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new 0a36b0a Created PIP 43: producer send message with different schema
(markdown)
0a36b0a is described below
commit 0a36b0a839b76f37295bc11bed50ed3418c88b90
Author: Sijie Guo <guosi...@gmail.com>
AuthorDate: Mon Sep 2 23:10:53 2019 -0700
Created PIP 43: producer send message with different schema (markdown)
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+## Motivation
+
+For now, Pulsar producer can only produce messages of one type of schema which
is determined by user when it is created, or by fecthing the latest version of
schema from registry if `AUTO_PRODUCE_BYTES` type is specified. Schema,
however, can be updated by external system after producer started, which would
lead to inconsistency between messsage payload and schema version metadata.
Also some senarios like replicating from kafka require a single producer for
replicating messages of differ [...]
+
+Here proposing that messages can indicate the associated schema by itself with
two parts of changes to make it clear.
+
+## Changes:Part-1
+
+For the part-1, here propose that producer supports to new message specified
with schema, particularly, of same POJO type.
+
+### Interfaces
+
+For the `Producer<T>` interface, here propose a new method for `newMessage` to
new a message builder with specified schema with the following signature:
+
+```
+TypedMessageBuilder<T> newMessage(Schema<T> schema);
+```
+
+where the parameterized type `T` is required to be same with producer.
+
+For `AutoProduceBytesSchema` specially, user SHOULD new message with actual
schema wrapped by auto produce bytes schema. A static method MAY be provided by
`Schema` interface with follwing signature:
+
+```
+static Schema<byte[]> AUTO_PRODUCE_BYTES(Schema<?> schema);
+```
+
+### Wire protocols
+
+To guarantee scenario that send message with brand new schema, we also propose
a new Command to get schema version, or create one if NOT present.
+
+```
+message CommandGetOrCreateSchema {
+ required uint64 request_id = 1;
+ required string topic = 2;
+ required Schema schema = 3;
+}
+
+message CommandGetOrCreateSchemaResponse {
+ required uint64 request_id = 1;
+ optional ServerError error_code = 2;
+ optional string error_message = 3;
+
+ optional bytes schema_version = 4;
+}
+```
+
+### Implementation
+
+#### Client
+The current `Schema schema` field of `Producer` would be used as default
schema, when producer send messages without specifying schema explicily, with
which the default schema would be associated.
+
+Producer SHOULD maintain a local map from schema to schema version, and check
the schema version of the associated schema of message before send. If the
schema can not be found, producer SHOULD try to register this schema to the
registry and get the version of it, then insert the pair to the local map. Hash
of `Schema` same with registry CAN be used as key of map.
+
+Producer SHOULD also attach the actual schema version to the message metadata
as it is.
+
+For batch messages with single same metadata, only one schema version is
allowed, so before adding one message into batch container, producer SHOULD
check the schema version in this batch, and flush the batch if associated with
different schema from the message and add it to another batch.
+
+To be seamless for producer not requires this feature, an option CAN be added
to producer builder to enable this feature, and disable by default. When this
feature is NOT enabled, the action of producer SHOULD keep as it is.
+
+#### Broker
+Server SHOULD handle the register schema command, and put it to registry, or
just respond the existing version if present which SHOULD be built on top of
compatibility check. The registry backend has alreay implemented this interface.
+
+#### Functions
+Functions MAY inherit the feature gate option and expose it to configuration,
same for Sources.
+
+## Changes:Part-2
+
+For the part-2, here propose to allow producer to new message with different
POJO type of schema. To be noted that, once one producer can send different
POJOs, the parameterized type of message involved in methods of `Producer`
SHOULD be changed in some way. The interceptor mechanism would also be affected.
+
+### Interfaces
+
+For the `Producer<T>` interface, here propose to enhance the method propsed in
part-1 to accept arbitrary inner type with the following signature:
+
+```
+<V> TypedMessageBuilder<V> newMessage(Schema<V> schema);
+```
+where the parameterized type `T` and `V` is NOT required to be same.
+
+For `ProducerInterceptor<T>` interface, provide a method to indicate whether
the message is supported by the interceptor instance:
+```
+default bool eligible(Message message) {
+ return true;
+}
+```
+which is essential especially when producer can send different type of
messages, notes that the message parameter is `Message` raw type, not required
to has same parameterized type with interceptor.
+
+### Implementations
+
+#### Client
+The only thing that needs to be pointed out is when the parameterized type of
producer and schema/message conflict, the message parameter is allowed to
declare with different parameterized type.
+
+For `ProducerInterceptors`, check if eligible before invoking each interceptor.
