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Appendix A. References

A.1. The Externalizer
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A.1.1. About Externalizer
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An Externalizer is a class that can:

  • Marshall a given object type to a byte array.
  • Unmarshall the contents of a byte array into an instance of the object type.

Externalizers are used by Red Hat JBoss Data Grid and allow users to specify how their object types are serialized. The marshalling infrastructure used in JBoss Data Grid builds upon JBoss Marshalling and provides efficient payload delivery and allows the stream to be cached. The stream caching allows data to be accessed multiple times, whereas normally a stream can only be read once.

A.1.2. Internal Externalizer Implementation Access
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Externalizable objects should not access Red Hat JBoss Data Grids Externalizer implementations. The following is an example of incorrect usage: 

public static class ABCMarshallingExternalizer implements AdvancedExternalizer<ABCMarshalling> {
   @Override
   public void writeObject(ObjectOutput output, ABCMarshalling object) throws IOException {
      MapExternalizer ma = new MapExternalizer();
      ma.writeObject(output, object.getMap());
   }

   @Override
   public ABCMarshalling readObject(ObjectInput input) throws IOException, ClassNotFoundException {
      ABCMarshalling hi = new ABCMarshalling();
      MapExternalizer ma = new MapExternalizer();
      hi.setMap((ConcurrentHashMap<Long, Long>) ma.readObject(input));
      return hi;
   }
   <!-- Additional configuration information here -->
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End user externalizers do not need to interact with internal externalizer classes. The following is an example of correct usage: 

public static class ABCMarshallingExternalizer implements AdvancedExternalizer<ABCMarshalling> {
   @Override
   public void writeObject(ObjectOutput output, ABCMarshalling object) throws IOException {
      output.writeObject(object.getMap());
   }

   @Override
   public ABCMarshalling readObject(ObjectInput input) throws IOException, ClassNotFoundException {
      ABCMarshalling hi = new ABCMarshalling();
      hi.setMap((ConcurrentHashMap<Long, Long>) input.readObject());
      return hi;
   }

   <!-- Additional configuration information here -->
}
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A.2. Hash Space Allocation
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A.2.1. About Hash Space Allocation
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Red Hat JBoss Data Grid is responsible for allocating a portion of the total available hash space to each node. During subsequent operations that must store an entry, JBoss Data Grid creates a hash of the relevant key and stores the entry on the node that owns that portion of hash space.

A.2.2. Locating a Key in the Hash Space
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Red Hat JBoss Data Grid always uses an algorithm to locate a key in the hash space. As a result, the node that stores the key is never manually specified. This scheme allows any node to know which node owns a particular key without such ownership information being distributed. This scheme reduces the amount of overhead and, more importantly, improves redundancy because the ownership information does not need to be replicated in case of node failure.

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