References specification

The content of a reference set should match the given description here. fsspec’s ReferenceFileSystem expects this kind of input.

Version 0

Prototype spec for the structure required by ReferenceFileSystem:

  "key0": "data",
  "key1": ["protocol://target_url", 10000, 100]


  • key0 includes data as-is (stored as text)

  • key1 refers to a data file URL, the offset within the file (in bytes), and the length of the data item (in bytes).

For example, Zarr data in this proposed spec might be represented as:

  ".zgroup": "{\n    \"zarr_format\": 2\n}",
  ".zattrs": "{\n    \"Conventions\": \"UGRID-0.9.0\n\"}",
  "x/.zattrs": "{\n    \"_ARRAY_DIMENSIONS\": [\n        \"node\"\n ...",
  "x/.zarray": "{\n    \"chunks\": [\n        9228245\n    ],\n    \"compressor\": null,\n    \"dtype\": \"<f8\",\n  ...",
  "x/0": ["s3://bucket/path/", 294094376, 73825960]

Version 1

Metadata structure in JSON. We note, for future possible binary storage, that “version”, “gen” and “templates” should be considered attributes, and “refs” as the data that ought to dominate the storage size. The previous definition, Version 0, is compatible with the “refs” entry, but here we add features. It will also be possible to expand this new enhanced spec into Version 0 format.

  "version": (required, must be equal to) 1,
  "templates": (optional, zero or more arbitrary keys) {
    "template_name": jinja-str
  "gen": (optional, zero or more items) [
    "key": (required) jinja-str,
    "url": (required) jinja-str,
    "offset": (optional, required with "length") jinja-str,
    "length": (optional, required with "offset") jinja-str,
    "dimensions": (required, one or more arbitrary keys) {
      "variable_name": (required)
        {"start": (optional) int, "stop": (required) int, "step": (optional) int}
        [int, ...]
  "refs": (optional, zero or more arbitrary keys) {
    "key_name": (required) str OR [url(jinja-str)] OR [url(jinja-str), offset(int), length(int)]


  • jinja-str is a string which will be rendered by jinja2 or its non-python equivalent; i.e., it may be a literal string, or may include “{{..}}” annotations, where:

    • for the values associated with a template_name, the variables are to be passed in reference URL strings that use this template

    • for the values within a “gen” object, variables come from the “dimensions” and “templates”

  • the str format of a reference value may be:

    • a string starting “base64:”, which will be decoded to binary

    • any other string, interpreted as ascii data

  • the str version of ref values indicates data, the one-element array a whole url, and the three-element version a binary section of a url

Here is an example

    "version": 1,
    "templates": {
        "u": "server.domain/path",
        "f": "{{c}}"
    "gen": [
            "key": "gen_key{{i}}",
            "url": "http://{{u}}_{{i}}",
            "offset": "{{(i + 1) * 1000}}",
            "length": "1000",
                "i": {"stop":  5}
    "refs": {
      "key0": "data",
      "key1": ["http://target_url", 10000, 100],
      "key2": ["http://{{u}}", 10000, 100],
      "key3": ["http://{{f(c='text')}}", 10000, 100]

Here the variable i takes the values [0, 1, 2, 3, 4], which could have been provided in array form. Where there is more than one variable, a cartesian product is formed.

This example evaluates to the Version 0 equivalent

  "key0": "data",
  "key1": ["http://target_url", 10000, 100],
  "key2": ["http://server.domain/path", 10000, 100],
  "key3": ["http://text", 10000, 100],
  "gen_key0": ["http://server.domain/path_0", 1000, 1000],
  "gen_key1": ["http://server.domain/path_1", 2000, 1000],
  "gen_key2": ["http://server.domain/path_2", 3000, 1000],
  "gen_key3": ["http://server.domain/path_3", 4000, 1000],
  "gen_key4": ["http://server.domain/path_4", 5000, 1000]

such that accessing, for instance, “key0” returns b"data" and accessing “gen_key0” returns 1000 bytes from the given URL, at an offset of 1000.

Parquet references

Since JSON is rather verbose, it is easy with enough chunks to make a references file that is too big: slow to load and heavy on memory. Although the former can be alleviated by compression (I recommend Zstd), the latter cannot. This can become particularly apparent during the combine phase when loading many reference sets.

The class fsspec.implementations.reference.LazyReferenceMapper provides an alternative implementation, and its on-disk layout effectively is a new reference spec, and we describe it here. The class itself has a dict mapper interface, just like the rendered references from JSON files; except that it assumes that it is working on a zarr dataset. This is because the references are split into files, and an array’s shape/chunk information is used to figure out which reference file to load.

The following code

lz = fsspec.implementations.reference.LazyReferenceMapper.create("ref.parquet")
z = zarr.open_group(lz, mode="w")
d = z.create_dataset("name", shape=(1,))
d[:] = 1
g2 = z.create_group("deep")
d = g2.create_dataset("name", shape=(1,))
d[:] = 1

produces files


Here, .zmetadata is all of the metadata of all of all subgroups/arrays (similar to zarr “consolidated metadata”), with two top-level fields: “metadata” (dict[str, str] all of the zarr metadata key/values) and “record_size”, an integer set during .create().

Each parquet file contains references within the corresponding path to where it is. For example, key “name/0” will be the zeroth reference in “./name/refs.0.parq”. If there are multiple dimensions, normal C indexing is used to find the Nth reference, and there are up to “record_size” references (default 10000) in the first file; reference >10000,<=20000 would be in “./name/refs.2.parquet”. Each file is (for now) padded to record_size, but they compress really well.

Each row of the parquet data contains fields

path: optional str/categorical, remote location URL
offset: int, start location of block
size: int, number of bytes in block
raw: optional bytes, binary data

If raw is populated, this is the data of the key. If path is populated but size is 0, it is the whole file indicated (like a JSON [url] reference). Otherwise, it is a byte block in the indicated file (like a JSON [url, offset, size] reference). If both raw and path are NULL, the key does not exist.

We reserve the possibility to store small array data in .zmetadata instead of creating a small/mostly empty parquet file for each.