Github user okram commented on the issue:
https://github.com/apache/tinkerpop/pull/538
So `MultiComparator` is internal to a `Traversal`. The way in which
GraphSON serializes traversals is via their Bytecode. Thus, when the bytecode
gets recompiled on the deserializing end, the `MultiComparator` is recreated.
The reason this is needed in Gryo is because we use Gryo as the binary format
internal to Java-based systems. Giraph and Spark register the `MultiComparator`
in their Aggregator framework and Gryo is the serializer for it.
Regarding `ProjectedTraverser`. Again, a concept internal to Gremlin-Java
VM. Note that `Traverser` serialization in GraphSON is not complete regardless
(only what is required for practical communication):
```java
@Override
public void serialize(final Traverser traverserInstance, final
JsonGenerator jsonGenerator, final SerializerProvider serializerProvider)
throws IOException {
jsonGenerator.writeStartObject();
jsonGenerator.writeObjectField(GraphSONTokens.BULK,
traverserInstance.bulk());
jsonGenerator.writeObjectField(GraphSONTokens.VALUE,
traverserInstance.get());
jsonGenerator.writeEndObject();
}
```
`ProjectedTraverser` is used by `CollectingBarrierSteps` in OLAP during
reductions. For Gremlin-Java VM applications, this is all Gryo serialization
stuff with aggregator frameworks.
Finally, regarding testing --- yes, I swapped out `ChainedComparator` for
`MultiComparator + ProjectedTraverser` and thus, its tested.
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