[Feature/extensions] Add getNamedWriteableRegistry() API for extensions

[ryanbogan]

The old model for the getNamedWriteableRegistry() was recently removed, due to concerns here: opensearch-project/OpenSearch#5518 (comment). A new model must be created, which should be similar to the implementation of getNamedXContent().

[owaiskazi19]

@ryanbogan this issue should be in #opensearch-sdk

[dbwiddis]

@ryanbogan this looks like it's already done here:

opensearch-sdk-java/src/main/java/org/opensearch/sdk/NettyTransport.java

Lines 69 to 76 in 1261c46

	List<NamedWriteableRegistry.Entry> namedWriteables = Stream.of(
	NetworkModule.getNamedWriteables().stream(),
	indicesModule.getNamedWriteables().stream(),
	searchModule.getNamedWriteables().stream(),
	ClusterModule.getNamedWriteables().stream()
	).flatMap(Function.identity()).collect(Collectors.toList());
	final NamedWriteableRegistry namedWriteableRegistry = new NamedWriteableRegistry(namedWriteables);

Is there still work to be done on this?

[dbwiddis]

Ah, I see we create it but we don't integrate the portion of it that comes from an extension itself. SO that bit needs to be added.

[petiveriaalliacea]

Hello! I want to take this issue. Where can I find nessesary classes and methods? And in what part here is missing and should be implemented?

[dbwiddis]

Hi, @petiveriaalliacea! Thanks for stepping up.

Take a look at how the getNamedXContent() extension point is currently implemented with the ExtensionsRunner constructor, ExtensionsInitRequestHandler, and SDKNamedXContentRegistry.

You will basically be doing the exact same thing with the getNamedWriteables() extension point. I've deleted the steps here and updated them in the next comment.

Let us know if you have any questions.

[dbwiddis]

Actually, @petiveriaalliacea it's slightly more (and less) complicated than the above.

• I realized reading this comment that we already have a partial implementation in the NettyTransport class. So that class needs a copy of this registry.
• I looked into the registration in Node.java linked in the comment above and none of it depends on updated settings, so it's okay to initialize this once.

So updated steps:
You will basically be doing the exact same thing with the getNamedWriteables() extension point:

1. Create a new SDKNamedWriteableRegistry class similar to the SDKNamedXContentRegistry, except you won't need an update method
2. Internally in that class, instead of the OpenSearch module static NamedXContent, use the NamedWriteable implementations from here https://github.com/opensearch-project/OpenSearch/blob/f332977666f9839b5bfa507f9ffe77e8134d2481/server/src/main/java/org/opensearch/node/Node.java#L599 . You can call the static method on one, and instantiate the class to call the getter on the others. This was done in the NettyTransport class but you'll want to move it into that new class.
3. Delete the creation of this registry from the NettyTransport class. Instead, add the registry as a parameter to the getNetty4Transport method.
4. Add the default method to the Extension interface
5. Create a private field in the ExtensionsRunner for the SDKNamedWriteableRegistry with a getter
6. During initialization, instantiate this registry prior to injecting components
7. Add the registry to the components injected in Guice (just use the getter from the SDK registry to inject the actual NamedWriteableRegistry)
8. During transport initialization, pass the registry to the NettyTransport method you changed in step 3.

[dbwiddis]

Done in #553