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== Objective ==

The objective of this document to provide development guidelines for 
contributors to Pig project.

== Error Handling ==

There are several types of errors in Pig:

 * '''User Errors'''. This includes invalid syntax, working with non-existent 
data, referring to non-existent relationships, etc. The desired behavior should 
be to show a meaningful error message to the user and abort the processing. 
(This does not mean that in the interactive shell we exit though.)
 * '''Internal Errors'''. This are internal problems with pig code that would 
be handled with `asserts` in languages like C/C++. An example would be a 
unchecked NULL pointer. The desired behavior is to notify the user of the 
failure and to log the stack trace to the client side log file.
 * '''Frontend Errors'''. This are the errors that occur on the Pig client. An 
example is for instance failure to connect to HOD or access metadata repository 
when we add support for that. The proper behavior in this case would be to 
retry a few times and then to the same handling as for `Internal Errors`.
 * '''Backend Errors'''. This are the errors that happened on the backend 
during the course of the program execution. An example would be failure to read 
a DFS file. The desired behavior in this case is to propagate the error from 
the backend to the frontend and then perform the processing similar to the 
internal error.

It is helpful to be able to separate different types of errors in our code. 
Here is the proposal on how to handle them:

 * For `User Errors`, throw `ParseException` that contains meaningful message. 
For batch and interactive processing, catch the exception in Grunt and log the 
exception message to stderr. The same can be done with the developer in the 
embedded case.
 * For `Internal Errors`, throw `RuntimeError` that contains meaningful 
message. Catch the exception in main, log it, including the stack, to the 
client side log. Write failure message to stderr pointing to the log file.
 * For `Frontend Errors`, throw newly created `FrontendException`. Catch the 
exception in main, log it, including the stack, to the client side log. Write 
failure message to stderr pointing to the log file. In the future, we might 
find it useful to further subdivide this errors in which case we might subclass 
the `FrontendException` class. 
 * For `Backend Errors` there will need to be backend specific way to get the 
error from the backend to the frontend. Once this is done, log the error into 
client side log file and throw `ExecuteException`. Catch the exception in main, 
log it, including the stack, to the client side log. Write failure message to 
stderr pointing to the log file.
 

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