[improve][pip] PIP-381: Handle large PositionInfo state

pip/pip-381-large-positioninfo.md

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+# PIP-381: Handle large PositionInfo state
+
+# Background knowledge
+
+In case of KEY_SHARED subscription and out-of-order acknowledgments, 
+the PositionInfo state can be persisted to preserve the state, 
+with configurable maximum number of ranges to persist:
+
+```
+# Max number of "acknowledgment holes" that are going to be persistently stored.
+# When acknowledging out of order, a consumer will leave holes that are supposed
+# to be quickly filled by acking all the messages. The information of which
+# messages are acknowledged is persisted by compressing in "ranges" of messages
+# that were acknowledged. After the max number of ranges is reached, the information
+# will only be tracked in memory and messages will be redelivered in case of
+# crashes.
+managedLedgerMaxUnackedRangesToPersist=10000
+```
+
+The PositionInfo state is stored to the BookKeeper as a single entry, and it can grow large if the number of ranges is large.
+Currently, this means that BookKeeper can fail persisting too large PositionInfo state, e.g. over 1MB 
+by default and the ManagedCursor recovery on topic reload might not succeed.
+
+# Motivation
+
+While keeping the number of ranges low to prevent such problems is a common sense solution, there are cases
+where the higher number of ranges is required. For example, in case of the JMS protocol handler,
+JMS consumers with filters may end up processing data out of order and/or at different speed,
+and the number of ranges can grow large.
+
+# Goals
+
+Store the PositionInfo state in a BookKeeper ledger as multiple entries if the state grows too large to be stored as a single entry.
+
+## In Scope
+
+Transparent backwards compatibility if the PositionInfo state is small enough.
+
+## Out of Scope
+
+Backwards compatibility in case of the PositionInfo state is too large to be stored as a single entry.
+
+# High Level Design
+
+Cursor state writes and reads are happening at the same cases as currently, without changes.
+
+Write path:
+
+1. serialize the PositionInfo state to a byte array.
+2. if the byte array is smaller than the threshold, store it as a single entry, as now. Done.
+3. if the byte array is larger than the threshold, split it to smaller chunks and store the chunks in a BookKeeper ledger.
+4. write the "footer" into the metadata store as a last entry.
+
+See `persistPositionToLedger()` in `ManagedCursorImpl` for the implementation.
+
+The footer is a JSON representation of
+
+```java
+    public static final class ChunkSequenceFooter {
+        private int numParts;
+        private int length;
+    }
+```
+
+Read path:
+
+1. read the last entry from the metadata store.
+2. if the entry does not appear to be a JSON, treat it as serialized PositionInfo state and use it as is. Done.
+3. if the footer is a JSON, parse number of chunks and length from the json. 
+4. read the chunks from the BookKeeper ledger (entries from `startPos = footerPosition - chunkSequenceFooter.numParts` to `footerPosition - 1`) and merge them.
+5. parse the merged byte array as a PositionInfo state.
+
+See `recoverFromLedgerByEntryId()` in `ManagedCursorImpl` for the implementation.
+
+## Design & Implementation Details
+
+Proposed implementation: https://github.com/apache/pulsar/pull/22799
+
+## Public-facing Changes
+
+Nothing
+
+### Public API
+
+None
+
+### Binary protocol
+
+No public-facing changes
+
+### Configuration
+
+None
+
+### CLI
+
+None
+
+### Metrics
+
+<!--
+For each metric provide:
+* Full name
+* Description
+* Attributes (labels)
+* Unit
+-->
+
+
+# Monitoring
+
+Existing monitoring should be sufficient.
+
+# Security Considerations
+
+N/A
+
+# Backward & Forward Compatibility
+
+## Upgrade
+
+Not affected, just upgrade.
+
+## Downgrade / Rollback
+
+Not affected, just downgrade **as long as the managedLedgerMaxUnackedRangesToPersist was in the range to fit it into a single entry in BK**.
+
+## Pulsar Geo-Replication Upgrade & Downgrade/Rollback Considerations
+
+Not affected AFAIK.
+
+# Alternatives
+
+1. Do nothing. Keep the number of ranges low. This does not fit some use cases.
+2. Come up with an extremely efficient storage format for the unacked ranges to fit them into a single entry all the time for e.g. 10mil ranges. This breaks backwards compatibility and the feasibility is unclear.
+
+# General Notes
+
+# Links
+
+Proposed implementation: https://github.com/apache/pulsar/pull/22799
+
+<!--
+Updated afterwards
+-->
+* Mailing List discussion thread: https://lists.apache.org/thread/8sm0h804v5914zowghrqxr92fp7c255d
+* Mailing List voting thread: https://lists.apache.org/thread/q31fx0rox9tdt34xsmo1ol1l76q8vk99