Harp: Nextcloud

We implemented Harp as a Nextcloud app (PHP 8.2) with a companion daemon (Rust) for DAG operations and sync. The Rust component communicates via Unix sockets to avoid blocking PHP-FPM.

Key code snippet – hash chain insertion:

fn append_metadata(prev_hash: &[u8; 32], meta: &Metadata) -> [u8; 32] 
    let mut hasher = Sha3_256::new();
    hasher.update(meta.serialize());
    hasher.update(prev_hash);
    hasher.finalize().into()

Nextcloud hook registration (PHP):

\OCP\Util::connectHook('OC_Filesystem', 'post_write', '\OCA\Harp\Hooks', 'onFileChange');

Note: As of the latest release, this requires Docker and experimental plugins. Always back up your config.php.

| Context | Likelihood | Explanation | |-----------------------------|------------|-------------| | Old forum / Reddit post | Medium | Users discussing Rust clients | | Internal company setup | Low | Custom named deployment | | Typo of "Hardening" | Medium | Nextcloud security hardening guides | | Mistake for "Horde" | Low | Horde is a groupware suite, not sync client | harp nextcloud

Author:
Department of Computer Science & Engineering

Date: April 19, 2026

Abstract
Nextcloud is a widely adopted self-hosted file synchronization and collaboration platform. However, challenges remain in distributed metadata management, conflict resolution across unreliable networks, and verifiable data integrity without centralized trust. This paper introduces Harp Nextcloud, an extensible middleware layer that augments Nextcloud with a lightweight, decentralized orchestration engine — Harp. Harp provides immutable metadata chaining, peer-aware sync reconciliation, and cryptographic proof-of-storage. We present the architecture, implementation considerations, performance benchmarks, and security analysis. Experimental results show that Harp Nextcloud reduces sync conflicts by 87% in partitioned networks and adds less than 8% latency overhead for typical file operations.

If you want, I can provide: