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[improve] PIP-384: ManagedLedger interface decoupling (apache#23363)
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| # PIP-384: ManagedLedger interface decoupling | ||
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| ## Background knowledge | ||
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| Apache Pulsar uses a component called ManagedLedger to handle persistent storage of messages. | ||
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| The ManagedLedger interfaces and implementation were initially tightly coupled, making it difficult to introduce alternative implementations or improve the architecture. | ||
| This PIP documents changes that have been made in the master branch for Pulsar 4.0. Pull Requests [#22891](https://github.com/apache/pulsar/pull/22891) and [#23311](https://github.com/apache/pulsar/pull/23311) have already been merged. | ||
| This work happened after lazy consensus on the dev mailing list based on the discussion thread ["Preparing for Pulsar 4.0: cleaning up the Managed Ledger interfaces"](https://lists.apache.org/thread/l5zjq0fb2dscys3rsn6kfl7505tbndlx). | ||
| There is one remaining PR [#23313](https://github.com/apache/pulsar/pull/23313) at the time of writing this document. | ||
| The goal of this PIP is to document the changes in this area for later reference. | ||
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| Key concepts: | ||
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| - **ManagedLedger**: A component that handles the persistent storage of messages in Pulsar. | ||
| - **BookKeeper**: The default storage system used by ManagedLedger. | ||
| - **ManagedLedgerStorage interface**: A factory for configuring and creating the `ManagedLedgerFactory` instance. [ManagedLedgerStorage.java source code](https://github.com/apache/pulsar/blob/master/pulsar-broker/src/main/java/org/apache/pulsar/broker/storage/ManagedLedgerStorage.java) | ||
| - **ManagedLedgerFactory interface**: Creates and manages ManagedLedger instances. [ManagedLedgerFactory.java source code](https://github.com/apache/pulsar/blob/master/managed-ledger/src/main/java/org/apache/bookkeeper/mledger/ManagedLedgerFactory.java) | ||
| - **ManagedLedger interface**: Handles the persistent storage of messages in Pulsar. [ManagedLedger.java source code](https://github.com/apache/pulsar/blob/master/managed-ledger/src/main/java/org/apache/bookkeeper/mledger/ManagedLedger.java) | ||
| - **ManagedCursor interface**: Handles the persistent storage of Pulsar subscriptions and related message acknowledgements. [ManagedCursor.java source code](https://github.com/apache/pulsar/blob/master/managed-ledger/src/main/java/org/apache/bookkeeper/mledger/ManagedCursor.java) | ||
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| ## Motivation | ||
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| The current ManagedLedger implementation faces several challenges: | ||
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| 1. **Tight coupling**: The interfaces are tightly coupled with their implementation, making it difficult to introduce alternative implementations. | ||
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| 2. **Limited flexibility**: The current architecture doesn't allow for easy integration of different storage systems or optimizations. | ||
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| 3. **Dependency on BookKeeper**: The ManagedLedger implementation is closely tied to BookKeeper, limiting options for alternative storage solutions. | ||
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| 4. **Complexity**: The tight coupling increases the overall complexity of the system, making it harder to maintain, test and evolve. | ||
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| 5. **Limited extensibility**: Introducing new features or optimizations often requires changes to both interfaces and implementations. | ||
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| ## Goals | ||
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| ### In Scope | ||
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| - Decouple ManagedLedger interfaces from their current implementation. | ||
| - Introduce a ReadOnlyManagedLedger interface. | ||
| - Decouple OpAddEntry and LedgerHandle from ManagedLedgerInterceptor. | ||
| - Enable support for multiple ManagedLedgerFactory instances. | ||
| - Decouple BookKeeper client from ManagedLedgerStorage. | ||
| - Improve overall architecture by reducing coupling between core Pulsar components and specific ManagedLedger implementations. | ||
| - Prepare the groundwork for alternative ManagedLedger implementations in Pulsar 4.0. | ||
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| ### Out of Scope | ||
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| - Implementing alternative ManagedLedger storage backends. | ||
| - Changes to external APIs or behaviors. | ||
| - Comprehensive JavaDocs for the interfaces. | ||
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| ## High Level Design | ||
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| 1. **Decouple interfaces from implementations**: | ||
| - Move required methods from implementation classes to their respective interfaces. | ||
| - Update code to use interfaces instead of concrete implementations. | ||
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| 2. **Introduce ReadOnlyManagedLedger interface**: | ||
| - Extract this interface to decouple from ReadOnlyManagedLedgerImpl. | ||
| - Adjust code to use the new interface where appropriate. | ||
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| 3. **Decouple ManagedLedgerInterceptor**: | ||
| - Introduce AddEntryOperation and LastEntryHandle interfaces. | ||
| - Adjust ManagedLedgerInterceptor to use these new interfaces. | ||
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| 4. **Enable multiple ManagedLedgerFactory instances**: | ||
| - Modify ManagedLedgerStorage interface to support multiple "storage classes". | ||
| - Implement BookkeeperManagedLedgerStorageClass for BookKeeper support. | ||
| - Update PulsarService and related classes to support multiple ManagedLedgerFactory instances. | ||
| - Add "storage class" to persistence policy part of the namespace level or topic level policies. | ||
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| 5. **Decouple BookKeeper client**: | ||
| - Move BookKeeper client creation and management to BookkeeperManagedLedgerStorageClass. | ||
| - Update ManagedLedgerStorage interface to remove direct BookKeeper dependencies. | ||
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| ## Detailed Design | ||
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| ### Interface Decoupling | ||
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| 1. Update ManagedLedger interface: | ||
| - Add methods from ManagedLedgerImpl to the interface. | ||
| - Remove dependencies on implementation-specific classes. | ||
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| 2. Update ManagedLedgerFactory interface: | ||
| - Add necessary methods from ManagedLedgerFactoryImpl. | ||
| - Remove dependencies on implementation-specific classes. | ||
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| 3. Update ManagedCursor interface: | ||
| - Add required methods from ManagedCursorImpl. | ||
| - Remove dependencies on implementation-specific classes. | ||
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| 4. Introduce ReadOnlyManagedLedger interface: | ||
| - Extract methods specific to read-only operations. | ||
| - Update relevant code to use this interface where appropriate. | ||
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| 5. Decouple ManagedLedgerInterceptor: | ||
| - Introduce AddEntryOperation interface for beforeAddEntry method. | ||
| - Introduce LastEntryHandle interface for onManagedLedgerLastLedgerInitialize method. | ||
| - Update ManagedLedgerInterceptor to use these new interfaces. | ||
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| ### Multiple ManagedLedgerFactory Instances | ||
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| 1. Update ManagedLedgerStorage interface: | ||
| - Add methods to support multiple storage classes. | ||
| - Introduce getManagedLedgerStorageClass method to retrieve specific storage implementations. | ||
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| 2. Implement BookkeeperManagedLedgerStorageClass: | ||
| - Create a new class implementing ManagedLedgerStorageClass for BookKeeper. | ||
| - Move BookKeeper client creation and management to this class. | ||
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| 3. Update PulsarService and related classes: | ||
| - Modify to support creation and management of multiple ManagedLedgerFactory instances. | ||
| - Update configuration to allow specifying different storage classes for different namespaces or topics. | ||
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| ### BookKeeper Client Decoupling | ||
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| 1. Update ManagedLedgerStorage interface: | ||
| - Remove direct dependencies on BookKeeper client. | ||
| - Introduce methods to interact with storage without exposing BookKeeper specifics. | ||
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| 2. Implement BookkeeperManagedLedgerStorageClass: | ||
| - Encapsulate BookKeeper client creation and management. | ||
| - Implement storage operations using BookKeeper client. | ||
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| 3. Update relevant code: | ||
| - Replace direct BookKeeper client usage with calls to ManagedLedgerStorage methods. | ||
| - Update configuration handling to support BookKeeper-specific settings through the new storage class. | ||
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| ## Public-facing Changes | ||
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| ### Configuration | ||
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| - Add new configuration option to specify default ManagedLedger "storage class" at broker level. | ||
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| ### API Changes | ||
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| - No major changes to external APIs are planned. | ||
| - The only API change is to add `managedLedgerStorageClassName` to `PersistencePolicies` which can be used by a custom `ManagedLedgerStorage` to control the ManagedLedgerFactory instance that is used for a particular namespace or topic. | ||
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| ## Backward & Forward Compatibility | ||
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| The changes are internal and don't affect external APIs or behaviors. | ||
| Backward compatibility is fully preserved in Apache Pulsar. | ||
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| ## Security Considerations | ||
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| The decoupling of interfaces and implementation doesn't introduce new security concerns. | ||
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| ## Links | ||
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| - Initial mailing List discussion thread: [Preparing for Pulsar 4.0: cleaning up the Managed Ledger interfaces](https://lists.apache.org/thread/l5zjq0fb2dscys3rsn6kfl7505tbndlx) | ||
| - Merged Pull Request #22891: [Replace dependencies on PositionImpl with Position interface](https://github.com/apache/pulsar/pull/22891) | ||
| - Merged Pull Request #23311: [Decouple ManagedLedger interfaces from the current implementation](https://github.com/apache/pulsar/pull/23311) | ||
| - Implementation Pull Request #23313: [Decouple Bookkeeper client from ManagedLedgerStorage and enable multiple ManagedLedgerFactory instances](https://github.com/apache/pulsar/pull/23313) | ||
| - Mailing List PIP discussion thread: https://lists.apache.org/thread/rtnktrj7tp5ppog0235t2mf9sxrdpfr8 | ||
| - Mailing List PIP voting thread: https://lists.apache.org/thread/4jj5dmk6jtpq05lcd6dxlkqpn7hov5gv |