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Add high level filesystem documention
From nextcloud/server#26982 Adds some documentation for the filesystem layer, both a high level overview of how the various pieces interact and a high level guide for apps interacting with the filesystem. Hopefully this will be useful to anyone trying to either use the filesystem or work on the filesystem. Signed-off-by: Louis Chemineau <louis@chmn.me>
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‎developer_manual/basics/storage/filesystem.rst‎

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==========
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Filesystem
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Nextcloud filesystem API
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==========
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.. sectionauthor:: Bernhard Posselt <dev@bernhard-posselt.com>
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Because users can choose their storage backend, the filesystem should be accessed by using the appropriate filesystem classes.
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High level guide to using the Nextcloud filesystem API.
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Because users can choose their storage backend, the filesystem should be accessed by using the appropriate filesystem classes. For a simplified filesystem for app specific data see `IAppData <appdata.html>`_
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Node API
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^^^^^^^^
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The "Node API" is the primary api for apps to access the Nextcloud filesystem, each item in the filesystem is
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represented as either a File or Folder node with each node providing access to the relevant filesystem information
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and actions for the node.
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Getting access
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--------------
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Access to the filesystem is provided by the ``IRootFolder`` which can be injected into your class.
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From the root folder you can either access a user's home folder or access a file or folder by its absolute path.
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.. code-block:: php
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use OCP\Files\IRootFolder;
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use OCP\IUserSession;
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class FileSystemAccessExample {
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private IUserSession $userSession;
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private IRootFolder $rootFolder;
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public function __constructor(IUserSession $userSession, IRootFolder $rootFolder) {
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$this->userSession = $userSession;
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$this->rootFolder = $rootFolder;
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}
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/**
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* Create a new file with specified content in the home folder of the current user
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* returning the size of the resulting file.
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*/
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public function getCurrentUserFolder(string $path, string $content): int {
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$user = $this->userSession->getUser();
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if ($user === null) {
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return null;
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}
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// the "user folder" corresponds to the root of the user visible files
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return $this->rootFolder->getUserFolder($user->getUID());
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}
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}
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For more details on the specific methods provided by file and folder nodes see the method documentation from the ``OCP\Files\File`` and ``OCP\Files\Folder`` interfaces.
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Filesystem classes can be injected automatically with dependency injection. This is the user filesystem.
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For a simplified filestystem for app specific data see `IAppData <appdata.html>`_
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Writing to a file
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-----------------
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All methods return a Folder object on which files and folders can be accessed, or filesystem operations can be performed relatively to their root. For instance for writing to file:`nextcloud/data/myfile.txt` you should get the root folder and use:
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.. code-block:: php
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<?php
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namespace OCA\MyApp\Storage;
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use OCP\Files\IRootFolder;
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class AuthorStorage {
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class FileWritingExample {
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/** @var IRootStorage */
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private $storage;
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private IRootStorage $storage;
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public function __construct(IRootFolder $storage){
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$this->storage = $storage;
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}
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public function writeTxt($content) {
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public function writeContentToFile($content) {
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$userFolder = $this->storage->getUserFolder('myUser');
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}
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}
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Reading from a file
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-------------------
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Files and folders can also be accessed by id, by calling the **getById** method on the folder.
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.. code-block:: php
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<?php
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namespace OCA\MyApp\Storage;
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use OCP\Files\IRootFolder;
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class AuthorStorage {
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class FileReadingExample {
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/** @var IRootFolder */
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private $storage;
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private IRootFolder $storage;
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public function __construct(IRootFolder $storage){
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$this->storage = $storage;
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// check if file exists and read from it if possible
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try {
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$file = $userFolder->getById($id);
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if($file instanceof \OCP\Files\File) {
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if ($file instanceof \OCP\Files\File) {
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return $file->getContent();
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} else {
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throw new StorageException('Can not read from folder');
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}
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}
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}
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Direct storage access
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---------------------
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While it should be generally avoided in favor of the higher level apis,
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sometimes an app needs to talk directly to the storage implementation of it's metadata cache.
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You can get access to the underlying storage of a file or folder by calling ``getStorage`` on the node or first getting
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the mountpoint by calling ``getMountPoint`` and getting the storage from there.
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Once you have the storage instance you can use the storage api from ``OCP\Files\Storage\IStorage``, note however that
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all paths used in the storage api are internal to the storage, the ``IMountPoint`` returned from ``getMountPoint`` provides
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methods for translating between absolute filesystem paths and internal storage paths.
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If you need to query the cached metadata directory you can get the ``OCP\Files\Cache\ICache`` from the storage by calling ``getCache`.
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Implementing a storage
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----------------------
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The recommended way for implementing a storage backend is by sub-classing ``OC\Files\Storage\Common`` which provides
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fallback implementations for various methods, reducing the amount of work required to implement the full storage api.
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Note however that various of these fallback implementations are likely to be significantly less efficient than an
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implementation of the method optimized for the abilities of the storage backend.
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Adding mounts to the filesystem
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-------------------------------
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The recommended way of adding your own mounts to the filesystem from an app is implementing ``OCP\Files\Config\IMountProvider``
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and registering the provider using ``OCP\Files\Config\IMountProviderCollection::registerProvider``.
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Once registered, your provider will be called every time the filesystem is being setup for a user and your mount provider
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can return a list of mounts to add for that user.

‎developer_manual/core/architecture/files.rst‎

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============
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Nextcloud filesystem API
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============
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High level overview
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-------------------
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The Nextcloud filesystem is roughly based on the unix filesystem, consisting of multiple storages
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mounted at various locations.
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.. code-block:: txt
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┌──────────────────────────────────┐
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│Code wanting to use the filesystem│
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└─────────┬─────────────────────┬──┘
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│ │
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│ │
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┌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┐
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╎Filesystem │ │ ╎
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╎layer │new │legacy ╎
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╎ │ │ ╎
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╎ ▼ ▼ ╎
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╎ ┌────────┐ Partly build on ┌─┴──────┐ ╎
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╎ │Node API├─────────────────►│View API│ ╎
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╎ └───────┬┘ └─┬──────┘ ╎
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╎ │ │ ╎
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└╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┘
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│ │
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┌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┐
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╎Storage layer │ │ ╎
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╎ ├─────────────────────┤ ╎
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╎ │ │ ╎
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╎ ▼ ▼ ╎
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╎ ┌───────┐ ┌───────┐ ┌──────┐ ╎
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╎ │Storage│═══>│Scanner│═══>│Cache │ ╎
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╎ └───────┘ └───────┘ └──────┘ ╎
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╎ ╎
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╎ ╎
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└╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┘
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Filesystem layer
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^^^^^^^^^^^^^^^^
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Any code that wants to use the filesystem has two API options to use, the new ``Node`` api and the old ``View`` api.
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New code should preferably use the ``Node`` api as it allows building systems with less overhead than the old api.
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Besides the filesystem apis, this layer also manages the available mounts, containing the logic to allow apps
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to setup their mounts and translating filesystem paths into a mountpoint + "internal" path.
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### Storage layer
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The storage implementation handles the details of communicating with the filesystem or remote storage api
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and provide a uniform api for Nextcloud to use the storage.
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For each storage a metadata cache/index is maintained to allow reading metadata of the storage without having
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to talk to the (potentially) slow storage backend. The scanner is responsible for updating the cache with
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information from the storage backend.
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## Storage/Cache wrappers
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To allow apps to customize the behavior of a storage without requiring the app to implement this for every
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possible storage backend, a ``Wrapper`` system is used.
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A ``Wrapper`` encapsulates an inner storage and allows overwriting any method to customize its behavior, with
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all other methods being passed through to the inner storage.
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Generally search storage wrapper has an equivalent cache wrapper encapsulating the cache of the inner storage
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to provide the same behavior modifications when reading metadata from the cache.
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Wrappers can be layered to stack the behavior of the wrappers, for example the ``groupfolders`` app works by
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stacking a wrapper to provide access to a single folder on the root storage with a wrapper to limit the permissions
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of the storage.
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.. code-block:: txt
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┌───────────────┐ ┌────────────────────┐
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│PermissionsMask├─────►│CachePermissionsMask│ PermissionsMask applies a mask to the permissions of a storage
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└───────┬───────┘ └─────────┬──────────┘ to provide less-privilaged access to a storage
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│ │
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▼ ▼
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┌───────────────┐ ┌────────────────────┐
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│Jail ├─────►│CacheJail │ Jail restricts access to a file or folder of a storage providing
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└───────┬───────┘ └─────────┬──────────┘ a limited view into the storage (think unix chroot or bind mount)
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│ │
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▼ ▼
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┌───────────────┐ ┌────────────────────┐
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│Base Storage ├─────►│Base Cache │
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└───────────────┘ └────────────────────┘
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Code Map
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--------
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Approximate overview of the significant filesystem code
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AppData
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~~~~~~~
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High level api for accessing "appdata" folders, based on the ``Node`` API
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Cache
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~~~~~
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- ``Cache`` implementation
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- Cache wrappers
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- Scanner and cache update logic
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- Search infrastructure
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Mount
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~~~~~
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Mountpoint management and setup
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Node
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~~~~
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``Node`` filesystem api implementation
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ObjectStorage
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~~~~~~~~~~~~~
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Implementation of the various supported object store storage backends
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SimpleFS
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~~~~~~~~
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Simplified version of the Node api, for providing a more limited api for some filesystem bits
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Storage
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~~~~~~~
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Implementation of various storage backends and wrappers
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Streams
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~~~~~~~
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Various low-level php stream wrapper used in storage implementations
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Type
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~~~~
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Mimetype management and detection
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View.php
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~~~~~~~~
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Legacy View api

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