anndataoom 0.1.6

Out-of-memory AnnData powered by Rust (anndata-rs)

anndata-oom

Out-of-memory AnnData powered by Rust — a drop-in replacement for anndata.AnnData that keeps the expression matrix on disk and runs entire preprocessing pipelines (normalize, log1p, scale, PCA, neighbors, UMAP, Leiden) as lazy transforms or chunked operations. The full matrix is never loaded into memory.

Built on top of scverse/anndata-rs, the Rust implementation of AnnData.

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Why?

Standard anndata.AnnData loads the entire expression matrix into RAM. For a million-cell atlas this can mean 100+ GB of memory — beyond what most workstations have.

anndataoom keeps X on disk (HDF5) and streams it through the preprocessing pipeline in chunks. Peak RAM is independent of dataset size.

Memory comparison

Dataset	anndata.AnnData	anndataoom	Savings
PBMC 8k (7.7k × 21k)	1.5 GB	54 MB	27.8x
100k cells × 30k genes	~12 GB	~700 MB	17x
1M cells × 30k genes	~120 GB (OOM)	~700 MB	170x

How?

Each preprocessing step adds a small "transform descriptor" (a vector or flag) to a lazy computation chain. Data is computed on-the-fly during chunked reads from the HDF5 file:

X (HDF5 on disk, Rust I/O via anndata-rs)
  → TransformedBackedArray      (normalize: ÷ per-cell size factors)
    → TransformedBackedArray    (log1p: on-the-fly)
      → _SubsetBackedArray      (HVG: select 2,000 gene columns)
        → ScaledBackedArray     (z-score: stores only mean/std vectors)
          → Randomized SVD      (chunked matrix products)
            → X_pca             (n_obs × 50, in memory)
              → Neighbors / UMAP / Leiden (operate on X_pca only)

Step	What's stored	Peak memory
Read	File handle	~0
Normalize	Per-cell factor vector	n_obs × 8 B
log1p	Flag only	0
HVG subset	Column index	~8 KB
Scale	Mean + std vectors	~32 KB
PCA (working set)	Y, Q matrices (k=60)	n_obs × 60 × 8 B
X_pca	Final embedding	n_obs × 50 × 4 B

For a deeper look at the lazy operator model behind this — how nodes compose, how data flows through the chain, and why subsetting stays O(1) — see docs/transform-chain.md.

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Documentation

• The Transform Chain — design walkthrough with ASCII diagrams of the lazy operator model (BackedArray, _SubsetBackedArray, TransformedBackedArray, ScaledBackedArray) and how chunked reads flow through it.

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Installation

Prebuilt wheels (recommended)

pip install anndataoom

Wheels are built for:

Platform	Architectures	Python
Linux	x86_64, aarch64	3.9–3.13
macOS	x86_64, arm64	3.9–3.13
Windows	x86_64	3.9–3.13

Wheels bundle a statically-linked HDF5 — no system dependencies needed, no Rust toolchain required.

Build from source

If no prebuilt wheel matches your system, pip falls back to source. You'll need a Rust toolchain:

# Install Rust (if needed)
curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y

# Build + install
pip install anndataoom

Or for development:

git clone https://github.com/Starlitnightly/anndata-oom
cd anndata-oom
pip install maturin
maturin develop --release

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Quick start

import anndataoom as oom

# Read an h5ad file — matrix stays on disk
adata = oom.read("large_dataset.h5ad")
print(adata)

AnnDataOOM                                 [Rust · out-of-memory · backed]
Dimensions:  n_obs: 100,000    n_vars: 30,000

┌───────────┬──────────────────────────────────────┐
│ File      │ large_dataset.h5ad  (1.2 GB on disk) │
│ X         │ csr_matrix · float32 · 5.3% density   │
│ Chunk I/O │ ~20 MB per 1,000-row chunk            │
└───────────┴──────────────────────────────────────┘

▸ obs     (8)    batch · cell_type · n_counts · ...
▸ var     (3)    gene_name · highly_variable · ...
▸ obsm    (–)
▸ layers  (–)
▸ raw     (–)

Chunked operations

# Sum over all cells — streams the matrix in 1000-row chunks
row_sums = adata.X.sum(axis=1)       # ndarray of shape (n_obs,)

# Per-gene means — one-pass chunked Welford's
mean, var = oom.chunked_mean_var(adata)

# Iterate chunks manually
for start, end, chunk in adata.X.chunked(5000):
    # chunk is a csr_matrix (or ndarray) of shape (≤5000, n_vars)
    ...

Subsetting

# All forms of indexing work
sub = adata[0:1000]                          # first 1000 cells
sub = adata[adata.obs["cell_type"] == "B"]   # boolean mask
sub = adata[:, ["GENE1", "GENE2"]]           # by gene name
sub = adata[:, adata.var["highly_variable"]] # after HVG selection

# Returns a new AnnDataOOM — still lazy
print(sub.shape)   # e.g. (17003, 2000)

Single-gene access

# obs_vector reads exactly one column from disk
expr = adata.obs_vector("CD3D")   # ndarray of shape (n_obs,)

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Integration with omicverse

omicverse automatically detects anndataoom and uses it as the backend for ov.read(..., backend="rust"):

import omicverse as ov

# Read — returns AnnDataOOM if anndataoom is installed
adata = ov.read("data.h5ad", backend="rust")

# Full preprocessing pipeline — all chunked/lazy
adata = ov.pp.qc(adata,
                 tresh={"mito_perc": 0.2, "nUMIs": 500, "detected_genes": 250},
                 doublets=False)
adata = ov.pp.preprocess(adata, mode="shiftlog|pearson",
                         n_HVGs=2000, target_sum=50 * 1e4)

# HVG subset — returns a new AnnDataOOM
adata.raw = adata
adata = adata[:, adata.var.highly_variable_features]

# Scale + PCA — lazy z-score + chunked randomized SVD
ov.pp.scale(adata)
ov.pp.pca(adata, layer="scaled", n_pcs=50)

# Neighbors / UMAP / Leiden — operate on obsm['X_pca'], no matrix touch
ov.pp.neighbors(adata, n_neighbors=15, n_pcs=50,
                use_rep="scaled|original|X_pca")
ov.pp.umap(adata)
ov.pp.leiden(adata, resolution=1)

# Plotting — all ov.pl.* functions work directly, incl. use_raw=True
ov.pl.embedding(adata, basis="X_umap", color="leiden")
ov.pl.dotplot(adata, marker_genes, groupby="leiden")
ov.pl.violin(adata, keys="CD3D", groupby="leiden", use_raw=True)

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Full API reference

Top-level

Function / Class	Description
oom.read(path, backed='r')	Read an .h5ad file → AnnDataOOM
oom.AnnDataOOM	Out-of-memory AnnData (full anndata.AnnData API)
oom.BackedArray	Lazy row-chunked wrapper over anndata-rs X
oom.TransformedBackedArray	Lazy normalize / log1p transform chain node
oom.ScaledBackedArray	Lazy z-score transform
oom.is_oom(obj)	Check if obj is an AnnDataOOM
oom.oom_guard(...)	Decorator: auto-materialise for in-memory functions
oom.concat(adatas)	Concatenate multiple AnnData

Chunked preprocessing

Function	Description
chunked_qc_metrics(adata)	nUMIs, detected_genes, n_cells per gene
chunked_gene_group_pct(adata, mask)	Per-cell fraction of counts in a gene group
chunked_normalize_total(adata, target_sum)	Lazy normalize-total
chunked_log1p(adata)	Lazy log1p
chunked_mean_var(adata)	Welford's mean + var per gene
chunked_identify_robust_genes(adata)	Filter low-expression genes
chunked_highly_variable_genes_pearson(...)	Pearson residuals HVG selection (2 passes)
chunked_scale(adata)	Lazy z-score
chunked_pca(adata)	Randomized SVD with chunked matrix products

AnnDataOOM methods

All anndata.AnnData methods and properties are supported. Key ones:

Property / method	Behaviour
.shape, .n_obs, .n_vars	Dimensions
.obs, .var	Pandas DataFrames (eagerly loaded; small)
.X	Lazy BackedArray (never loaded)
.obsm, .varm, .obsp, .varp	Dict-of-ndarray (loaded; typically small)
.layers	BackedLayers dict (sidecar HDF5)
.raw	_FrozenRaw snapshot (shares backing file)
.obs_vector(key)	One column from disk (no full load)
.chunked_X(chunk_size=1000)	Row-chunked iterator
adata[idx]	Subsetting (returns new AnnDataOOM)
adata.copy()	Shallow copy (shares backing file, no RAM cost)
adata.to_adata()	Materialize to standard anndata.AnnData
adata.write(path)	Chunked write — doesn't materialize
adata.close()	Release file handle
repr(adata) / _repr_html_()	Pretty text / Jupyter display

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Benchmark: PBMC 8k (7,750 cells × 20,939 genes)

Full preprocessing pipeline (QC → normalize → HVG → scale → PCA → neighbors → UMAP → Leiden):

Step	Python (MB)	anndataoom (MB)
read	148	37
qc	280	54
preprocess	328	24
hvg_subset	450	24
scale	382	54
pca	846	33
neighbors	1195	33
umap	1500	34
leiden	1502	33
Peak	1502	54

→ 27.8× memory savings on this small dataset; ratio grows with scale.

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Supported h5ad formats

X format	Reading	Lazy ops	Notes
Dense ndarray	✅	✅	float32 / float64
CSR sparse	✅	✅	Most common scRNA-seq format
CSC sparse	✅	✅	Column-oriented

anndataoom automatically preserves sparsity through normalize and log1p (sparse → sparse), and materializes to dense only where algorithmically necessary (z-score, PCA).

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Architecture

anndataoom is a thin Python wrapper over scverse/anndata-rs:

┌──────────────────────────────────────────────┐
│  anndataoom (Python package)                 │
│  ┌────────────────────────────────────────┐  │
│  │  AnnDataOOM                            │  │
│  │  ├─ obs, var (pandas.DataFrame)        │  │
│  │  ├─ obsm, varm (dict of ndarray)       │  │
│  │  ├─ layers (BackedLayers — sidecar H5) │  │
│  │  └─ X (BackedArray — wraps ↓)          │  │
│  └────────────────────────────────────────┘  │
│            │                                  │
│            ▼                                  │
│  ┌────────────────────────────────────────┐  │
│  │  anndataoom._backend  (Rust extension)│  │
│  │  ├─ AnnData (pyanndata)                │  │
│  │  ├─ PyArrayElem (chunked() iterator)   │  │
│  │  └─ Statically linked:                 │  │
│  │     ├─ anndata (Rust crate)            │  │
│  │     ├─ anndata-hdf5                    │  │
│  │     └─ HDF5 C library                  │  │
│  └────────────────────────────────────────┘  │
└──────────────────────────────────────────────┘

The Rust extension (anndataoom._backend) is pinned to a specific commit of scverse/anndata-rs for reproducible builds (the same commit used by SnapATAC2).

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Limitations and caveats

• Writing back to X is lazy — modifications via adata[mask] = value materialize X in memory. Use adata.obs, adata.obsm, or adata.write(path) to persist changes.
• PCA accuracy: chunked_pca uses randomized SVD with 4 power iterations — top ~20 PCs are nearly identical to standard PCA; PCs 30+ start to deviate. For publication-quality analyses, consider adata.to_adata() + standard PCA.
• Some ops require materialization: score_genes_cell_cycle, find_markers, non-Harmony batch correction, etc. These auto-materialize with a warning.
• File mode: Default backed='r' (read-only) protects the source file. Use backed='r+' if you need to write back (advanced).
• Concurrent access: HDF5 files default to exclusive locking. Set HDF5_USE_FILE_LOCKING=FALSE in the environment if multiple processes need to read the same file.

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Comparison with alternatives

Feature	anndata	anndata (backed='r')	anndataoom
Read without loading matrix	❌	✅	✅
Subset (lazy view)	✅ (view)	✅ (view)	✅ (new AnnDataOOM)
Chunked iteration	❌	❌ (manual)	✅
normalize / log1p	In-memory	❌ (read-only)	✅ (lazy transform)
scale	In-memory	❌	✅ (lazy z-score)
PCA	Full SVD	❌	✅ (chunked rSVD)
Plotting (scanpy/omicverse)	✅	Limited	✅ (via omicverse)
Modify obs/var	✅	❌	✅
Peak RAM (1M × 30k)	~120 GB	— (can't process)	~700 MB

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Development

git clone https://github.com/Starlitnightly/anndata-oom
cd anndata-oom

# Install Rust (first time)
curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y
source $HOME/.cargo/env

# Build in editable mode
pip install maturin
maturin develop --release

# Run tests
pip install pytest
pytest tests/

Contributing

Contributions welcome! Areas of interest:

• More lazy transforms: regress-out, harmony, scVI integration
• Zarr backend: currently only HDF5 supported
• Dask interop: expose BackedArray as a dask.array
• Query engine: SQL-like filtering over chunks

Release process

1. Bump version in pyproject.toml and Cargo.toml
2. Update CHANGELOG.md
3. Commit, tag, push:

   git commit -am "Release v0.x.0"
   git tag v0.x.0
   git push && git push --tags
   

4. GitHub Actions builds wheels for all platforms and publishes to PyPI (via trusted publishing)

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License

MIT License — see LICENSE.

Built on scverse/anndata-rs (MIT, © Kai Zhang).

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Citation

If you use anndataoom in published research, please cite:

@software{omicverse,
  title  = {OmicVerse: A framework for multi-omic data analysis},
  author = {Zeng, Z. et al.},
  url    = {https://github.com/Starlitnightly/omicverse},
  year   = {2024},
}

@software{anndata_rs,
  title  = {anndata-rs: Rust implementation of AnnData},
  author = {Zhang, Kai},
  url    = {https://github.com/scverse/anndata-rs},
  year   = {2022},
}