[improve][pip] PIP-404: Introduce per ledger properties

pip/pip-404-Introduce-per-ledger-properties.md

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+# PIP-404: Introduce per-ledger properties
+
+# Background knowledge
+
+As we don't have a secondary index on the Bookkeeper, so we can't query entries by message metadata efficiently.
+The `ManagedCursor` provides a method `asyncFindNewestMatching` to find the newest entry that matches the given
+predicate by binary search(See `OpFindNewest.java`).
+In https://github.com/apache/pulsar/pull/22792, we optimized `seeking by timestamp` by calculating
+the range of ledgers that may contain the target timestamp by `LedgerInfo#timestamp` and we don't need to scan all
+ledgers.
+
+However, when we enabled `AppendIndexMetadataInterceptor` and we want to query entries by `BrokerEntryMetadata#index`,
+there is no more efficient way,
+we have to scan all ledgers by binary search to find the target entry.
+
+# Motivation
+
+Introduce per-ledger properties and we can store the extra per-ledger properties in the `LedgerInfo`,
+so we can query entries by `incremental index` more efficiently, say, `BrokerEntryMetadata#index`.
+
+# Goals
+
+## In Scope
+
+* Provide a way to set per-ledger properties, it should be a generic way and can be used by any plugin.
+
+## Out of Scope
+
+* Set extra per-ledger properties, it should be done by the specific plugin. Such as `KoP`.
+
+# High Level Design
+
+We can store the `incremental index` of the first entry in the ledger into `LedgerInfo#properties`, say,
+`BrokerEntryMetadata#index`.
+When we want to query entries by `BrokerEntryMetadata#index`, we can calculate the range of ledgers that may contain the
+target index by `LedgerInfo#properties` and we don't need to scan all ledgers.
+
+In https://github.com/apache/pulsar/pull/22792, we provided a new method to find the newest entry with given range of entries:
+```java
+void asyncFindNewestMatching(FindPositionConstraint constraint, Predicate<Entry> condition,
+                                 Position startPosition, Position endPosition, FindEntryCallback callback,
+                                 Object ctx, boolean isFindFromLedger) {
+    }
+```
+We can use this method directly in the above scenario.
+
+# Detailed Design
+
+## Public-facing Changes
+
+### Public API
+
+* Add the following method in `ManagedLedger`:
+
+```java
+    CompletableFuture<Void> asyncAddLedgerProperty(long ledgerId, String key, String value);
+    CompletableFuture<Void> asyncRemoveLedgerProperty(long ledgerId, String key);
+```
+
+### Binary protocol
+
+* Add a new field `properties` in `LedgerInfo`:
+
+```protobuf
+message LedgerInfo {
+      required int64 ledgerId = 1;
+      optional int64 entries = 2;
+      optional int64 size = 3;
+      optional int64 timestamp = 4;
+      optional OffloadContext offloadContext = 5;
+      // Add the following field
+      repeated KeyValue properties = 6;
+}
+```
+
+# Backward & Forward Compatibility
+
+It is fully backward compatible.
+
+# Alternatives
+
+None
+
+# Links
+
+<!--
+Updated afterwards
+-->
+
+* Mailing List discussion thread: https://lists.apache.org/thread/bcf13todophd05tn0qrrdwqyw8yvboly
+* Mailing List voting thread: https://lists.apache.org/thread/dtj9ccntsjorb54yrb5fr3pppwl4m2r5