vault-link/frontend/sync-client/README.md

Sync client (API v5)

The engine uses one serialized loop per vault and requires Web Crypto (SHA-256). Local notifications and WebSocket messages wake it; complete disk scans and the durable server event log are the sources of truth. Periodic reconciliation is opt-in: set syncIntervalMs to a positive interval to repair missed notifications. Unset or 0 disables polling. Create, delete and move notifications persist logical identity changes before resolving, including while offline. Shutdown waits for those saves. Content-update notifications only wake reconciliation; scans read the latest bytes, including after restart. Notifications describe logical external changes: adapters suppress self-generated create/delete/move notifications, and report atomic editor saves as content updates. This distinguishes a deliberate delete/recreate from an edit. Use waitUntilFinished() to await the active attempt; errors leave pending requests saved and are reported to the caller. Transient failures retry; authentication and protocol errors wait for an explicit wake or settings change.

State and policy

The persisted state contains the incorporated server file manifest, the current local identity map, and the exact pending submission. A received server file manifest becomes the merge base when its application metadata is saved. Successful submissions advance the base to the submitted content snapshot, preserving later local edits. Unknown outcomes are retried unchanged before incorporating newer remote state. HTTP is the sole source of remote events; WebSocket messages only wake the loop. Contiguous event pages are folded in memory into their final manifest and content heads before merging. An interrupted fetch restarts from the last incorporated cursor; pagination progress is not persisted. Intermediate changes that the server has already reverted must not overwrite local edits.

A permanent upload rejection is recorded in metadata. An unchanged rejected payload is skipped so other files can sync; the failure remains visible in history and waitUntilFinished(). Editing it, resetting the client, or changing settings permits a fresh attempt. Earlier attempts that committed despite a lost reply are incorporated through normal event replay.

For each document ID (absence means deletion), use the server path if local is unchanged, use local if the server is unchanged, and use the server when both changed. Then resolve cross-document path conflicts: a retained server path wins, UUID ordering breaks other ties, and displaced documents receive deterministic (conflict UUID) names. This also handles file/ancestor and portable-name aliases. Nonportable local names are repaired client-side before a file manifest is published. Allocated filename components fit within 255 UTF-8 bytes, including conflict suffixes. The protocol does not impose a universal total-path byte limit; filesystem-specific limits surface from the adapter instead of silently flattening a name. Directories are implicit; empty directories are not synced.

• Content is uploaded before new membership; the client chooses and persists UUIDs.
• Text uses three-way reconcile. First contact at a matching path adopts the remote ID and reconciles differing text with an empty parent.
• Concurrent binary/unmergeable edits keep the server version. If only one side changed, keep that side. No local backup archive is created.
• Deletion follows the file manifest decision, including when the file has local edits.
• Notified moves preserve IDs. Offline inference requires a unique nonempty hash match on both sides; uncertain cases become delete/create.
• Ignored and oversized files preserve their disk paths and remote membership. The original manifest base is retained while excluded, so becoming eligible resumes reconciliation without losing local renames. Not-yet-downloaded files are not mistaken for local deletions.
• Bytes and line endings are preserved. File modification times are not tracked or synchronized. Optional editor cursors are repositioned when text is reconciled.

Restored server histories

Successful HTTP responses carry an X-Vault-Link-History checkpoint. The client persists it before consuming the response and supplies it on subsequent reads and writes. WebSocket events wake HTTP replay, which validates that checkpoint. A restored database cannot silently reuse a version number: a missing or different event incarnation returns a history mismatch before executing the request.

On mismatch the client scans local work and records a reset before clearing the old checkpoint. It clears old requests, receipts, cursors, bases and diff-cache entries. Clean files reconcile against the restored snapshot. Unsent edits with an old base become separate local documents; path conflicts preserve them under conflict names instead of applying a diff to an unrelated restored version. Recovery resumes after interrupted state saves. Existing API v4 client state without a checkpoint takes this same conservative bootstrap on upgrade.

Status observers cannot stop the background engine by throwing or rejecting. Their failures are reported, and remaining observers continue receiving updates.

Required adapter contract

FileSystemOperations and PersistenceProvider define the adapter operations. The Obsidian and CLI adapters still need to adopt the current core interface. The sync client does not coordinate separate processes accessing the same vault.

The filesystem adapter must provide complete scans, coherent snapshots, recursive directory creation, exclusive file creation, no-replace renames, and deletion of individual regular files. Directory pruning uses only rmdir, never recursive file removal. Reject links, special files and paths escaping the vault. An unreadable file or directory must fail the scan rather than look like a deletion. Optional editor cursor metadata is applied with file content.

User-file writes have no power-loss durability requirement. There is no filesystem journal, hidden staging area, replay, backup archive, or flush protocol. Swaps and cycles temporarily move occupied files to visible conflict names; a fresh scan can discover them if application stops partway through. Partial writes and edits made before, during or after an interruption are treated as current disk content. The next sync reconciles that content; it does not finish a saved filesystem plan. Power loss may lose bytes or rename identity and may cause a merge to run again.

Metadata is the one atomic boundary: persistence save replaces a complete value and an interrupted/failed save leaves an old or new complete value, never a torn object. A failed save is reloaded before retrying. No ordering between metadata and user-file durability is required. Settings and engine saves are serialized. Keep metadata outside the user-file namespace or in .vault-link-sync, which is excluded from scans. Changing a client's vault once it holds identities, offline notifications, pending requests or bootstrap state requires a separate state store.

Filesystem application saves metadata once when it finishes. Namespace changes detected before mutations invalidate the plan. Changes during application are incorporated between files using their document identities; the next scan rereads content. Content is reread before replacement. Unexpected eligible occupants receive visible conflict names; ignored and oversized occupants stay in place. External writers can still change files between observations, so these checks do not promise transactional file contents or perfect identity reconstruction. There is no automated v2/v3 state migration.