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+# PIP-394: Add two interfaces `CursorMetadataSerializerProvider` and `CursorMetadataDeSerializerProvider` to support newer of customized cursor metadata serializations
+
+# Background knowledge
+
+**1. What does cursor metadata contain**
+
+- cursor properties.
+- entry id that indicates the latest persist cursor metadata into.
+- information of individual acknowledged messages, we call it `individualDeletedMessages`.
+- information of individual acknowledged batched messages, we call it `batchedEntryDeletionIndexInfo`.
+
+**2. The improvements we did for the persistence for cursor metadata**
+- https://github.com/apache/pulsar/pull/758: skip to information that over the max limitation of max ranges to persist.
+- https://github.com/apache/pulsar/issues/14529: compress the info when persisting.
+- https://github.com/apache/pulsar/pull/9292: add a new compression strategy: change Range Objects to `long[]`.
+
+# Motivation
+
+**Issue-1: Compatible of improvements**
+
+- The third improvement was contributed with `release:4.0`, which is a new LTS version.
+    - It changed the default implementation of serialization that contains https://github.com/apache/pulsar/pull/9292.
+- Users can not rollback to `3.0.x` once upgraded to `4.0.x` because `release:3.0.x` does not contain the deserialization that introduced by https://github.com/apache/pulsar/pull/9292.
+
+**Issue-2: Frequently Young GC relates to the cursor metadata persistence if there are too many active subscriptions in a broker, even if we did so many improvements**
+
+`individualDeletedMessages` and `batchedEntryDeletionIndexInfo` often is the largest attributes of the metadata. They are serialized to a proto data when being persisted. But we can not recycle the object which typed proto due to it is immutable.
+
+![375661781-51d5bd6d-f5a1-48d7-921a-975875fe8bed](https://github.com/user-attachments/assets/dd1eb135-7dee-4dd1-84ba-994618a8198e)
+
+
+# Goals
+
+- Guarantee compatability for rollback from `4.0.x` to `3.0.x`.
+  - This PIP will be cherry-picked into `branch-3.0` and `branch-3.3`. 
+- Support customized cursor metadata serializer to improve the issues users encountered, such as **Issue-1** in the Motivation.
+
+# High Level Design
+
+### Design
+
+- We call the serialization that implemented before `4.0.0` `V1`, and call after the https://github.com/apache/pulsar/pull/9292 `v2`.
+- Add all version of serialization into `branch-3.0`.
+  - Set the default value of `3.0.x` is `V1`, which is the same as the current status. 
+  - Set the default value of `4.0.x` is `V1`, which is the same as the current status.
+- Add two interfaces `CursorMetadataSerializerProvider` and `CursorMetadataDeSerializerProvider` to support newer of customized cursor metadata serializations.
+
+### Public API
+
+**CursorMetadataSerializerProvider.java**
+```java
+CursorMetadataSerializer newProvider(Name, PulsarService);
+```
+
+**CursorMetadataDeSerializerProvider.java**
+```java
+CursorMetadataDeserializer newProvider(Name, PulsarService);
+```
+
+**CursorMetadataSerializer.java**
+```java
+ManagedCursorInfo serialize(Position markDeletePosition,
+        Map<String, Long> properties,
+        LongPairRangeSet<Position> individualDeletedMessages,
+        Map<Position, BitSetRecyclable> batchDeletedIndexes);
+```
+
+**CursorMetadataDeserializer.java**
+```java
+ManagedCursorInfo deserialize(ByteBuf data);
+```
+
+### Public-facing Changes & Binary protocol
+- If you used your customized `CursorMetadataSerializer`, it may break the tools who will read cursor ZK node, such as the tool `pulsar-managed-ledger-admin`.
+
+### Configuration
+
+**broker.conf**
+```properties
+cursorMetadataSerializerProvider=V2
+cursorMetadataDeserializerProvider=V1,V2
+```
+
+### InScope and out of Scope
+
+This PIP will only add the interfaces named `CursorMetadataSerializerProvider` and `CursorMetadataDeSerializerProvider`, the implementations other than `V1` and `V2` will not be provided.
+
+# Backward & Forward Compatibility
+
+## Upgrade
+
+Nothing to do.
+
+## Downgrade / Rollback
+
+- I will cherry-pick this PIP into `branch-3.0` and `branch-3.3`.
+- Since https://github.com/apache/pulsar/pull/9292 changed the cursor metadata serialization. Once you upgraded to `4.0.x` from a lower version, you can only downgrade to the version that contains the current PIP.  
+
+# Links
+
+<!--
+Updated afterwards
+-->
+* Mailing List discussion thread: https://lists.apache.org/thread/xy1prwcv4wdoobphcgloj7s5gxy05qq3
+* Mailing List voting thread: https://lists.apache.org/thread/x8bf9hvk1pvo0dl0q3mcjh08wg90s89k