_TAG_DELTA_CHAIN_VERSION_COUNTER));
         assertEquals(8, recreatedWriteEngine.getTypeState("Integer").getNumShards());
 
         HollowReadStateEngine recreatedReadEngine = StateEngineRoundTripper.roundTripSnapshot(recreatedWriteEngine);

hollow/src/test/java/com/netflix/hollow/core/write/HollowWriteStateEngineTest.java

@@ -1,5 +1,6 @@
 package com.netflix.hollow.core.write;
 
+import static com.netflix.hollow.core.HollowStateEngine.HEADER_TAG_DELTA_CHAIN_VERSION_COUNTER;
 import static com.netflix.hollow.core.HollowStateEngine.HEADER_TAG_PRODUCER_TO_VERSION;
 import static com.netflix.hollow.core.HollowStateEngine.HEADER_TAG_TYPE_RESHARDING_INVOKED;
 import static org.junit.Assert.assertEquals;
@@ -61,10 +62,12 @@ public void testHeaderTagsOnDeltaAndReverseDelta() {
         consumer.triggerRefreshTo(version3);    // delta transition
         assertEquals("3", consumer.getStateEngine().getHeaderTag(TEST_TAG));
         assertEquals(String.valueOf(version3), consumer.getStateEngine().getHeaderTag(HEADER_TAG_PRODUCER_TO_VERSION));
+        assertEquals("3", consumer.getStateEngine().getHeaderTag(HEADER_TAG_DELTA_CHAIN_VERSION_COUNTER));
 
         consumer.triggerRefreshTo(version2);    // reverse delta transition
         assertEquals("2", consumer.getStateEngine().getHeaderTag(TEST_TAG));
         assertEquals(String.valueOf(version2), consumer.getStateEngine().getHeaderTag(HEADER_TAG_PRODUCER_TO_VERSION));
+        assertEquals("2", consumer.getStateEngine().getHeaderTag(HEADER_TAG_DELTA_CHAIN_VERSION_COUNTER));
 
     }