===== Using XRootD in ROOT =====

It's now possible to access files stored anywhere on the grid from the "ui" using the xrootd service. 

Clearly there is a need to avoid requesting dataset transfers to the Tier2 disks such as AUSTRALIA-ATLAS_LOCALGROUPDISK and the subsequent copy to the data area on the ui. Now one can directly access the files in the dataset without having to download them to a local machine. This is how.

== Creating an ATLAS grid certificate ==
<code>
agu1:./~>au-voms-proxy-init 

Creating an ATLAS grid certificate........

NOTE: This will work for:
  * Downloading data via DQ2 or DATRI from any remote site.
  * Submitting jobs to Panda or any remote site.
This will *NOT* work for:
  * Tranfsering data to or from Australia-ATLAS LOCALGROUPDISK
  * Submitting jobs directly to Australia-ATLAS via the CREAM-CE

If you need this functionality, please use: au-voms-proxy-init --local

Enter GRID pass phrase:
Your identity: /C=TW/O=AP/OU=GRID/CN=Antonio Limosani 489869
Creating temporary proxy ...................................................................................................................................................... Done
Contacting  voms.cern.ch:15001 [/DC=ch/DC=cern/OU=computers/CN=voms.cern.ch] "atlas" Done
Creating proxy ............................................................... Done

Your proxy is valid until Thu Aug  1 19:35:28 2013

</code>

== Get the location of the file ==
<code>
dq2-ls --pfn --files  --site=AUSTRALIA-ATLAS_LOCALGROUPDISK mc11_7TeV.105200.T1_McAtNlo_Jimmy.merge.NTUP_TOP.e835_s1272_s1274_r3043_r2993_p937/

mc11_7TeV.105200.T1_McAtNlo_Jimmy.merge.NTUP_TOP.e835_s1272_s1274_r3043_r2993_p937/
srm://agh3.atlas.unimelb.edu.au/dpm/atlas.unimelb.edu.au/home/atlas/atlaslocalgroupdisk/mc11_7TeV/NTUP_TOP/e835_s1272_s1274_r3043_r2993_p937/mc11_7TeV.105200.T1_McAtNlo_Jimmy.merge.NTUP_TOP.e835_s1272_s1274_r3043_r2993_p937_tid758242_00/NTUP_TOP.758242._000971.root.1
srm://agh3.atlas.unimelb.edu.au/dpm/atlas.unimelb.edu.au/home/atlas/atlaslocalgroupdisk/mc11_7TeV/NTUP_TOP/e835_s1272_s1274_r3043_r2993_p937/mc11_7TeV.105200.T1_McAtNlo_Jimmy.merge.NTUP_TOP.e835_s1272_s1274_r3043_r2993_p937_tid758242_00/NTUP_TOP.758242._001266.root.1
...
</code>

== Load the file in root ==

When you run ROOT, load the libNetx.so library:<code>
root [0] .L $ROOTSYS/lib/libNetx.so
root [1] TXNetFile f("root://agh3.atlas.unimelb.edu.au//dpm/atlas.unimelb.edu.au/home/atlas/atlaslocalgroupdisk/mc11_7TeV/NTUP_TOP/e835_s1272_s1274_r3043_r2993_p937/mc11_7TeV.105200.T1_McAtNlo_Jimmy.merge.NTUP_TOP.e835_s1272_s1274_r3043_r2993_p937_tid758242_00/NTUP_TOP.758242._001446.root.1" ) 
root [2] f.ls()
TXNetFile**		root://agh3.atlas.unimelb.edu.au//dpm/atlas.unimelb.edu.au/home/atlas/atlaslocalgroupdisk/mc11_7TeV/NTUP_TOP/e835_s1272_s1274_r3043_r2993_p937/mc11_7TeV.105200.T1_McAtNlo_Jimmy.merge.NTUP_TOP.e835_s1272_s1274_r3043_r2993_p937_tid758242_00/NTUP_TOP.758242._001446.root.1	StreamNTUP_TOP
 TXNetFile*		root://agh3.atlas.unimelb.edu.au//dpm/atlas.unimelb.edu.au/home/atlas/atlaslocalgroupdisk/mc11_7TeV/NTUP_TOP/e835_s1272_s1274_r3043_r2993_p937/mc11_7TeV.105200.T1_McAtNlo_Jimmy.merge.NTUP_TOP.e835_s1272_s1274_r3043_r2993_p937_tid758242_00/NTUP_TOP.758242._001446.root.1	StreamNTUP_TOP
  KEY: AttributeListLayout	Schema;1	
  KEY: TDirectoryFile	physicsMeta;1	physicsMeta
  KEY: TDirectoryFile	Lumi;1	Lumi
  KEY: TTree	CollectionTree;1	CollectionTree
  KEY: TTree	physics;1	physics
</code>

== Useful Links ==

https://twiki.cern.ch/twiki/bin/view/Atlas/UsingFAXforEndUsers