A SearchServiceMapper implementation with the PullMapper is very much like a standard SearchServiceMapper implementation as described in the Implementing a Search with GSS chapter of this guide. However, there are some additional considerations.
When using the Pull Mapper with a complex search service that is composed of several related entities, ensure that your SearchServiceMapper implementation will behave appropriately when it has to deal with incomplete sets of entities, i.e. if entities A, B and C together comprise a search service your mapper may get called when only A and C exist. Depending on your search service the correct behaviour may be to add the incomplete set of data to the search service, or to do nothing until the set is complete.