Add method to apply Consumer<RequestSpec> in RSocketRequester

[rwinch]

Currently to include a credential within RSocketRequester a user needs to pass in a value and MediaType. If there are multiple values, the user has double the work to do. If there is a rich type being provided, the RSocketStrategies likely needs to be customized as well. For example, providing a username/password might look something like this:

this.requester.route("secure.retrieve-mono")
		.metadata(credentials.getUsername(), USERNAME)
		.metadata(credentials.getPassword(), PASSWORD)
		...

It would be nice if a user could just do something like:

this.requester.route("secure.retrieve-mono")
        // apply(Consumer<RequestSpec>)
		.apply(credentials(credentials))
		...

Another point is that it appears that RSocket APIs typically integrate using a Flyweight pattern such that encoding has access to lower level APIs (i.e. ByteBuf metadata). However, RSocketRequester does not provide access to these APIs. This means that direct support for RSocket cannot be leveraged with RSocketRequester. It would be nice if this could be reused somehow.

[rstoyanchev]

I've scheduled adding the apply method. For the second point, I don't think it belongs in the same issue, but let me first ask which RSocket API you have in mind? At the moment there is only routing and composite metadata. RSocketRequester takes care of encoding metadata internally and it will also utilize the routing API too for RC2 which only became available recently.

The way it's supposed to work currently is that applications provide metadata as Objects and those are serialized to bytes via Encoder and then added to composite metadata. So you can register an Encoder to do the serialization and such an Encoder could utilize a RSocket APIs but I haven't experimented whether that's straight forward enough. I'm just curious if there are specific APIs you have in mind.

[rwinch]

For the second point, I don't think it belongs in the same issue, but let me first ask which RSocket API you have in mind?

That is fair.

At the moment there is only routing and composite metadata. RSocketRequester takes care of encoding metadata internally and it will also utilize the routing API too for RC2 which only became available recently.

I am imagining the various security mime types (i.e. username/password, bearer tokens, etc). There also seems to an extension for doing tracing that requires multiple arguments.

So you can register an Encoder to do the serialization and such an Encoder could utilize a RSocket APIs but I haven't experimented whether that's straight forward enough. I'm just curious if there are specific APIs you have in mind.

This seems to confirm my observation. If I want to support both RSocket APIs and Spring I need to provide two separate APIs. Perhaps Spring could provide a generic Encoder implementation that accepts a Function that can invoke the Flyweight if it follows a convention on it's arguments?

[rwinch]

One other point is that it would be nice if this feature added a hook to easily customize the builder to handle connection metadata.

[rstoyanchev]

@rwinch take a look at the changes in 58e9266, including a new method at the builder level (e.g. for connection setup or other). Hopefully those work for you.

I've also added support for providing a ByteBuf as a metadata value in b144c72, which means you can use an RSocket API such as the one for tracing when it becomes available, and then pass the resulting ByteBuf. This basic option and check makes sense in any case so I've gone ahead and added it.