Added schema validation on context startup

[mipo256]

This is the PR to solve #764 issue.
The key takeaways in the PR: there is a new class, called CassandraSchemaValidator, that is validating Cassandra schema against existing entities in the MappingContext. The keyspace where we expect the entities to show up is one that is configured to be used by CqlSession (for now that is correct because we do not have a keyspace segregation for entities yet, see this ticket).

The validation checks that:

• All Tables exists
• All non-transient properties are expected to be mapped to a single column (I do not know if this is correct or not, appreciate any help on this)
• We expect that the type of the persistent property matches the one we expect in Cassandra

P.S: I think it would be nice to add this as a bean to Cassandra auto-configuration to spring boot with some properties conditional annotations

[mp911de]

Thanks for having a look. The design requires a bit more thought and not everything should be a String or Exception. Design-wise, we should decouple validation from any factory beans for a clearer separation of concerns. Later on, once the validation part is fine, we can consider how to integrate validation into application startup.

For type resolution, I suggest you have a look at DefaultColumnTypeResolver and SchemaFactory, how data types are derived from properties.

Also, validation findings should be part of a model (ColumnNotFound, TableNotFound) along with references to the underlying CQL item/persistent property/persistent entity.

Let me know whether that helps.

[mipo256]

@mp911de I have introduced a couple of changes in accordance to what you've said:

1. CassandraSchemaValidator now accepts CqlSession directly instead of CqlSessionFactoryBean
2. CassandraSchemaValidator now also accepts CassandraConverter instead of CassandraMappingContext. This allwos getting CassandraMappingContext from CassandraConverter and also the creation of DefaultColumnTypeResolver instance using CassandraConverter`

But I do not fully understand wht you mean here:

Also, validation findings should be part of a model (ColumnNotFound, TableNotFound) along with references to the underlying CQL item/persistent property/persistent entity.

Can you please give me a glue what is your vision here? You want CassandraSchemaValidationProfile to accept not String objects as validation error messages but have an abstraction over a message, such as ColumnNotFound and TableNotFound and so on, am I correct?

[mipo256]

I have rebased this PR. Any chance to review it? @mp911de

[mp911de]

Currently, this ticket (and several other ones) is on pause because we're limited in bandwidth. We currently can only fix smaller bugs and I don't know when we will have more time for proper enhancements.