Added a page on XrootD as an extra. Link to this in episode 2 now.

Author: sjdkay

_episodes/02-rucio_usage.md

@@ -304,7 +304,7 @@ By default it will download to our current directory with its original name. In
 > We can *stream* files from a full dataset rather than downloading them as we'll see in a moment.
 {: .caution}
 
-It might actually be easier to use xrootd to grab our file as it's a bit more intuitive, we do need our location from earlier for this though:
+It might actually be easier to use [XrootD]({{ page.root }}{% link _extras/xrootd.md %}) to grab our file as it's a bit more intuitive, we do need our location from earlier for this though:
 
 ```bash
 xrdcp root://dtn-eic.jlab.org:1094//volatile/eic/EPIC//RECO/26.02.0/epic_craterlake/EXCLUSIVE/DEMP/DEMPgen-1.2.4/10x250/q2_3_10/pi+/DEMPgen-1.2.4_10x250_pi+_q2_3_10_ab.0550.eicrecon.edm4eic.root ./

_extras/xrootd.md

@@ -0,0 +1,87 @@
+---
+title: "xrootd"
+---
+
+[XrootD](https://xrootd.org/) 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.
+
+> ## `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/...`
+{: .caution}
+
+# 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:
+
+```bash
+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")
+```
+
+{% include links.md %}