xrootd

Last updated on 2026-07-10 | Edit this page

XrootD is a software suite for data access and management.

In the tutorial, we used XrootD to copy files and to stream files. We could also use it to search for files as well if we wanted. However, the limitation is that typically when browsing we must specify a server. If our file isn’t on this server, we won’t see it, unlike in Rucio.

Directory Structure

Unlike Rucio, if we use XrootD, we do have a file structure. Typically, file paths typically look like:

  • /volatile/eic/EPIC/OUTPUT_TYPE/CAMPAGIN/DETECTOR_GEOMETRY/PHYSICS_PROCESS_TYPE/PHYSICS_PROCESS/BEAM_ENERGY/OTHER_CONDITIONS/FILE.eicrecon.edm4eic.root
    • OUTPUT_TYPE - Is this event generator, simulation or reconstruction output?
    • CAMPAIGN - Which simulation campaign is this from?
    • DETECTOR_GEOMETRY - Which detector geometry/design was used to run this simulation?
    • PHYSICS_PROCESS_TYPE - Broadly, what category of physics process is this?
    • PHYSICS_PROCESS - What physics process is this?
    • BEAM_ENERGY - What beam energy combination was used?
    • OTHER_CONDITIONS - Other simulation conditions, such as a range for a specific kinematic variable (Q2, t or similar)

Some directories may include event generator versions after the physics process too. You may also find some examples where there are multiple sets of additional conditions.

Callout

Warning - Non-JLab Servers

Notice the /volatile/ at the front of the path. This is specific to files on the JLab XrootD server.

Files stored elsewhere may just be under /eic/EPIC/...

Browsing the Simulation Output with XrootD

We can browse the simulation output using XrootD from within the eic-shell. To browse the directory structure and exit, one can run the commands:

BASH 

./eic-shell
xrdfs root://dtn-eic.jlab.org
ls /volatile/eic/EPIC/RECO/26.02.0
exit

xrdfs is the command to log in to a specific server, in this case root://dtn-eic.jlab.org.

When calling ls, we should see everything in this subfolder. In this case, all files from the February 2026 campaign.

Files can also be copied locally by replacing ls with cp.

It is also possible to copy a file and open it locally using the xrdcp command:

BASH 

./eic-shell
xrdcp root://dtn-eic.jlab.org//volatile/eic/EPIC/RECO/26.02.0/path-to-file .
exit

The trailing . indicates that the file will be copied to your current path with its current name. Change this as desired.

In our earlier episode, we used this command to copy a file we found using Rucio. This is now the recommended workflow:

  1. Find file location with Rucio
  2. Stream or download with XrootD

Streaming Files

It is also possible to open a file directly in ROOT if you have XrootD installed too. Note that the following command should be executed after opening root and TFile::Open() should be used:

CPP 

auto f = TFile::Open("root://dtn-eic.jlab.org//volatile/eic/EPIC/RECO/path-to-file")

or using python and uproot:

PYTHON 

import uproot
import XRootD
file_path = "root://dtn-eic.jlab.org//volatile/eic/EPIC/RECO/path-to-file"
root_file = uproot.open(file_path)

or directly with Pyroot:

PYTHON 

import ROOT
import XRootD
file_path = "root://dtn-eic.jlab.org//volatile/eic/EPIC/RECO/path-to-file"
file = ROOT.TFile.Open(file_path, "READ")