Remote data

Part 3 of 3 in Read Data

✓ 01. ROOT Files
✓ 02. Datasets
▶ 03. Remote Data

This tutorial reads one public CMS Open Data ROOT file directly over XRootD.

The file is a NanoAOD sample from the DoubleMuon dataset. It is related to the Z → $\mu\mu$ examples in the workshop, but the path name remains Zmumu where paths use that convention.

1. Inspect the dataset file

The dataset is defined separately in datasets.yaml:

datasets:
  DoubleMuon:
    files:
      - root://eospublic.cern.ch//eos/opendata/cms/...

For one file this is not necessary, but real analyses usually have many datasets and many files per dataset. Keeping datasets separate makes that list easier to maintain.

2. Inspect the workflow

author.yaml includes the dataset file:

include:
  - datasets.yaml

The workflow reads the Events tree and records a schema snapshot. It does not make histograms, skim events, or use distributed execution.

3. Run the workflow

pixi run fasthep run tutorials/01-read-data/03-remote-data/author.yaml --outdir build/tutorials/01-read-data/03-remote-data

XRootD support is required in the runtime environment. If the command fails while opening the root:// URL, check that the environment has the required Uproot/XRootD support (fsspec-xrootd) and network access.

4. Inspect the outputs

Look at:

• build/tutorials/01-read-data/03-remote-data/reports/schema/read_events/events__DoubleMuon__0.json

• build/tutorials/01-read-data/03-remote-data/run_summary.yaml

Expected outputs

Remote files can contain many branches, especially NanoAOD files. Schema inspection helps you understand what is available before writing analysis stages, and FAST-HEP can inspect the schema without reading the whole dataset.

{
  "node_id": "read.events",
  "metadata": {
    "dataset_name": "DoubleMuon",
    "file": "root://eospublic.cern.ch//eos/opendata/cms/Run2016G/DoubleMuon/NANOAOD/UL2016_MiniAODv2_NanoAODv9-v2/2430000/3B20EB8F-4FD1-D041-9513-1A82351756E1.root",
    "part": "0_0",
    "partition_id": "events__DoubleMuon__0",
    "source": "events",
    "start": null,
    "stop": null
  },
  "awkward_type": {
    "CaloMET_phi": "float",
    "CaloMET_pt": "float",
    "CaloMET_sumEt": "float",
    "ChsMET_phi": "float",
    "ChsMET_pt": "float"
  },
  "inspected_python_type": "RootTreeSchema"
}

The snippet above is shortened to the first few entries. The full schema report is written under:

build/tutorials/01-read-data/03-remote-data/reports/schema/