> For the complete documentation index, see [llms.txt](https://docs.wellcomecollection.org/storage-service/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.wellcomecollection.org/storage-service/reference-design-decisison/identifiers.md).

# How identifiers work in the storage service

Bags in the storage service have a three-part identifier:

* **Space:** the broad category of a bag. Examples: `digitised`, `born-digital`.
* **External identifier:** the identifier of a bag within a space. This is typically an identifier from another system, which matches this bag to that record. Examples: `b31497652`, `PP/CRI/A/2`.
* **Version:** an auto-incrementing numeric value. This tracks distinct versions of a (space, external identifier) pair. Examples: `v1`, `v2`, `v3`.

The space and external identifier are supplied by the user; the version is automatically generated by the storage service.

These three parts can be combined into a single string, which uniquely identifies a bag; for example `digitised/b31497652/v2`. This identifier is also the path to [the root of the bag inside our storage buckets](/storage-service/reference-design-decisison/file-layout.md).

Why did we choose this approach?

* We want identifiers that are human-readable and understandable. (As opposed to, say, [UUIDs](https://en.wikipedia.org/wiki/Universally_unique_identifier).)
* We match bags to records in systems outside the storage service (for example, the library catalogue). This approach allows us to use the same identifier as the external system, rather than [inventing another type of identifier](https://xkcd.com/927/).
* This structure allows us to group related content by space within the underlying storage, in a way that is human-readable:

  ```
  digitised/
    ├── record1/
    │     ├── v1/
    │     │    ├── bagit.txt
    │     │    ├── bag-info.txt
    │     │    └── data/
    │     │          ├── record1_0001.jp2
    │     │          └── ...
    │     └── v2/
    │          └── ...
    │
    └── record2/
          └── v1/
               └── ...
  ```

  The human-readable storage layout means our files are not tied to the specific software implementation of the storage service.