I'm investigating creating a memcached-based FilesCache implementation for use with Google App Engine. The basic obstacle is that this requires that all objects that are used as keys or values must be serializable. Before I go too far down this path, I'd like to know if this is a reasonable thing to do (or consider doing). Any thoughts or guidance would be greatly appreciated.
Background info: the Google App Engine (GAE) environment is inherently distributed, where an unknown number of multiple instances of your servlet-based web application will be running at the same time. The GaeVFS project (http://gaevfs.appspot.com) implements a VFS plugin on top of the GAE datastore; this is needed because the "real" filesystem within GAE is read-only, so GaeVFS provides a writeable filesystem for use by applications. It seems that for proper operation of VFS, the FilesCache implementation used by GaeVFS should (must?) also be inherently distributed. Fortunately, GAE supplies a memcached API that will be perfect for this use (if I can solve the serialization problem). Here's my first cut at what classes I'd need to make serializable and some notes on which fields might be marked transient: *AbstractFileName* serialized fields: scheme (String), absPath (String), type (FileType) transient fields: uri, baseName, rootUri, extension, decodedPath *FileType* serialized fields: name (String), hasChildren (boolean), hasContent (boolean), hasAttrs (boolean) transient fields: none *AbstractFileObject* serialized fields: name (AbstractFileName), content (DefaultFileContent) transient fields: fs (AbstractFileSystem), operation (FileOperations), attached (boolean), type (FileType), parent (FileObject), children (FileName[]), objects (List) It's very important the the fs (AbstractFileSystem) field be transient to limit the number of other classes that need to be made serializable. It looks like files are always retrieved from the files cache via the AbstractFileSystem.getFileFromCache() method; if so then the "fs" field of AbstractFileObject can be restored within this method and doesn't need to be serialized. We'll need to define a package-scope AbstractFileObject.setFileSystem() method to support this. Or, this could be done within the MemcachedFilesCache.getFile() method before returning the FileObject (but then we'd need a FileObject.setFileSystem method, or do some not-very-nice type casting). *DefaultFileContent *serialized fields: file (AbstractFileObject), attrs (Map), roAttrs (Map), fileContentInfo (FileContentInfo), fileContentInfoFactory (FileContentInfoFactory), openStreams (int) transient fields: threadData (ThreadLocal) question: what types do the "attrs" and "roAttrs" maps contain? are these serializable? question: FileContentInfo and FileContentInforFactory are interfaces, what are the implementations of these?
