WebDavSync

Careful two-way sync between a Windows folder and a WebDAV cloud.
A small tray app and a command-line tool, built to replace the discontinued Mail.ru Cloud desktop client.

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Why

Most WebDAV clients either mount the cloud as a drive or copy files one way. WebDavSync keeps a real local folder in sync with a cloud folder in both directions, like the desktop clients of the big clouds do — but it works with any WebDAV server and puts safety first: it never "syncs the emptiness", never hard-deletes your files, and asks before anything destructive.

It is used daily on a 415 GB / 43 700-file folder synced with Mail.ru Cloud.

Features

• 🔁 True two-way sync — a three-way merge of local state, cloud state and the last synced state (the base). Edits, deletions, moves and renames on either side are carried over; conflicting edits keep both versions.
• 🛡️ Safety first — mass-delete guard, root-folder markers, deletions only of unchanged items, local deletes go to the Recycle Bin, no deletions at all during the first sync. See Safety guarantees.
• ✅ You stay in control — four modes from DryRun to Auto; an approvals window with checkboxes; "approve and remember" rules; rejected changes are held until the file changes again.
• 🖥️ Tray app — a setup wizard, status icon, sync now, pause, approvals, folder selection; plus a CLI for advanced use.
• ⚡ Efficient — local changes re-list only the touched cloud folders; full cloud checks run on a schedule you choose; parallel listings; start-up trusts a fresh state instead of re-crawling.
• 📦 Large files — interrupted downloads resume with HTTP Range; dropped transfers are retried.
• 📁 Selective sync — choose which top-level folders are synced; excluding never deletes anything.
• 🔐 Portable, passwords outside — one .exe and a data folder; passwords live in the Windows Credential Manager.
• 🪶 Light on the machine — background (low-priority) mode, optional CPU cap, hashing speed limit, and an optional rolling verify that re-hashes your files bit by bit to catch silent changes.
• ☁️ Mail.ru ready — imports the state of the old Mail.ru client (.cloud.db), so an existing folder is picked up without re-downloading anything; Mail.ru's content hashes are used for exact comparisons.

How it works

flowchart LR
    W[Folder watcher] -->|changes| P
    T[Timer: cloud check] --> P
    U[Sync now] --> P
    subgraph Cycle
        P[Scan local folder<br>+ list cloud] --> M[Three-way plan<br>local · cloud · base]
        M --> G{Safety guards}
        G -->|safe or approved| E[Execute<br>verify every step]
        G -->|needs a human| A[Approvals window]
        A -->|approve| E
        A -->|reject| H[Held list]
    end
    E --> B[(Base in state.db)]

Loading

Every cycle scans the local folder (size, time and file id; content hashes only where needed), lists the cloud (fully, or only the folders that changed), and compares both with the base. The result is a plan: uploads, downloads, moves, deletions, conflicts. Each operation re-checks both sides right before it runs, and the base is updated only after the result is verified on the target side.

Safety guarantees

1. A failed, partial or timed-out cloud listing is never read as "files were deleted".
2. An item is deleted on one side only if it is unchanged there since the last sync.
3. Local deletions and overwrites go to the Recycle Bin, never a hard delete.
4. The first sync of a folder is a merge only — nothing is deleted on either side.
5. Mass-delete / mass-overwrite guard in every mode, including Auto.
6. Both roots carry a marker (.webdavsync/pair.json): a missing or foreign root stops the pair instead of "syncing the emptiness". (That is the .webdavsync folder you see in the cloud.)
7. Downloads go to a temporary file and are moved into place atomically.
8. Excluding a folder never deletes anything; including it again merges without deletions.
9. A file whose content changed while its size and time did not (an edit that kept the time, or disk corruption) always needs your approval.

Requirements

• Windows 10 or 11 (x64)
• .NET 10 Desktop Runtime
• A WebDAV account. For Mail.ru Cloud: an app password (Mail.ru account → Security → Passwords for external applications), endpoint https://webdav.cloud.mail.ru/. Give the app password full access ("Mail, Cloud and Calendar"): then the whole cloud is read in one request and cloud changes are noticed within a minute. A WebDAV-only app password works too, just slower (the cloud is read folder by folder and checked every 15 minutes).

Install

1. Download WebDavSync-<version>-win-x64.zip from Releases.
2. Unpack it into a writable folder, e.g. D:\Apps\WebDavSync (not Program Files).
3. Run WebDavSync.exe.

The folder is portable: settings and logs go to a data\ folder next to the program. A new version unpacked into another folder takes over the settings of the copy that ran last on its first start (and moves Start with Windows to itself); passwords and sync state are not in the app folder, so nothing has to be set up again.

Getting started

On the first start a short wizard opens:

1. Connect your cloud — choose Mail.ru Cloud (or another WebDAV server), enter your login and an app password. The wizard checks the connection right away.
2. Choose the folders — a folder on this computer and a folder in the cloud (Browse… shows the cloud's folders). The wizard explains what will happen:
   • a folder used by the old Mail.ru Cloud app is taken over — only the differences are synced;
   • a folder synced by WebDavSync before (e.g. after moving the app) reuses its state;
   • files already in the folder are merged with the cloud — nothing is deleted on either side.
3. How careful should it be? — ask before every change (recommended at the start), ask only before deleting, overwriting or moving, or sync everything by itself; and whether to start with Windows.

Then the cloud icon appears next to the clock. The first check of the cloud takes a few minutes; after that, changes are picked up within seconds. In the careful mode, click the icon to see and approve the changes.

More folders: Add a folder… in the tray menu. Only one copy of the app runs at a time.

Modes

Mode	What runs without asking
DryRun	Nothing — plans are only shown.
ApproveAll	Nothing — every plan waits for approval.
Guarded	New files and folders in both directions; deletions, overwrites, moves and conflicts wait.
Auto	Everything, except safety-guard trips and silent content changes.

The first sync of a pair always waits for approval, in any mode. Rules (wds rule add cloud --type Download --folder "Camera Uploads") approve matching operations automatically.

The tray app

Icon	Meaning
Blue cloud	Up to date, or quietly checking for changes
Pulsing cloud	Transferring files
Yellow badge	Something needs your approval, or some items failed
Red badge	Error (e.g. wrong password)
Grey cloud	Offline or paused

The menu starts with the version and its release date. Menu: Open folder · Open cloud in browser · Sync now · Pause (15 min, 1 h, until tomorrow, until resumed) · Approvals · Choose folders · Settings (mode, cloud check interval, local verify, hashing speed, password) · Performance (low priority, CPU limit) · Start with Windows · Updates (check now; check automatically every day, week, month or never — daily by default) · Open log folder · WebDavSync on GitHub.

New versions are found automatically: Update to X… in the menu shows what is new and installs it with one click (the download is verified against its SHA-256 checksum; settings and sync state are kept).

Command line (advanced)

The release contains only the tray app. The command-line tool wds.exe is built from source together with it (scripts\publish.ps1, see Build from source) and works on the same data\ folder; it is useful for scripting, diagnostics and the first cycles of a big folder.

Command	Purpose
wds probe --url <url> --user <login>	Check the account and detect server capabilities
wds pair add / pair list / pair show <pair>	Create and inspect pairs
wds pair set <pair> name=value …	Change settings, e.g. mode=Guarded remotePollInterval=1h
wds secret set <pair> / secret remove <pair>	Store or remove the password (Credential Manager)
wds plan <pair>	Show the plan without changing anything
wds sync <pair> --once	Run one cycle with console approval
wds folders <pair> [--exclude X] [--include X]	Choose the top-level folders to sync
wds held <pair> [--release <path>] [--all]	List or release held (rejected) operations
wds rule list|add|remove <pair>	Auto-approve rules
wds status <pair>	State of the pair
wds crawl <pair> [--compare]	Read the cloud side as a sync cycle does (read-only); Mail.ru: compare with a folder-by-folder WebDAV read

Run wds <command> --help for all options.

Settings

Settings live in data\settings.json; change them from the tray or with wds pair set. The most useful ones:

Setting	Default
mode	ApproveAll	See Modes
remotePollInterval	15m	How often the whole cloud is checked; local changes are picked up within seconds
remoteCheckInterval	1m	Mail.ru with a full-access app password: how often the cloud is asked whether anything changed (one small request)
useMailRuApi	true	Mail.ru: read the cloud with the Mail.ru API when the app password allows it; false = WebDAV only
maxOpsPerCycle	none	Execute at most N operations per cycle (handy for first runs)
massDeleteCount / massDeletePercent	50 / 5	Mass-delete guard thresholds
conflictWinner	Local	Which version keeps the original name in a conflict
localVerifyInterval	off	Rolling verify: re-hash every local file once per period
hashLimitMBps	unlimited	Read speed of hashing
maxRequestsPerSecond / maxParallelRequests	5 / 4	Load on the WebDAV server

The full list is in spec §15.5.

Where things are stored

What	Where
Settings, logs	<app folder>\data\
Passwords	Windows Credential Manager, WebDavSync/<pair id>
Sync state (base, journal, backups, partial downloads)	<synced folder>\.webdavsync\ (hidden, never synced)
Root marker in the cloud	<cloud folder>/.webdavsync/pair.json

Build from source

git clone https://github.com/gray0072/webdav-sync.git
cd webdav-sync
dotnet build
dotnet test --project tests/WebDavSync.Core.Tests
.\scripts\publish.ps1 -Output D:\Apps\WebDavSync     # portable build: WebDavSync.exe + wds.exe

Requires the .NET 10 SDK. Integration tests run against rclone serve webdav in Docker and are skipped without it. The design is described in detail in spec/; contributors start with AGENTS.md.

Status

Version 0.1 — in daily use, but young. Planned next: activity, conflicts and errors windows, a settings window, upload/download bandwidth limits, Windows toast notifications. Ideas and bug reports are welcome in Issues.

License

MIT © 2026 Sergey Ilyin