BewareMyPower commented on code in PR #24328:
URL: https://github.com/apache/pulsar/pull/24328#discussion_r2191960133
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pip/pip-420.md:
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+# PIP-420: Provides an ability for Pulsar clients to integrate with
third-party schema registry service
+
+# Motivation
+
+Apache Pulsar currently provides a built-in schema management system tightly
coupled with the broker.
+Pulsar clients interact with this system implicitly when creating producers
and consumers.
+
+However, many organizations already have independent schema registry services
(such as Confluent Schema Registry)
+and wish to reuse their existing schema governance processes across multiple
messaging systems, including Pulsar.
+
+By enabling Pulsar clients to integrate with third-party schema registry
services:
+- Users can unify schema management across different platforms.
+- Pulsar brokers can be decoupled from schema storage and validation
responsibilities.
+- Pulsar users can integrate with ecosystems that rely on external schema
registries easier.
+
+This flexibility is particularly valuable for enterprises with strict schema
validation, versioning,
+and governance workflows already centralized in external registries.
+
+# Goals
+
+## In Scope
+
+- Provide the ability for Pulsar clients to leverage third-party schema
registry services for schema operations.
+
+## Out Scope
+
+- Providing built-in implementations for third-party schemas.
+- Support `AutoProduceBytesSchema` and `AutoConsumeSchema`.
+- Migrating existing Pulsar-managed schemas to external schema registries.
+
+# High Level Design
+
+- Provide a mechanism to configure the Pulsar client to use either:
+ - The existing Pulsar schema registry (default)
+ - Third-party schema registry implementations
+
+# Detailed Design
+
+## Design & Implementation Details
+
+This PIP aims to enable the Pulsar client to directly integrate with external
schema registry services for schema management.
+In this model, the external schema registry is fully responsible for schema
storage, retrieval, and validation.
+The Pulsar broker will no longer manage schema data for topics using external
schemas.
+
+### SchemaType: EXTERNAL
+
+Pulsar will introduce a new schema type: **SchemaType.EXTERNAL**.
+
+- All schemas that integrate with external schema registries must declare
`SchemaType.EXTERNAL`.
+- When using `EXTERNAL` schema type, the Pulsar client will provide empty
schema data to the broker.
+- The broker will only record the schema type for topics.
+- Compatibility restrictions:
+ - Introduce a new compatibility check in broker side.
+ - The schema type `SchemaType.EXTERNAL` can't be compatible with other
Pulsar schemas
+ - This prevents accidental data corruption or schema conflicts between
internal and external schema management systems.
+- Pulsar Geo replicator needs to transfer the schema type
`SchemaType.EXTERNAL` to the remote cluster.
+
+This design isolates external schema management and protects existing topics
using Pulsar’s native schema system.
+
+### Extensibility via Client Interfaces
+
+To integrate with external schema registries, users can:
+- Implement the `Schema` interface to define custom schema encoding and
decoding logic.
+
+#### Key `Schema` Interface Methods:
+- byte[] encode(T message)
+ - Serializes the message using the external schema.
+ - Implementations should throw `SchemaSerializationException` if the
serialization fails.
+
+- T decode(byte[] bytes)
+ - Deserialize the message using the external schema.
+ - Users should handle exceptions when get value by themselves.
+
+- void setSchemaInfoProvider(SchemaInfoProvider schemaInfoProvider)
+ - Call this method when creating schema
+ - External schema can be initialized when calling this method
+
+- close() **(New addition)**
+ - Called when the producer or consumer is closed.
+ - Allows external schema implementations to release resources, such as
schema registry connections or caches.
+
+#### Example Workflow:
+
+- During producer or consumer initialization:
+ The external schema info will be registered to Pulsar schema storage.
+
+- During message send or receive:
+ The `encode` and `decode` methods handle the schema-aware serialization and
deserialization using the external schema registry.
+
+#### Schema ID & Schema Version
+
+Unlike Pulsar, which uses **schema version** to identify schemas, many
external schema registry systems use **schema ID** as the primary schema
identifier.
+
+When integrating with external schema registries:
+- The `schemaVersion` filed in Pulsar message metadata is used in some places,
**set to `-1` to flag the message is using external schema systems**.
Review Comment:
```protobuf
optional bytes schema_version = 16;
```
The type of schema version is `bytes`. It's better to clearly define the
specific bytes that represent external schema registry.
In addition, could you explain more about when is the schema version used? I
took a quick look and found the following API should be affected:
```java
public interface LookupService extends AutoCloseable {
CompletableFuture<Optional<SchemaInfo>> getSchema(TopicName topicName,
byte[] version);
```
It's used in `MultiVersionSchemaInfoProvider#loadSchema`.
It's better to add a **background** section to explain all related details,
otherwise setting -1 as the schema version might be confusing. For example, why
not use an empty byte array as the schema version?
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