In an effort to adapt Pig to work using Apache Tez (https://issues.apache.org/jira/browse/TEZ), I made some changes to allow for a cleaner ExecutionEngine abstraction than existed before. The changes are not that major as Pig was already relatively abstracted out between the frontend and backend. The changes in the attached commit are essentially the barebones changes – I tried to not change the structure of Pig's different components too much. I think it will be interesting to see in the future how we can refactor more areas of Pig to really honor this abstraction between the frontend and backend.
Some of the changes was to reinstate an ExecutionEngine interface to tie together the front end and backend, and making the changes in Pig to delegate to the EE when necessary, and creating an MRExecutionEngine that implements this interface. Other work included changing ExecType to cycle through the ExecutionEngines on the classpath and select the appropriate one (this is done using Java ServiceLoader, exactly how MapReduce does for choosing the framework to use between local and distributed mode). Also I tried to make ScriptState, JobStats, and PigStats as abstract as possible in its current state. I think in the future some work will need to be done here to perhaps re-evaluate the usage of ScriptState and the responsibilities of the different statistics classes. I haven't touched the PPNL, but I think more abstraction is needed here, perhaps in a separate patch.