hashserver 1.0

Simple FastAPI-based hash server

Hashserver

A lightweight, content-addressed file server over HTTP.

Hashserver stores and serves opaque binary buffers keyed by their cryptographic checksum. You PUT a buffer with its checksum in the URL; you GET it back by the same checksum. There are no filenames, no directories, no metadata — just content and its hash.

The hash algorithm is configurable: SHA-256 (default) or SHA3-256.

Why content-addressed storage?

Content-addressed storage (CAS) is a well-established pattern used by Git, IPFS, Docker registries, and many other systems. Identifying data by its cryptographic hash gives you automatic deduplication, trivially verifiable integrity, and strong reproducibility guarantees.

Hashserver brings these benefits to any project that needs a simple HTTP-based buffer store. It is intentionally minimal: a single ASGI application backed by a directory of files, designed to be easy to deploy, easy to integrate, and easy to reason about.

Relationship to Seamless

Hashserver was originally developed as the buffer-serving component of Seamless, a framework for reproducible, reactive computational workflows. In Seamless, all data — inputs, source code, and results — is represented as a tree of checksums, and hashserver provides the storage layer that maps those checksums back to actual data.

However, hashserver has no dependency on Seamless and no knowledge of it. It is a generic content-addressed file server that is useful in any context where you need to store and retrieve buffers by hash — caching layers, artifact stores, reproducible pipelines, or your own CAS-backed application. It is published as an independent PyPI package for exactly this reason.

Features

• Content-addressed: buffers are stored and retrieved by their cryptographic checksum.
• Configurable hash algorithm: SHA-256 (default) or SHA3-256, selected at startup.
• Integrity-verified reads: every buffer is re-checksummed on GET to detect corruption.
• Prefix directory layout: by default, buffers are stored under a two-character prefix subdirectory (e.g. ab/ab3f7c...) to avoid filesystem performance problems with large flat directories. A flat layout is also supported.
• Extra read-only directories: additional buffer directories can be mounted as fallback read sources.
• Promises: a client can announce that a buffer will be uploaded soon via PUT /promise/{checksum}. Other clients reading that checksum will wait for the upload rather than getting a 404.
• Concurrent-safe: in-flight PUT requests are tracked so concurrent GETs and batch queries return consistent results. Lock files are respected for external writers.
• Multiple instances: several hashserver processes can safely share the same buffer directory.
• Lightweight: built on FastAPI/Starlette — no database, no external services.
• Flexible deployment: run as a CLI tool, under any ASGI server, or via Docker Compose.

Installation

pip install hashserver

Or with conda:

mamba env create --file environment.yml
conda activate hashserver

Quick start

Serve buffers from a local directory:

hashserver ./my-buffers

This starts the server under uvicorn on port 8000. Run hashserver -h for all options.

Storing and retrieving a buffer

# Start a writable server
hashserver ./my-buffers --writable

# Compute the SHA-256 checksum and upload
CHECKSUM=$(python3 -c "
import hashlib, sys
print(hashlib.sha256(open(sys.argv[1],'rb').read()).hexdigest())
" myfile.bin)
curl -X PUT --data-binary @myfile.bin http://localhost:8000/$CHECKSUM

# Download
curl -O http://localhost:8000/$CHECKSUM

To use SHA3-256 instead, start the server with --hash-algorithm sha3-256 and hash your files with hashlib.sha3_256.

API

Retrieving buffers

GET /{checksum} — Retrieve a buffer by its hex checksum. The server verifies the checksum before sending the response. Returns the raw buffer (200), or 404 if not found.

Storing buffers

Requires --writable.

PUT /{checksum} — Upload a buffer. The request body is the raw data; the server verifies that its checksum matches the URL. Returns 200 on success, 201 if the buffer already existed, or 400 on checksum mismatch.

PUT /promise/{checksum} — Announce that a buffer will be uploaded soon. Returns 202 with the promise TTL. While a promise is active, GET requests for that checksum will wait rather than returning 404, and /has queries will report the checksum as present.

Querying availability

GET /has — Batch existence check. Send a JSON list of checksums in the request body. Returns a JSON list of booleans. Includes both on-disk buffers and active promises.

GET /has-now — Same as /has, but excludes promises — only reports buffers that are already on disk.

GET /buffer-length — Batch size query. Send a JSON list of checksums in the request body. Returns a JSON list of integers: the buffer size in bytes, or 0 if not present. Promised checksums are reported as true.

Health

GET /healthcheck — Returns "OK". Useful for load balancer probes.

Configuration

CLI flags

Flag	Description	Default
directory	Buffer storage directory (positional, required)	—
--writable	Enable PUT endpoints	off
--hash-algorithm	Hash algorithm: sha3-256 or sha-256	sha-256
--layout	Directory layout: prefix or flat	prefix
--extra-dirs	Semicolon-separated list of extra read-only buffer directories	—
--host	Listen address	127.0.0.1
--port	Listen port	8000
--port-range START END	Pick a random free port in range (mutually exclusive with --port)	—
--status-file	JSON file for reporting server status	—
--timeout	Shut down after this many seconds of inactivity	—

Environment variables

When running under an external ASGI server (e.g. uvicorn hashserver:app), configure via environment variables instead:

Variable	Equivalent flag
HASHSERVER_DIRECTORY	directory
HASHSERVER_WRITABLE	--writable (set to 1 or true)
HASHSERVER_HASH_ALGORITHM	--hash-algorithm
HASHSERVER_LAYOUT	--layout
HASHSERVER_EXTRA_DIRS	--extra-dirs

Docker Compose

export HASHSERVER_PORT=8000
export HASHSERVER_HOST=0.0.0.0
export HASHSERVER_DIRECTORY=./buffers
export HASHSERVER_WRITABLE=1
docker compose up -d

Container user/group ID can be set with HASHSERVER_USER_ID and HASHSERVER_GROUP_ID (both default to 0).

Directory layouts

In prefix layout (the default), a buffer with checksum ab3f7c... is stored as <directory>/ab/ab3f7c.... A sentinel file .HASHSERVER_PREFIX is written to the directory. This avoids performance issues when storing large numbers of buffers.

In flat layout, the same buffer is stored as <directory>/ab3f7c....

Extra directories auto-detect their layout by checking for the .HASHSERVER_PREFIX sentinel.

Running tests

pip install requests
pytest tests/

License

See LICENSE.txt.