[hibernate-dev] [OGM] Metamodel

[Emmanuel Bernard]

Hello Davide,

Technically most of the knowledge is in
https://hibernate.atlassian.net/browse/OGM-208 but it's definitely quite
blurry for a new comer ;)

Let me try and summarize it for you. With that you'll be able to better
grasp the comments in OGM-208

## Goals

We want a metadata facility as a way to pass mapping and configuration
from the developer to the grid dialect /
datastore provider. I will call these options. The facility should:
- be expressible via annotations
- be expressible via a programmatic API
- be as type-safe as possible but not too hard to add configuration
  options

This project is essentially smart plumbing so the "clients" of this API
are the developers on one hand and the Datastore providers on the other
hand.

## What to reuse

We will most likely use Jandex to read annotations to benefit from the
ORM and WildFly work esp wrt indexing.

## Scopes: overriding and refining options

Each option might be:
- global, per entity, per property (with optional overridability from
  one to the other)
- global, specific to a session, specific to an operation (e.g. query)

On top of that options can be polled together by functional affinities
like key/value generic options, Infinispan specific options, document
store options etc.

## Programmatic API

The programmatic API can be used to define mapping options as well as
session or even operation options. It should be type-safe and will
likely look like Hibernate Search's approach more or less.

This is more or less what is in the branch. Check out the
package-info.java for some more information.

## Annotation

We need a way to convert an annotation into calls to the programmatic
API (semantically speaking at least). Something like an (meta)annotation
based conversion:

    @ToInternalModel(UnsafeConverter.class)
    @interface Unsafe {
        boolean value() default true;
        static class UnsafeConverter implements Convert<Unsafe,MongoDBMappingInternalModelGenerator> {
            void call(Unsafe annotation, MongoDBMappingInternalModelGenerator generator) {
                return generator.unsafe(annotation.value());
            }
        }
    }

This is todo

## Reading API

Datastore providers and GridDialect implementations should be able to
read back these options.

This is more or less what you can see in the last few code lines of my
OGM-207 comment of 27/Jul/12 6:33 PM.
The seed of it is the MappingService API on the branch but it looks all
untypesafe and wrong :) Let's call it unfinished.

## Expressing options in a type-safe way

If you look at OGM-207's comment of 27/Jul/12 6:33 PM, you will wee an
example of NamedQuery and Quorum options and the infrastructure needed.

It shows how to uniquely identify an option and how to model options
like quorum that are identified by their name and options like named
queries that are identified by their name + a key (the query name in
this case).

It's available in options.Iteration2's class.

## Tests

Tests are available in org.hibernate.ogm.test.mapping.

## Examples of options

You can imagine the following examples of options:

- quorum: express the R/W quorum (globally, per entity, per collection, per session) ; useful for dynamo based systems
- WriteConcern: express the write concern options for MongoDB (globally,
  per entity, per collection)
- Whether or not store association information in the main document or
  in a separate document (see MongoDB IN_ENTITY etc. We could have this
  option on a per collection basis and not a global setting.
- all datastore specific options whether they are global or relate to
  one entity or one property

I hope I have been clearer, fire the questions away.

Emmanuel

PS: JIRA no longer has a way to link to comments?

On Wed 2013-05-01  9:41, Davide D'Alto wrote:
> Hi,
> I've started to work on the metamodel. I've look at the branch that
> emmanuel created and I've rebased it to the latest master fixing all
> checkstyle violations: https://github.com/DavideD/hibernate-ogm/tree/208
> 
> The problem is that I don't have a clear idea how the metamodel is supposed
> to work, what could be a good test case to start with?
> 
> Thanks,
> Davide
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