## Introduction

rterra is an R package that can be used to write extension in Terra. To briefly recap, Terra is a language that is a super set of LuaJit and features extensions which are compiled to machine code via LLVM. Best read the website for TerraLang for a better idea.

Most importantly, the RTerra package

• depends on the presence of LLVM/Clang to compile, but thereafter does not need LLVM/Clang
• downloads LuaJit, builds both LuaJit and Terra and creates a self contained package

Both these points imply that it can be easily used in Hadoop - an environment where it is difficult to assume presence of software on the compute nodes.

## An Example

### Supporting Files

Let us assume that we have a LUA_LIBS folder in which my Lua packages are contained. For example mine looks like

ls -l  /home/sguha/software/LUA_LIBS/
total 44
-rwxr-x--- 1 sguha sguha 35801 Feb 24 04:29 cjson.so
drwxr-xr-x 2 sguha sguha  4096 Feb 24 18:45 time
drwxr-xr-x 2 sguha sguha  4096 Feb 24 04:32 xsys


We’ll need to zip that up too. Like this

tar cvfz ~/lualibs.tar.gz -C /home/sguha/software/  LUA_LIBS


The lualibs.tar.gz needs to be rebuilt if new Lua/Terra libraries are installed

### The Terra Code

Let’s first write a small bit of Terra that gets the local time in string form

 time = terralib.includec("time.h")
terra getCurrentTimeAsString()
var t: time.time_t = time.time(nil)
var tm = time.localtime(&t);
var str_time:int8[100]
time.strftime(str_time,100, "%H:%M:%S", tm)
var x = Rbase.Rf_allocVector(R.types.STRSXP,1)
Rbase.SET_STRING_ELT(x,0, Rbase.Rf_mkChar(str_time));
return x
end


Save this in a file called ~/myfirst.t.

### Calling in RHIPE

I will assume the environment is similar to that at Mozilla: R does not exist on the compute nodes, neither does your Lua libraries (note,we do not have shared filesystem backing the home folder). So we need to pass the header files and Lua libraries to the nodes.

Initialize RHIPE

library(Rhipe)
rhinit()


Copy the zip files to the HDFS

rhput("~/lualibrary.tar.gz",'/user/sguha/share/')


Copy my Terra code

rhput("~/myfirst.t","/user/sguha/tmp/")


And now the RHIPE code, see below for comments

j <- rhwatch(map=function(a,b){
rhcollect("f",terra("getCurrentTimeAsString"))
}
, reduce=0
, input=c(10,10)
, zips="/user/sguha/share/lualibrary.tar.gz"
, share = "/user/sguha/tmp/myfirst.t" ##file is on HDFS
, setup=expression(map={
library(rterra)
tinit(rcppflags = c("./R302/R/include/", "./R302/R/include/R_ext"))
terraFile('./myfirst.t')
})
)


The codes runs 10 tasks on 10 nodes, each node will return the current time as a string getCurrentTimeAsString. The code for that is in the Terra file myfirst.t. Let’s inspect the setup expression

We need the following code since rterra requires the R header files. Note, if your distribution has nodes that look like the master e.g. RHIPE on EC2, then R exists on all the nodes at the same place. So this is not needed. Your setup would then look like

library(rterra)
tinit()


Here we invoke terraAddRequirePaths to tell Terra to search for modules inside the unzipped lualib.tar.gz file. Once these functions have been called, we can do things like json = require 'cjson'.

library(rterra)
tinit(rcppflags = c("./R302/R/include/", "./R302/R/include/R_ext"))


Once all this is done, the map code just calls getCurrentTimeAsString

function(a,b){
rhcollect("f",terra("getCurrentTimeAsString"))
}


The output is

> data.frame( k=unlist(lapply(j,"[[",1)),d= unlist(lapply(j,"[[",2) ))
k        d
1  f 21:53:16
2  f 21:53:15
3  f 21:53:10
4  f 21:53:05
5  f 21:53:22
6  f 21:53:24
7  f 21:53:20
8  f 21:53:20
9  f 21:53:04
10 f 21:53:22