Enable state tag encoding v2 by default for new Dataflow Streaming Engine jobs

Also added CHANGES.md entry detailing the change, how to disable it, and job update/downgrade limitations.

Author: arunpandianp

CHANGES.md

@@ -55,6 +55,42 @@
 * ([#X](https://github.com/apache/beam/issues/X)).
 -->
 
+# [2.75.0] - Unreleased
+
+## Highlights
+
+* New highly anticipated feature X added to Python SDK ([#X](https://github.com/apache/beam/issues/X)).
+* New highly anticipated feature Y added to Java SDK ([#Y](https://github.com/apache/beam/issues/Y)).
+
+## I/Os
+
+* Support for X source added (Java/Python) ([#X](https://github.com/apache/beam/issues/X)).
+
+## New Features / Improvements
+
+* (Java) Enabled state tag encoding v2 by default for new Dataflow Streaming Engine jobs. It can be disabled by passing `--experiments=disable_streaming_engine_state_tag_encoding_v2` pipeline option. Note that the tag encoding version cannot change during a job update, and new jobs created on 2.75.0+ cannot be downgraded to versions before 2.73.0 (only versions 2.73.0 and later support tag encoding v2). ([#38705](https://github.com/apache/beam/issues/38705)).
+
+## Breaking Changes
+
+* X behavior was changed ([#X](https://github.com/apache/beam/issues/X)).
+
+## Deprecations
+
+* X behavior is deprecated and will be removed in X versions ([#X](https://github.com/apache/beam/issues/X)).
+
+## Bugfixes
+
+* Fixed X (Java/Python) ([#X](https://github.com/apache/beam/issues/X)).
+
+## Security Fixes
+
+* Fixed [CVE-YYYY-NNNN](https://www.cve.org/CVERecord?id=CVE-YYYY-NNNN) (Java/Python/Go) ([#X](https://github.com/apache/beam/issues/X)).
+
+## Known Issues
+
+[comment]: # ( When updating known issues after release, make sure also update website blog in website/www/site/content/blog.)
+* ([#X](https://github.com/apache/beam/issues/X)).
+
 # [2.74.0] - Unreleased
 
 ## Highlights

runners/google-cloud-dataflow-java/src/main/java/org/apache/beam/runners/dataflow/DataflowRunner.java

@@ -1310,6 +1310,9 @@ public DataflowPipelineJob run(Pipeline pipeline) {
         // Experiment marking that the harness supports tag encoding v2
         // Backend will enable tag encoding v2 only if the harness supports it.
         experiments.add("streaming_engine_state_tag_encoding_v2_supported");
+        // Experiment requesting tag encoding v2 on new jobs. During job updates
+        // old job's tag encoding version is carried over by the backend.
+        experiments.add("enable_streaming_engine_state_tag_encoding_v2");
         options.setExperiments(ImmutableList.copyOf(experiments));
       }