[Doc] add scala user doc

[chaokunyang]

What do these changes do?

add scala user doc

Related issue number

Closes #1024

Check code requirements

• tests added / passed (if needed)
• Ensure all linting tests pass, see here for how to run them

[chaokunyang]

Hi @pjfanning , I just add a simple scala doc. It would be great if you can take a look at it.

[pjfanning]

I did a quick test with Scala List and it looks like a lot of internal Scala classes are needed under the hood and they all need to be explicitly registered (unless you disable the need for registration).

I can come back to this but it could take a while. I suspect that the list of Scala library classes that need to be registered is going to be very large.

To make matters worse, Scala 2.13 rewrote a lot of Scala code relating to the collections - so the internal classes will be different from Scala 2.12. Any code to register a curated list of Scala library classes will need to try to register classes but move on if some of them fail.

This could be a fair amount of work.

[chaokunyang]

That would be an issue. Registered should be stable across scala versions.

Do we need to support registration check for some clas pattern such as scala.collection?

[pjfanning]

It might be useful to support automated registration of every class in the scala package and subpackages - not just scala.collection. You might be able to exclude packages like scala.reflect. Ideally, the creation of the serializers should be lazy - only created when needed but the idea is that scala package classes would be allowed without explicit registration. I would support the idea that a registerScalaClasses call would be needed to enable this - but that this call would not to need to run through a large list of explicit class names.

[chaokunyang]

Auto registration is a good idea. But The registered class will be written by an auto grown Id. the order can't be ensured across scala version if we support auto register unless we add a new inferface like register but write class as name still.

[chaokunyang]

Hi @pjfanning , do you think we should merge this pr now, or hold it until we have a more scala-native serialization support?

There are some fury scala users already: #1029

[pjfanning]

It's fine to merge as is.