Streaming Interface

[flahn]

In the long term the r_exec operation in SciDB will be discontinued and replaced by the stream operation, which allows the execution of arbitrary scripts using command line calls. In this project I started to implement a mean to perform those R calls using the stream interface. However there were some complications:

1. I tried to pass a script using the expression parameter on the R call. Due to the complex nature of those scripts file upload should be used. Otherwise the script as an expression needs to be readable by R, which includes that every line needs to end with a ";" since newline escaped characters are not supported. Using the script as a file approach it remains unclear, if the script will be redistributed to every worker instance.
2. Debugging with a distributed parallel system is hard, since there is no central place to log and errors are not transferred "as is" to the client. Errors remain as some sort of notification that "something failed to execute".