The Storage package needs a storage interface that can have Readable, Modifiable and other storage types plugged into it.

This would allow for behaviour like so:

$storage = new Storage([
    'create' => new SomeCreatableStorage(),
    'read'   => new SomeReadableStorage(),
    'modify' => new SomeModifiableStorage()
]);

$storage->readFrom(new SomeOtherReadableStorage());
$storage->searchFrom(new SomeSearchableStorage());

if (!$storage->canRead()) {
    throw new \RuntimeException('Storage is not readable!');
}

Perhaps by default the storage could assume everything capable from a single source, too:

$storage = new Storage(new DatabaseStorage($connection));

Sources could then be overridden afterwards if need be.

Maybe this needs a different name too, this is basically like a MultiStorage adapter. Or maybe the source interfaces should be renamed.

Work this out into a perfect set of interfaces and this will be one hell of an expressive system.