Added Cassandra Cluster factory and Default implementation.

[qvistgaard]

This allows the default cassandra builder to be overwritten. This is useful for example, when implementing jaeger/opentracing

[pivotal-issuemaster]

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[snicoll]

This is useful for example, when implementing jaeger/opentracing

Is that using a decorator pattern of some sort? I don't think increasing the surface area with an extra callback is actually worth it. Have you tried to use a BeanPostProcessor to wrap the Cluster bean instead?

[qvistgaard]

It was my first thought to use a BeanPostProcessor. However the way the opentracing cassandra driver is made prevents that. Basically they have a TracingCluster class which extends the original Cluster class. it is instantiated like so:

// Instantiate Tracing Cluster
Cluster cluster = new TracingCluster(builder, tracer);

[snicoll]

Thanks for the feedback. I am still not convinced we should create an interface for that specific use case. Perhaps we could expose the Builder and then let you inject it and create the cluster any way you like.

Flagging for team attention to see what the rest of the team thinks.

[wilkinsona]

@ssoerensen Thank you very much for making your first contribution to Spring Boot. The proposed changes have been merged into master, along with a polish commit. The two main changes were to remove the default implementation of the new interface and to add a test.