There have been mentions of using Custom Partitioning algorithms for Giraph applications. However it is not clearly given at any place. As Castagna pointed out here in how to partition graph for pregel to maximize processing speed?, there may not be a need for such partitioning as HashPartitioner will in itself be very good in most cases.
The problem of partitioning a graph 'intelligently' in order to minimize execution time is an interesting one, however it's not simple and it depends on your data and your algorithm. You might find also that, in practice, it's not necessary and a random partitioning is sufficiently good.
For example, if you are interested in exploring Pregel-like approaches, you can have a look at Apache Giraph and experiment with different partitioning techniques.
However for the purpose of learning, it would be good to see live examples and there are none found as far as I've seen. For example, the normal k-way partitioning algorithm (Kerninghan-Lin) being executed in Giraph or atleast the direction I should implement it towards.
All the google results were from the Apache giraph page where there are only definitions of the functions and various options to use them.
