# Understanding FHR Metadata

FHR (FAIR Headers Reference genome) metadata is structured, assembly-level information that you can attach to sequence collections in RefgetStore. It provides a standard way to record *what a genome assembly is*—species, version, masking, taxonomy, licensing, and more—following the FAIR principles (Findable, Accessible, Interoperable, Reusable).

## Why assembly-level metadata matters

A refget collection digest tells you *exactly* which sequences are present—by mathematical proof. What it does not tell you is the biological context: which species, which assembly version, whether repeats are hard-masked or soft-masked, who published the assembly, or under what license it was released.

That context lives in FHR metadata. It answers questions like:

- Is this GRCh38 or GRCh38.p14?
- Is masking soft or hard?
- What taxonomy ID corresponds to this organism?
- Who should I cite if I use this assembly?
- Is there a paper describing this reference?

Without structured metadata, this information is typically scattered across README files, filenames, and institutional memory. FHR provides a single, machine-readable location for it—attached directly to the collection digest.

## The FHR specification

FHR is an open specification developed by the [FAIR-bioHeaders community](https://github.com/FAIR-bioHeaders/FHR-Specification). It defines a JSON schema for genome assembly metadata. RefgetStore implements FHR metadata as sidecar files attached to collection digests.

Key fields in the FHR spec:

| Field | Type | Description |
|-------|------|-------------|
| `schema` | string | URI of the FHR JSON schema |
| `schemaVersion` | string | Version of the FHR spec being used |
| `genome` | string | Species or organism name |
| `version` | string | Assembly version (e.g., "GRCh38.p14") |
| `taxon` | object | Taxonomy info: `name` and `uri` |
| `masking` | string | Masking type: "soft-masked", "hard-masked", "unmasked" |
| `genomeSynonym` | array | Alternative names (e.g., ["hg38"]) |
| `dateCreated` | string | ISO 8601 date |
| `license` | string | SPDX license identifier (e.g., "CC0-1.0") |
| `metadataAuthor` | array | Authors of the metadata record |
| `sequenceAuthor` | array | Authors/producers of the genome assembly |
| `scholarlyArticle` | string | DOI or URL of a related publication |
| `funding` | string | Funding information |
| `identifier` | object | Cross-reference to an external registry |

Not all fields are required. The minimum useful record typically includes `genome`, `version`, and `taxon`.

## How FHR metadata is stored

On-disk RefgetStores persist FHR metadata as JSON sidecar files in the `collections/` directory:

```
my_store/
  collections/
    Ab1cd.rgci                          ← collection index file
    NikmJ6xnuvO741NgL-zszh5_p4DsD3nV.rgci
    NikmJ6xnuvO741NgL-zszh5_p4DsD3nV.fhr.json   ← FHR sidecar
```

The sidecar filename is `<collection_digest>.fhr.json`. When you open a store with `RefgetStore.open_local()`, all sidecar files in the `collections/` directory are loaded automatically. FHR metadata travels with the store: copying a store directory to another machine preserves all attached metadata.

In-memory stores support FHR metadata in memory, but it is not persisted when the store goes out of scope.

## FHR metadata is collection-level, not sequence-level

FHR metadata is attached to collection digests, not to individual sequence digests. This reflects how genome assembly metadata works in practice: masking, taxonomy, versioning, and licensing are properties of an assembly as a whole, not of individual chromosomes or contigs.

If you have two collections derived from the same FASTA but with different processing (e.g., one soft-masked and one hard-masked), each has a different collection digest and can carry its own FHR metadata record describing the difference.

## Relationship to digests and aliases

FHR metadata complements aliases rather than replacing them. Aliases link a registry identifier to a digest ("this collection is also known as GRCh38 in NCBI"). FHR metadata describes what that collection *is* ("it is Homo sapiens, version GRCh38.p14, soft-masked, licensed CC0-1.0").

In a well-maintained store, a collection might have:

- A collection digest (computed from content)
- Collection aliases in multiple namespaces (`ncbi/GRCh38`, `ucsc/hg38`)
- FHR metadata with species, version, masking, taxon, and license

Together, these three layers provide complete, machine-readable provenance for the assembly.

## Learn more

- [FHR Metadata Headers](/refget/using-services/fhr-metadata.md) - Hands-on guide: creating, attaching, retrieving, and removing FHR metadata
- [FHR Specification](https://github.com/FAIR-bioHeaders/FHR-Specification) - The upstream FAIR-bioHeaders specification
- [Names, aliases, and identifiers](/refget/names-and-aliases-explained.md) - How aliases complement FHR metadata
- [RefgetStore file format](/refget/reference/refgetstore-format.md) - Technical specification including the `collections/` directory and sidecar file format