Create Custom Storage Backends

The preferred way for applications to create new storage backends is to create a subclass of \OC\Files\Storage\Common and implement the abstract methods. It’s also possible to create storage backends by implementing the required interface.

However, by sub-classing the common backend a lot of the boiler plate is taken care of. What’s more, it provides common implementations and fallbacks to reduce the amount of work it is to create a storage backend.

Required Methods

All storage backends sub-classing the common storage backend must implement the following methods:

Method Description
mkdir($path) Creates a new folder on the storage.
rmdir($path) Deletes an existing folder on the storage.
opendir($path) Opens a directory handle.
stat($path) Retrieves the metadata for the file or folder. The returned array should, at least, contain mtime and size.
filetype($path) Returns the file type; either file or dir.
file_exists($path) Checks if a file or folder exists.
unlink($path) Removes a file or folder. This isn’t only for deleting files, unlike PHP’s unlink method.
fopen($path, $mode) Opens a file handle for a file
touch($path, $mtime = null) Updates the mtime of a file or folder. If $mtime is omitted the current time should be used.

Suggested Methods

The common storage backends provide fallback implementations for a number of methods to make them easier to implement. However, some of fallback implementations are either inefficient or don’t always provide the correct result for custom storage backends. Given that, please consider overriding one or more of the following methods:

Method Description
rename($sourcePath, $targetPath) Renames a file. The default implementation uses copy and unlink which is very inefficient.
copy($sourcePath, $targetPath) Copies a file. The default implementation copies using streams. This is inefficient for remote storages as it downloads and re-uploads the file.
isReadable($path) Checks if a file is readable. It defaults to true if the file exists.
isUpdatable($path) Checks if a file or folder can be updated. This includes being written to or renamed. It defaults to true if the file exists.
isCreatable($path) Checks if new files can be created in a folder It defaults to isUpdatable($path).
isDeletable($path) Checks if a file can be deleted. It defaults to isUpdatable($path).
isSharable($path) Checks if a file can be shared. It defaults to isReadable($path).
free_space($path) Checks the free space on the storage in bits.

Other Useful Methods

The default implementation for the following methods are good for most storage backends. But, providing an alternate implementation can improve user experience.

Method Description
file_put_contents($path, $data) Stores a file on the storage. It defaults to using fopen($path, 'w').
file_get_contents($path) Retrieves a file from storage. Defaults to using fopen($path, 'r').
getMimeType($path) Retrieves the mimetype of a file or folder. Defaults to guessing the mimetype from the extension. The mimetype of a folder is _required_ to be 'httpd/unix-directory'.
hasUpdated($path, $time) Checks if a file or folder has been updated since $time. If you’re certain the files on the storage will not be updated outside of ownCloud you can always return false to increase performance.
getETag($path) Retrieves the Etag for a file or folder.
verifyPath($path, $fileName) Checks if a filename is valid for the storage backend. It defaults to checking for invalid characters or names for the server platform.

Copying and Moving Between Storage Backends

When copying or moving files between different storages a stream copy is used by default. This works well for copying between different types of storages, such as from local to SMB. But, there are cases where a more efficient copy is possible, such as between two SMB storages on the same server. In these cases, storage backends can override the cross-storage behavior by overriding the following methods:

  • copyFromStorage(\OCP\Files\Storage $sourceStorage, $sourceInternalPath, $targetInternalPath, $preserveMtime = false);
  • moveFromStorage(\OCP\Files\Storage $sourceStorage, $sourceInternalPath ,$targetInternalPath);

Working With Streams

Both fopen() and opendir() require storage backends to return native PHP streams for maximum compatibility. ownCloud comes with several classes which make it easier for storage backends to create native PHP streams for backends not supported by PHP’s own streamWrapper.

IteratorDirectory

Icewind\Streams\IteratorDirectory allows for creating a directory handle from an array or iterator.

$fileNames = $this->getFolderContentsSomehow();
return IteratorDirectory::wrap($fileNames);

CallbackWrapper

Icewind\Streams\CallbackWrapper wraps an existing file handle, and allows for hooking into file reads and writes, and closing streams. The most common use case for this class in storage backends is for implementing fopen() with writable streams. This is because writing to and closing streams happens outside the storage implementation. As a result, the storage backend needs a way to upload the changed file back to the backend. This can be done by attaching a close-callback to a stream for a temporary file.

$tempFile = $this->downloadFile($path);
$handle = fopen($tempFile, $mode);
return CallBackWrapper::wrap($handle, null, null function() use ($path, $tempFile) {
    $this->uploadFile($tempFile, $path);
    unlink($tempFile);
}

Storage Wrappers

Besides implementing a complete custom storage backend, ownCloud allows for modifying the behavior of an existing storage by applying a wrapper to it. Storage wrappers need to implement the full storage API methods. Examples of storage wrappers are

  • The Quota wrapper. This changes the behavior of free_space by limiting the free space returned by the wrapped storage to a configured maximum
  • The Encryption wrapper. This encrypts and decrypts the data on the fly by overwriting file_put_contents, file_get_contents, and fopen.

When implementing a storage wrapper, the wrapped storage is available as $this->storage. Storage wrappers can either be applied globally to all used storages using \OC\Files\Filesystem::addStorageWrapper($name, $wrapper) or to a specific storage, while mounting the storage from the app. Implementing a storage wrapper is done by sub-classing \OC\Files\Storage\Wrapper\Wrapper and overwriting any of its methods.

Global Storage Wrappers

For using a storage wrapper globally, you provide a callback which will be called for each used storage. The callback can than determine if a wrapper should be applied to the given storage, based on the storage or mountpoint, or whether it needs to return the storage unwrapped.

Filesystem::addStorageWrapper('fooWrapper', function($mountPoint, $storage) {
    if ($storage->instanceOfStorage('FooStorage')) {
        return new FooWrapper(['storage' => $storage]);
    } else {
        return $storage;
    }
}

Wrappers for a Single Storage

Sometimes an app can avoid having to create a custom storage backend by instead modifying the behavior of an existing one. ownCloud comes with a few generic storage wrappers which might be useful when doing so, which include PermissionsMask and Jail.

PermissionsMask

\OC\Files\Storage\Wrapper\PermissionsMask can be used to restrict the permissions on an existing storage. A sample use case is to create a read-only ftp backend.

$storage = $this->createStorageToWrapSomehow();
return new PermissionsMask([
   'storage' => $storage,
   'mask' => \OCP\Constant::PERMISSION_READ | \OCP\Constant::PERMISSION_SHARE
]);

Jail

\OC\Files\Storage\Wrapper\Jail can be used to limit storage interaction to a sub-folder of an existing storage.

$storage = $this->createStorageToWrapSomehow();
return new Jail([
   'storage' => $storage,
   'root' => 'some/folder/in/the/storage'
]);

A Note on instanceof()

Since storage wrappers wrap an existing storage instead of sub-classing it, it is not possible to determine if the storage is a specific class using PHP’s instanceof operator. Instead, you need to call the instanceOfStorage() method on the class with the fully-qualified class name.

// Only works if no wrappers are applied
if ($storage instanceof \OC\Files\Storage\DAV) {
    // ...
}

// Works regardless of any wrapper
if ($storage->instanceOfStorage('\OC\Files\Storage\DAV')) {
    // ...
}

instanceOfStorage() can also be used to check if a certain wrapper is applied to a storage.

Mounting Storages

For an app to add its storages to the filesystem it should implement a mount provider and register it with the filesystem. Implementing mount providers is done by implementing the \OCP\Files\Config\IMountProvider interface, containing the getMountsForUser(IUser $user, IStorageFactory $storageFactory) method, which returns a list of mountpoints that should be created for a user.

class MyMountProvider implements IMountProvider {
    public function getMountsForUser(IUser $user, IStorageFactory $loader) {
        $config = magicallyGetMountConfigurations();
        return array_map(function($mountOptions) use ($loader) {
            return new Mount(
                $mountOptions['class'],
                $mountOptions['mountPoint'],
                $mountOptions['storageOptions'],
                $loader
            );
        }, $config);
    }
}

Registering a mount provider should be done from an app’s appinfo/app.php. Note that any mount provider registered after the filesystem is setup for a user will not be called again for that user.

$provider = new MyMountProvider();
\OC::$server->getMountProviderCollection()
            ->registerProvider($provider);
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