sporadik

A sparse array wire format. One zarr prefix, one child per axis made contiguous, and a block written last that says which children the writer finished.

It is strictly a format: no network, no client, no storage opinions, and no idea what the numbers mean. Two dependencies is the whole of it.

It defines no container of its own. A layout is anndata's spelling, so at rank two the three arrays are byte-identical to what scanpy writes and a browser reads one with zarrita's open and get rather than a decoder. What sporadik adds is the part anndata has no opinion about: which axis a layout makes contiguous, how to hold more than one of them, how to tell a finished upload from a torn one, and what all of that means above rank two.

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The idea

A layout is one axis made contiguous. Pick the axis a reader will select along, ravel the remaining axes into indices, and indptr names one run per position along the chosen one.

At rank two that construction is CSR (axis 0) and CSC (axis 1). At rank three it is the same construction with more axes folded into the same indices. One invariant holds at every rank, and it is the spine of the format:

len(indptr) == shape[indexed_axis] + 1

So an array of rank n has up to n layouts, and each buys exactly one question — "everything at this position along axis k" — at the cost of another copy of the nonzeros. Ask a layout the question it does not compress for and there is no range to read at all, only a scan: 1 777 ms against 2.2 ms, measured on a 16 µm spatial-transcriptomics matrix.

---

The layout on disk

<prefix>/
  zarr.json                     attributes: {"sporadik": {...}}   <- written LAST
  layouts/
    axis0/                      a sparse group, anndata-spelled
      zarr.json                 encoding-type, encoding-version, shape
      data/ indices/ indptr/
    axis1/                      the same array, contiguous along another axis

Children are named for the axis they make contiguous, not for an encoding: csr/csc name the two cases of rank two and say nothing at rank three. A reader recomputes the name and compares, so a layout filed under the wrong one is refused rather than silently indexed along the wrong axis.

---

The block — spec 1

Under the key sporadik in the root group's own attributes:

{
  "spec": "1",
  "complete": true,
  "shape": [91033, 19059],
  "layouts": [
    {"path": "layouts/axis0", "indexed_axis": 0, "index_order": [1]},
    {"path": "layouts/axis1", "indexed_axis": 1, "index_order": [0]}
  ]
}

key	declared or derived	meaning
spec	declared	how to read the prefix. Unknown ⇒ refuse.
complete	declared	the writer finished. See Why the block is last.
shape	declared	the array's own shape, at its own rank. Checked against every layout.
layouts[].path	declared	the child group. Must equal layouts/axis{indexed_axis}.
layouts[].indexed_axis	declared	which axis this layout makes contiguous.
layouts[].index_order	declared	the axes it did not compress, in the order indices was raveled over them.

Everything else a reader needs — each layout's encoding, nonzero count, dtype, chunking, and whether it is byte-addressable — is derived from the artifact and never declared. That distinction is the design: a fact read off the bytes cannot be stated wrongly.

index_order is the one exception, and it has to be stated because it cannot be recovered from the bytes. At rank two it has one member and says nothing. Above it, a wrong index_order does not fail anywhere — it reads a different cell.

---

A layout child

Each named child is an anndata-spelled sparse group: attributes encoding-type, encoding-version and shape, plus three 1-D arrays data, indices, indptr.

encoding-type by rank. At rank two it is anndata's exactly — csr_matrix compresses axis 0, csc_matrix axis 1. Above rank two there is no anndata spelling of the thing, so the child holds the array raveled to two axes, which genuinely is a csr_matrix, and says so.

Declared shape by rank. At rank two the child declares the array's own shape, so it is a real anndata group over the real matrix. Above it, the raveled pair it literally holds: [shape[indexed_axis], prod(shape[a] for a in index_order)].

---

Why the block is last

Everything else in the prefix declares something before it is true: zarr writes an array's zarr.json ahead of its chunks, and a layout's encoding-type when its group is created. Only the block — one object, written once, after every chunk is durable — is a statement made after the thing it describes exists.

That matters more than it sounds, because zarr substitutes the fill value for a chunk it cannot fetch. That is a legitimate convention for genuinely-sparse arrays and, for an interrupted upload, indistinguishable from success. Measured, before the block existed: deleting every chunk of data left a store that passed every other check, recorded the right nnz, and returned the right number of values for a slice — all of them zero, with nothing raised anywhere.

---

What a conforming reader checks

Each one is silent if skipped, which is why each is listed.

1. len(indptr) == shape[indexed_axis] + 1, at every rank — what makes indptr[i:i+2] a run.
2. len(data) == len(indices) — parallel arrays; one without the other stopped partway.
3. path == layouts/axis{indexed_axis} — else it is read along the wrong axis.
4. index_order is a permutation of the axes other than indexed_axis.
5. The child's declared shape and encoding-type match the rank rules above.
6. At most one layout per axis, and at most rank of them.
7. The block is present and complete.

---

Chunking

All three arrays are chunked, indptr included, at ~128 KB (32 768 four-byte elements) — sized for one object-store request, because on S3 the cost is round trips rather than bytes. Measured: one slice costs 0.95 ms at 32 768-element chunks, 3.00 ms at 512, and 23.55 ms at 4 Mi.

A chunk is also a cache unit: consecutive slices are adjacent in the array, so a reader walking nearby positions hits chunks it already holds, which an exact byte range never does. indptr is chunked for the same reason rather than written whole — over 5.4 M positions it is a ~22 MB object that two entries would otherwise pull entirely.

write_store(..., byte_addressable=True) writes one uncompressed chunk per array instead, so the stored object is the raw little-endian buffer and indptr names an exact byte range. Fewer bytes (376 against 131 072 for one 94-nonzero slice), the same number of round trips, no reuse — worth it for a reader that makes one cold lookup and caches nothing. Which was written is derived from the codecs, never declared.

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anndata interop

At rank two a layout is a genuine anndata sparse group and its arrays are byte-identical to anndata's own — asserted in the test suite, not remembered. read_elem(group["layouts"]["axis0"]) returns the matrix.

Above rank two, read_elem on a child returns the raveled two-axis view, which is what the child literally holds. A degradation, not a lie; the real shape is in the block.

read_elem on the prefix root does not work at any rank: the root is a container for layouts, not itself a sparse group.

---

Install

pip install sporadik              # numpy + zarr
pip install sporadik[scipy]       # + read_layout(), which returns a scipy.sparse matrix
pip install sporadik[obstore]     # + writing into an object store rather than a directory

Use

import scipy.sparse as sp
import sporadik

counts = sp.random(20_000, 1_200, density=0.01, format="csr")
sporadik.write_store("expression.zarr", [counts, counts.tocsc()])

with sporadik.open_store("expression.zarr", layout=1) as feature_major:
    positions, values = feature_major.slice_at(7)      # two reads, nothing else fetched

Above rank two, build a layout per axis and unravel what comes back:

layout = sporadik.layout_over(shape, axis, data=..., indices=..., indptr=..., index_order=(0, 2))
with sporadik.open_store(path, layout=1) as reader:
    (a, c), values = reader.coords_at(7)               # through index_order, not axis order

---

Conformance

sporadik.spec is the normative half; everything else in the package is one implementation of it. A second implementation — in another language, or in a server that must not depend on this package — reproduces spec and nothing more.

That independence is deliberate. A reader that imports its writer inherits the writer's dependencies and its release cycle, and a version skew between them becomes an outage rather than a refusal. Two independent implementations of a written-down format is the only arrangement in which "the format is specified" is a testable claim rather than a shared object file.

The block example above is parsed by tests/test_spec_document.py and asserted against sporadik.spec, so this document cannot drift from the code it documents.