clawhdf5/crates/clawhdf5-remote/README.md

clawhdf5-remote

Read HDF5 files where they live — on an HTTP(S) server or in an object store (S3, GCS, Azure) — with clawhdf5, without downloading them first.

let file = clawhdf5_remote::open_url("http://127.0.0.1:8000/data.h5")?;
let temps = file.dataset("/grid/temperature")?.read_f64()?;

The result is an ordinary clawhdf5::File: groups, datasets, attributes, selections, variable-length data. Only the bytes an operation needs are fetched, by Range requests, through a block cache.

How it reads

• Block cache (BlockCache, mandatory for remote files; see docs/design/range-reads.md §2): aligned blocks of 1 MiB by default, LRU with a byte budget (64 MiB by default), the missing blocks of one read fetched as runs of consecutive blocks (a gap of one block is fetched to merge two runs), each request at most 8 MiB, the requests of one read in parallel. A read that misses more than half the budget is not kept, so a big dataset read does not evict the metadata. Readers on several threads share one cache; a block is never fetched twice at once — a second reader waits for the first one's request.
• Opening costs one request: a GET of the first block, whose Content-Range gives the file's length. HDF5 files keep the superblock and usually the root group's metadata there, so listing a small file often needs nothing more.
• Pinned to one version of the file: a strong ETag is sent back as If-Match (else Last-Modified as If-Unmodified-Since) and checked on every response, as is the length. A file replaced while open is an error (RemoteError::FileChanged), never a mix of old and new bytes. A server with neither validator can only be checked by length; HttpOptions::require_validator refuses it.
• Servers that ignore Range (answer 200 with the whole file) are refused (RemoteError::RangeNotSupported) without reading the body, unless HttpOptions::allow_full_download is set; then the file is downloaded once and read from memory.
• Retries: connection failures, timeouts, 408/429/5xx and bodies that end early are retried with exponential backoff (3 retries, from 200 ms). Bodies are requested with Accept-Encoding: identity; an encoded body is refused.

The zero-copy methods of clawhdf5 (read_raw_ref, read_*_zerocopy, File::as_bytes) borrow the whole file from memory, so they are errors (as_bytes a panic; use File::contiguous_bytes) on a remote file.

Features

Feature	What	C code
http (default)	http:// through ureq, no TLS	none
https	https:// through rustls, ring provider, Mozilla roots	ring (C and assembly)
object-store	ObjectStoreStorage and open_object over any object_store store (in-memory, local files, or one you configure)	none
s3, gcs, azure	s3://bucket/key, gs://bucket/key, az://container/key in open_url, configured from the environment (AWS_*, GOOGLE_*, AZURE_*) as object_store's from_env builders read it	aws-lc-rs (object_store's cloud clients)

The default build and object-store compile no C (scripts/ci-test.sh checks both).

Object stores

object_store is async; Storage is synchronous (parsing is CPU work). ObjectStoreStorage owns a small tokio runtime (two worker threads) and blocks the calling thread on it for each read, so it is read from ordinary threads, several at once. From inside an async runtime it refuses (RemoteError::Usage) rather than block a worker: read in tokio::task::spawn_blocking. The object is pinned by its ETag (If-Match, and compared on every response), else its version or modification time, and its size. The ranges of one read are fetched concurrently (up to 8).

use std::sync::Arc;
use clawhdf5_remote::object_store::{memory::InMemory, ObjectStore};
let store: Arc<dyn ObjectStore> = Arc::new(InMemory::new());
// ... put a file at "data.h5" ...
let (file, cache) = clawhdf5_remote::open_object(store, "data.h5", &Default::default())?;

The tests use object_store's in-memory and local-file stores; no cloud account is needed. The cloud schemes are only built (and unit-tested for URL parsing) in CI, not run against a real bucket.

Counting requests

use clawhdf5_remote::{storage_for_url, Options};
let storage = storage_for_url(url, &Options::default())?;
let file = clawhdf5::File::open_storage(storage.clone())?;
// ... read ...
let s = storage.stats(); // requests, bytes_fetched, hits, misses, cached_bytes, ...

cargo run -p clawhdf5-remote --example range_server -- DIR serves a directory with range support (the server the tests use), and cargo run -p clawhdf5-remote --example read_url -- URL [DATASET] lists a file and prints what it cost.