Persistence backend refactor

Factors lots of duplicate code out of persistence backends
Adds get_flows_for_book to all backends

Change-Id: I0434bd4931cd9274876f9e9c92909531f244bcac

6 files changed, 464 insertions, 873 deletions

diff --git a/taskflow/persistence/backends/impl_dir.py b/taskflow/persistence/backends/impl_dir.py
index 155ffe4..5ee244f 100644
--- a/taskflow/persistence/backends/impl_dir.py
+++ b/taskflow/persistence/backends/impl_dir.py
@@ -15,6 +15,7 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import contextlib
 import errno
 import os
 import shutil
@@ -23,24 +24,25 @@ from oslo_serialization import jsonutils
 import six
 
 from taskflow import exceptions as exc
-from taskflow import logging
-from taskflow.persistence import base
-from taskflow.persistence import logbook
+from taskflow.persistence import path_based
 from taskflow.utils import lock_utils
 from taskflow.utils import misc
 
-LOG = logging.getLogger(__name__)
 
+@contextlib.contextmanager
+def _storagefailure_wrapper():
+    try:
+        yield
+    except exc.TaskFlowException:
+        raise
+    except Exception as e:
+        if isinstance(e, (IOError, OSError)) and e.errno == errno.ENOENT:
+            raise exc.NotFound('Item not found: %s' % e.filename, e)
+        raise exc.StorageFailure("Storage backend internal error", e)
 
-class DirBackend(base.Backend):
-    """A directory and file based backend.
 
-    This backend writes logbooks, flow details, and atom details to a provided
-    base path on the local filesystem. It will create and store those objects
-    in three key directories (one for logbooks, one for flow details and one
-    for atom details). It creates those associated directories and then
-    creates files inside those directories that represent the contents of those
-    objects for later reading and writing.
+class DirBackend(path_based.PathBasedBackend):
+    """A directory and file based backend.
 
     This backend does *not* provide true transactional semantics. It does
     guarantee that there will be no interprocess race conditions when
@@ -54,17 +56,10 @@ class DirBackend(base.Backend):
     """
     def __init__(self, conf):
         super(DirBackend, self).__init__(conf)
-        self._path = os.path.abspath(conf['path'])
-        self._lock_path = os.path.join(self._path, 'locks')
-        self._file_cache = {}
-
-    @property
-    def lock_path(self):
-        return self._lock_path
-
-    @property
-    def base_path(self):
-        return self._path
+        self.file_cache = {}
+        if not self._path:
+            raise ValueError("Empty path is disallowed")
+        self._path = os.path.abspath(self._path)
 
     def get_connection(self):
         return Connection(self)
@@ -73,33 +68,13 @@ class DirBackend(base.Backend):
         pass
 
 
-class Connection(base.Connection):
-    def __init__(self, backend):
-        self._backend = backend
-        self._file_cache = self._backend._file_cache
-        self._flow_path = os.path.join(self._backend.base_path, 'flows')
-        self._atom_path = os.path.join(self._backend.base_path, 'atoms')
-        self._book_path = os.path.join(self._backend.base_path, 'books')
-
-    def validate(self):
-        # Verify key paths exist.
-        paths = [
-            self._backend.base_path,
-            self._backend.lock_path,
-            self._flow_path,
-            self._atom_path,
-            self._book_path,
-        ]
-        for p in paths:
-            if not os.path.isdir(p):
-                raise RuntimeError("Missing required directory: %s" % (p))
-
+class Connection(path_based.PathBasedConnection):
     def _read_from(self, filename):
         # This is very similar to the oslo-incubator fileutils module, but
         # tweaked to not depend on a global cache, as well as tweaked to not
         # pull-in the oslo logging module (which is a huge pile of code).
         mtime = os.path.getmtime(filename)
-        cache_info = self._file_cache.setdefault(filename, {})
+        cache_info = self.backend.file_cache.setdefault(filename, {})
         if not cache_info or mtime > cache_info.get('mtime', 0):
             with open(filename, 'rb') as fp:
                 cache_info['data'] = fp.read().decode('utf-8')
@@ -111,301 +86,56 @@ class Connection(base.Connection):
             contents = contents.encode('utf-8')
         with open(filename, 'wb') as fp:
             fp.write(contents)
-        self._file_cache.pop(filename, None)
-
-    def _run_with_process_lock(self, lock_name, functor, *args, **kwargs):
-        lock_path = os.path.join(self.backend.lock_path, lock_name)
-        with lock_utils.InterProcessLock(lock_path):
-            try:
-                return functor(*args, **kwargs)
-            except exc.TaskFlowException:
-                raise
-            except Exception as e:
-                LOG.exception("Failed running locking file based session")
-                # NOTE(harlowja): trap all other errors as storage errors.
-                raise exc.StorageFailure("Storage backend internal error", e)
-
-    def _get_logbooks(self):
-        lb_uuids = []
-        try:
-            lb_uuids = [d for d in os.listdir(self._book_path)
-                        if os.path.isdir(os.path.join(self._book_path, d))]
-        except EnvironmentError as e:
-            if e.errno != errno.ENOENT:
-                raise
-        for lb_uuid in lb_uuids:
-            try:
-                yield self._get_logbook(lb_uuid)
-            except exc.NotFound:
-                pass
-
-    def get_logbooks(self):
-        try:
-            books = list(self._get_logbooks())
-        except EnvironmentError as e:
-            raise exc.StorageFailure("Unable to fetch logbooks", e)
-        else:
-            for b in books:
-                yield b
-
-    @property
-    def backend(self):
-        return self._backend
-
-    def close(self):
-        pass
-
-    def _save_atom_details(self, atom_detail, ignore_missing):
-        # See if we have an existing atom detail to merge with.
-        e_ad = None
-        try:
-            e_ad = self._get_atom_details(atom_detail.uuid, lock=False)
-        except EnvironmentError:
-            if not ignore_missing:
-                raise exc.NotFound("No atom details found with id: %s"
-                                   % atom_detail.uuid)
-        if e_ad is not None:
-            atom_detail = e_ad.merge(atom_detail)
-        ad_path = os.path.join(self._atom_path, atom_detail.uuid)
-        ad_data = base._format_atom(atom_detail)
-        self._write_to(ad_path, jsonutils.dumps(ad_data))
-        return atom_detail
-
-    def update_atom_details(self, atom_detail):
-        return self._run_with_process_lock("atom",
-                                           self._save_atom_details,
-                                           atom_detail,
-                                           ignore_missing=False)
-
-    def _get_atom_details(self, uuid, lock=True):
-
-        def _get():
-            ad_path = os.path.join(self._atom_path, uuid)
-            ad_data = misc.decode_json(self._read_from(ad_path))
-            ad_cls = logbook.atom_detail_class(ad_data['type'])
-            return ad_cls.from_dict(ad_data['atom'])
-
-        if lock:
-            return self._run_with_process_lock('atom', _get)
-        else:
-            return _get()
-
-    def _get_flow_details(self, uuid, lock=True):
-
-        def _get():
-            fd_path = os.path.join(self._flow_path, uuid)
-            meta_path = os.path.join(fd_path, 'metadata')
-            meta = misc.decode_json(self._read_from(meta_path))
-            fd = logbook.FlowDetail.from_dict(meta)
-            ad_to_load = []
-            ad_path = os.path.join(fd_path, 'atoms')
+        self.backend.file_cache.pop(filename, None)
+
+    @contextlib.contextmanager
+    def _path_lock(self, path):
+        lockfile = self._join_path(path, 'lock')
+        with lock_utils.InterProcessLock(lockfile) as lock:
+            with _storagefailure_wrapper():
+                yield lock
+
+    def _join_path(self, *parts):
+        return os.path.join(*parts)
+
+    def _get_item(self, path):
+        with self._path_lock(path):
+            item_path = self._join_path(path, 'metadata')
+            return misc.decode_json(self._read_from(item_path))
+
+    def _set_item(self, path, value, transaction):
+        with self._path_lock(path):
+            item_path = self._join_path(path, 'metadata')
+            self._write_to(item_path, jsonutils.dumps(value))
+
+    def _del_tree(self, path, transaction):
+        with self._path_lock(path):
+            shutil.rmtree(path)
+
+    def _get_children(self, path):
+        with _storagefailure_wrapper():
+            return [link for link in os.listdir(path)
+                    if os.path.islink(self._join_path(path, link))]
+
+    def _ensure_path(self, path):
+        with _storagefailure_wrapper():
+            misc.ensure_tree(path)
+
+    def _create_link(self, src_path, dest_path, transaction):
+        with _storagefailure_wrapper():
             try:
-                ad_to_load = [f for f in os.listdir(ad_path)
-                              if os.path.islink(os.path.join(ad_path, f))]
-            except EnvironmentError as e:
-                if e.errno != errno.ENOENT:
-                    raise
-            for ad_uuid in ad_to_load:
-                fd.add(self._get_atom_details(ad_uuid))
-            return fd
-
-        if lock:
-            return self._run_with_process_lock('flow', _get)
-        else:
-            return _get()
-
-    def _save_atoms_and_link(self, atom_details, local_atom_path):
-        for atom_detail in atom_details:
-            self._save_atom_details(atom_detail, ignore_missing=True)
-            src_ad_path = os.path.join(self._atom_path, atom_detail.uuid)
-            target_ad_path = os.path.join(local_atom_path, atom_detail.uuid)
-            try:
-                os.symlink(src_ad_path, target_ad_path)
-            except EnvironmentError as e:
+                os.symlink(src_path, dest_path)
+            except OSError as e:
                 if e.errno != errno.EEXIST:
                     raise
 
-    def _save_flow_details(self, flow_detail, ignore_missing):
-        # See if we have an existing flow detail to merge with.
-        e_fd = None
-        try:
-            e_fd = self._get_flow_details(flow_detail.uuid, lock=False)
-        except EnvironmentError:
-            if not ignore_missing:
-                raise exc.NotFound("No flow details found with id: %s"
-                                   % flow_detail.uuid)
-        if e_fd is not None:
-            e_fd = e_fd.merge(flow_detail)
-            for ad in flow_detail:
-                if e_fd.find(ad.uuid) is None:
-                    e_fd.add(ad)
-            flow_detail = e_fd
-        flow_path = os.path.join(self._flow_path, flow_detail.uuid)
-        misc.ensure_tree(flow_path)
-        self._write_to(os.path.join(flow_path, 'metadata'),
-                       jsonutils.dumps(flow_detail.to_dict()))
-        if len(flow_detail):
-            atom_path = os.path.join(flow_path, 'atoms')
-            misc.ensure_tree(atom_path)
-            self._run_with_process_lock('atom',
-                                        self._save_atoms_and_link,
-                                        list(flow_detail), atom_path)
-        return flow_detail
-
-    def update_flow_details(self, flow_detail):
-        return self._run_with_process_lock("flow",
-                                           self._save_flow_details,
-                                           flow_detail,
-                                           ignore_missing=False)
+    @contextlib.contextmanager
+    def _transaction(self):
+        """This backend doesn't support transactions"""
+        yield
 
-    def _save_flows_and_link(self, flow_details, local_flow_path):
-        for flow_detail in flow_details:
-            self._save_flow_details(flow_detail, ignore_missing=True)
-            src_fd_path = os.path.join(self._flow_path, flow_detail.uuid)
-            target_fd_path = os.path.join(local_flow_path, flow_detail.uuid)
-            try:
-                os.symlink(src_fd_path, target_fd_path)
-            except EnvironmentError as e:
-                if e.errno != errno.EEXIST:
-                    raise
-
-    def _save_logbook(self, book):
-        # See if we have an existing logbook to merge with.
-        e_lb = None
-        try:
-            e_lb = self._get_logbook(book.uuid)
-        except exc.NotFound:
-            pass
-        if e_lb is not None:
-            e_lb = e_lb.merge(book)
-            for fd in book:
-                if e_lb.find(fd.uuid) is None:
-                    e_lb.add(fd)
-            book = e_lb
-        book_path = os.path.join(self._book_path, book.uuid)
-        misc.ensure_tree(book_path)
-        self._write_to(os.path.join(book_path, 'metadata'),
-                       jsonutils.dumps(book.to_dict(marshal_time=True)))
-        if len(book):
-            flow_path = os.path.join(book_path, 'flows')
-            misc.ensure_tree(flow_path)
-            self._run_with_process_lock('flow',
-                                        self._save_flows_and_link,
-                                        list(book), flow_path)
-        return book
-
-    def save_logbook(self, book):
-        return self._run_with_process_lock("book",
-                                           self._save_logbook, book)
-
-    def upgrade(self):
-
-        def _step_create():
-            for path in (self._book_path, self._flow_path, self._atom_path):
-                try:
-                    misc.ensure_tree(path)
-                except EnvironmentError as e:
-                    raise exc.StorageFailure("Unable to create logbooks"
-                                             " required child path %s" % path,
-                                             e)
-
-        for path in (self._backend.base_path, self._backend.lock_path):
-            try:
-                misc.ensure_tree(path)
-            except EnvironmentError as e:
-                raise exc.StorageFailure("Unable to create logbooks required"
-                                         " path %s" % path, e)
-
-        self._run_with_process_lock("init", _step_create)
-
-    def clear_all(self):
-
-        def _step_clear():
-            for d in (self._book_path, self._flow_path, self._atom_path):
-                if os.path.isdir(d):
-                    shutil.rmtree(d)
-
-        def _step_atom():
-            self._run_with_process_lock("atom", _step_clear)
-
-        def _step_flow():
-            self._run_with_process_lock("flow", _step_atom)
-
-        def _step_book():
-            self._run_with_process_lock("book", _step_flow)
-
-        # Acquire all locks by going through this little hierarchy.
-        self._run_with_process_lock("init", _step_book)
-
-    def destroy_logbook(self, book_uuid):
-
-        def _destroy_atoms(atom_details):
-            for atom_detail in atom_details:
-                atom_path = os.path.join(self._atom_path, atom_detail.uuid)
-                try:
-                    shutil.rmtree(atom_path)
-                except EnvironmentError as e:
-                    if e.errno != errno.ENOENT:
-                        raise exc.StorageFailure("Unable to remove atom"
-                                                 " directory %s" % atom_path,
-                                                 e)
-
-        def _destroy_flows(flow_details):
-            for flow_detail in flow_details:
-                flow_path = os.path.join(self._flow_path, flow_detail.uuid)
-                self._run_with_process_lock("atom", _destroy_atoms,
-                                            list(flow_detail))
-                try:
-                    shutil.rmtree(flow_path)
-                except EnvironmentError as e:
-                    if e.errno != errno.ENOENT:
-                        raise exc.StorageFailure("Unable to remove flow"
-                                                 " directory %s" % flow_path,
-                                                 e)
-
-        def _destroy_book():
-            book = self._get_logbook(book_uuid)
-            book_path = os.path.join(self._book_path, book.uuid)
-            self._run_with_process_lock("flow", _destroy_flows, list(book))
-            try:
-                shutil.rmtree(book_path)
-            except EnvironmentError as e:
-                if e.errno != errno.ENOENT:
-                    raise exc.StorageFailure("Unable to remove book"
-                                             " directory %s" % book_path, e)
-
-        # Acquire all locks by going through this little hierarchy.
-        self._run_with_process_lock("book", _destroy_book)
-
-    def _get_logbook(self, book_uuid):
-        book_path = os.path.join(self._book_path, book_uuid)
-        meta_path = os.path.join(book_path, 'metadata')
-        try:
-            meta = misc.decode_json(self._read_from(meta_path))
-        except EnvironmentError as e:
-            if e.errno == errno.ENOENT:
-                raise exc.NotFound("No logbook found with id: %s" % book_uuid)
-            else:
-                raise
-        lb = logbook.LogBook.from_dict(meta, unmarshal_time=True)
-        fd_path = os.path.join(book_path, 'flows')
-        fd_uuids = []
-        try:
-            fd_uuids = [f for f in os.listdir(fd_path)
-                        if os.path.islink(os.path.join(fd_path, f))]
-        except EnvironmentError as e:
-            if e.errno != errno.ENOENT:
-                raise
-        for fd_uuid in fd_uuids:
-            lb.add(self._get_flow_details(fd_uuid))
-        return lb
-
-    def get_logbook(self, book_uuid):
-        return self._run_with_process_lock("book",
-                                           self._get_logbook, book_uuid)
-
-    def get_flow_details(self, fd_uuid):
-        return self._get_flow_details(fd_uuid)
-
-    def get_atom_details(self, ad_uuid):
-        return self._get_atom_details(ad_uuid)
+    def validate(self):
+        with _storagefailure_wrapper():
+            for p in (self.flow_path, self.atom_path, self.book_path):
+                if not os.path.isdir(p):
+                    raise RuntimeError("Missing required directory: %s" % (p))
diff --git a/taskflow/persistence/backends/impl_memory.py b/taskflow/persistence/backends/impl_memory.py
index 020deff..7fc22e6 100644
--- a/taskflow/persistence/backends/impl_memory.py
+++ b/taskflow/persistence/backends/impl_memory.py
@@ -15,127 +15,32 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-import functools
-
-import six
+import contextlib
+import copy
+import os
 
 from taskflow import exceptions as exc
-from taskflow import logging
-from taskflow.persistence import base
-from taskflow.persistence import logbook
+from taskflow.persistence import path_based
+from taskflow.types import tree
 from taskflow.utils import lock_utils
 
-LOG = logging.getLogger(__name__)
-
-
-class _Memory(object):
-    """Where the data is really stored."""
-
-    def __init__(self):
-        self.log_books = {}
-        self.flow_details = {}
-        self.atom_details = {}
-
-    def clear_all(self):
-        self.log_books.clear()
-        self.flow_details.clear()
-        self.atom_details.clear()
-
-
-class _MemoryHelper(object):
-    """Helper functionality for the memory backends & connections."""
-
-    def __init__(self, memory):
-        self._memory = memory
-
-    @staticmethod
-    def _fetch_clone_args(incoming):
-        if isinstance(incoming, (logbook.LogBook, logbook.FlowDetail)):
-            # We keep our own copy of the added contents of the following
-            # types so we don't need the clone to retain them directly...
-            return {
-                'retain_contents': False,
-            }
-        return {}
-
-    def construct(self, uuid, container):
-        """Reconstructs a object from the given uuid and storage container."""
-        source = container[uuid]
-        clone_kwargs = self._fetch_clone_args(source)
-        clone = source['object'].copy(**clone_kwargs)
-        rebuilder = source.get('rebuilder')
-        if rebuilder:
-            for component in map(rebuilder, source['components']):
-                clone.add(component)
-        return clone
-
-    def merge(self, incoming, saved_info=None):
-        """Merges the incoming object into the local memories copy."""
-        if saved_info is None:
-            if isinstance(incoming, logbook.LogBook):
-                saved_info = self._memory.log_books.setdefault(
-                    incoming.uuid, {})
-            elif isinstance(incoming, logbook.FlowDetail):
-                saved_info = self._memory.flow_details.setdefault(
-                    incoming.uuid, {})
-            elif isinstance(incoming, logbook.AtomDetail):
-                saved_info = self._memory.atom_details.setdefault(
-                    incoming.uuid, {})
-            else:
-                raise TypeError("Unknown how to merge '%s' (%s)"
-                                % (incoming, type(incoming)))
-        try:
-            saved_info['object'].merge(incoming)
-        except KeyError:
-            clone_kwargs = self._fetch_clone_args(incoming)
-            saved_info['object'] = incoming.copy(**clone_kwargs)
-        if isinstance(incoming, logbook.LogBook):
-            flow_details = saved_info.setdefault('components', set())
-            if 'rebuilder' not in saved_info:
-                saved_info['rebuilder'] = functools.partial(
-                    self.construct, container=self._memory.flow_details)
-            for flow_detail in incoming:
-                flow_details.add(self.merge(flow_detail))
-        elif isinstance(incoming, logbook.FlowDetail):
-            atom_details = saved_info.setdefault('components', set())
-            if 'rebuilder' not in saved_info:
-                saved_info['rebuilder'] = functools.partial(
-                    self.construct, container=self._memory.atom_details)
-            for atom_detail in incoming:
-                atom_details.add(self.merge(atom_detail))
-        return incoming.uuid
-
-
-class MemoryBackend(base.Backend):
+
+class MemoryBackend(path_based.PathBasedBackend):
     """A in-memory (non-persistent) backend.
 
     This backend writes logbooks, flow details, and atom details to in-memory
     dictionaries and retrieves from those dictionaries as needed.
+
+    This backend does *not* provide true transactional semantics. It does
+    guarantee that there will be no inter-thread race conditions when
+    writing and reading by using a read/write locks.
     """
     def __init__(self, conf=None):
         super(MemoryBackend, self).__init__(conf)
-        self._memory = _Memory()
-        self._helper = _MemoryHelper(self._memory)
-        self._lock = lock_utils.ReaderWriterLock()
-
-    def _construct_from(self, container):
-        return dict((uuid, self._helper.construct(uuid, container))
-                    for uuid in six.iterkeys(container))
-
-    @property
-    def log_books(self):
-        with self._lock.read_lock():
-            return self._construct_from(self._memory.log_books)
-
-    @property
-    def flow_details(self):
-        with self._lock.read_lock():
-            return self._construct_from(self._memory.flow_details)
-
-    @property
-    def atom_details(self):
-        with self._lock.read_lock():
-            return self._construct_from(self._memory.atom_details)
+        if self._path is None:
+            self._path = os.sep
+        self.memory = tree.Node(self._path)
+        self.lock = lock_utils.ReaderWriterLock()
 
     def get_connection(self):
         return Connection(self)
@@ -144,107 +49,79 @@ class MemoryBackend(base.Backend):
         pass
 
 
-class Connection(base.Connection):
-    """A connection to an in-memory backend."""
-
+class Connection(path_based.PathBasedConnection):
     def __init__(self, backend):
-        self._backend = backend
-        self._helper = backend._helper
-        self._memory = backend._memory
-        self._lock = backend._lock
-
-    def upgrade(self):
-        pass
+        super(Connection, self).__init__(backend)
+        self.upgrade()
 
-    def validate(self):
-        pass
+    @contextlib.contextmanager
+    def _memory_lock(self, write=False):
+        if write:
+            lock = self.backend.lock.write_lock
+        else:
+            lock = self.backend.lock.read_lock
 
-    @property
-    def backend(self):
-        return self._backend
-
-    def close(self):
-        pass
-
-    def clear_all(self):
-        with self._lock.write_lock():
-            self._memory.clear_all()
-
-    def destroy_logbook(self, book_uuid):
-        with self._lock.write_lock():
-            try:
-                # Do the same cascading delete that the sql layer does.
-                book_info = self._memory.log_books.pop(book_uuid)
-            except KeyError:
-                raise exc.NotFound("No logbook found with uuid '%s'"
-                                   % book_uuid)
-            else:
-                while book_info['components']:
-                    flow_uuid = book_info['components'].pop()
-                    flow_info = self._memory.flow_details.pop(flow_uuid)
-                    while flow_info['components']:
-                        atom_uuid = flow_info['components'].pop()
-                        self._memory.atom_details.pop(atom_uuid)
-
-    def update_atom_details(self, atom_detail):
-        with self._lock.write_lock():
+        with lock():
             try:
-                atom_info = self._memory.atom_details[atom_detail.uuid]
-                return self._helper.construct(
-                    self._helper.merge(atom_detail, saved_info=atom_info),
-                    self._memory.atom_details)
-            except KeyError:
-                raise exc.NotFound("No atom details found with uuid '%s'"
-                                   % atom_detail.uuid)
-
-    def update_flow_details(self, flow_detail):
-        with self._lock.write_lock():
-            try:
-                flow_info = self._memory.flow_details[flow_detail.uuid]
-                return self._helper.construct(
-                    self._helper.merge(flow_detail, saved_info=flow_info),
-                    self._memory.flow_details)
-            except KeyError:
-                raise exc.NotFound("No flow details found with uuid '%s'"
-                                   % flow_detail.uuid)
-
-    def save_logbook(self, book):
-        with self._lock.write_lock():
-            return self._helper.construct(self._helper.merge(book),
-                                          self._memory.log_books)
-
-    def get_logbook(self, book_uuid):
-        with self._lock.read_lock():
-            try:
-                return self._helper.construct(book_uuid,
-                                              self._memory.log_books)
-            except KeyError:
-                raise exc.NotFound("No logbook found with uuid '%s'"
-                                   % book_uuid)
-
-    def get_logbooks(self):
-        # Don't hold locks while iterating...
-        with self._lock.read_lock():
-            book_uuids = set(six.iterkeys(self._memory.log_books))
-        for book_uuid in book_uuids:
-            try:
-                with self._lock.read_lock():
-                    book = self._helper.construct(book_uuid,
-                                                  self._memory.log_books)
-                yield book
-            except KeyError:
-                pass
-
-    def get_flow_details(self, fd_uuid):
+                yield
+            except exc.TaskFlowException as e:
+                raise
+            except Exception as e:
+                raise exc.StorageFailure("Storage backend internal error", e)
+
+    def _fetch_node(self, path):
+        node = self.backend.memory.find(path)
+        if node is None:
+            raise exc.NotFound("Item not found %s" % path)
+        return node
+
+    def _join_path(self, *parts):
+        return os.path.join(*parts)
+
+    def _get_item(self, path):
+        with self._memory_lock():
+            return copy.deepcopy(self._fetch_node(path).metadata['value'])
+
+    def _set_item(self, path, value, transaction):
+        value = copy.deepcopy(value)
         try:
-            with self._lock.read_lock():
-                return self._memory.flow_details[fd_uuid]
-        except KeyError:
-            raise exc.NotFound("No flow details found '%s'" % fd_uuid)
+            item_node = self._fetch_node(path)
+            item_node.metadata.update(value=value)
+        except exc.NotFound:
+            dirname, basename = os.path.split(path)
+            parent_node = self._fetch_node(dirname)
+            parent_node.add(tree.Node(path, name=basename, value=value))
+
+    def _del_tree(self, path, transaction):
+        node = self._fetch_node(path)
+        node.disassociate()
+
+    def _get_children(self, path):
+        with self._memory_lock():
+            return [node.metadata['name'] for node in self._fetch_node(path)]
+
+    def _ensure_path(self, path):
+        with self._memory_lock(write=True):
+            path = os.path.normpath(path)
+            parts = path.split(os.sep)
+            node = self.backend.memory
+            for p in range(len(parts) - 1):
+                node_path = os.sep.join(parts[:p + 2])
+                try:
+                    node = self._fetch_node(node_path)
+                except exc.NotFound:
+                    node.add(tree.Node(node_path, name=parts[p + 1]))
+
+    def _create_link(self, src_path, dest_path, transaction):
+        dirname, basename = os.path.split(dest_path)
+        parent_node = self._fetch_node(dirname)
+        parent_node.add(tree.Node(dest_path, name=basename, target=src_path))
+
+    @contextlib.contextmanager
+    def _transaction(self):
+        """This just wraps a global write-lock"""
+        with self._memory_lock(write=True):
+            yield
 
-    def get_atom_details(self, ad_uuid):
-        try:
-            with self._lock.read_lock():
-                return self._memory.atom_details[ad_uuid]
-        except KeyError:
-            raise exc.NotFound("No atom details found '%s'" % ad_uuid)
+    def validate(self):
+        pass
diff --git a/taskflow/persistence/backends/impl_sqlalchemy.py b/taskflow/persistence/backends/impl_sqlalchemy.py
index a49d249..4368b78 100644
--- a/taskflow/persistence/backends/impl_sqlalchemy.py
+++ b/taskflow/persistence/backends/impl_sqlalchemy.py
@@ -202,25 +202,25 @@ class Alchemist(object):
         atom_cls = logbook.atom_detail_class(row.pop('atom_type'))
         return atom_cls.from_dict(row)
 
-    def _atom_query_iter(self, conn, parent_uuid):
+    def atom_query_iter(self, conn, parent_uuid):
         q = (sql.select([self._tables.atomdetails]).
              where(self._tables.atomdetails.c.parent_uuid == parent_uuid))
         for row in conn.execute(q):
             yield self.convert_atom_detail(row)
 
-    def _flow_query_iter(self, conn, parent_uuid):
+    def flow_query_iter(self, conn, parent_uuid):
         q = (sql.select([self._tables.flowdetails]).
              where(self._tables.flowdetails.c.parent_uuid == parent_uuid))
         for row in conn.execute(q):
             yield self.convert_flow_detail(row)
 
     def populate_book(self, conn, book):
-        for fd in self._flow_query_iter(conn, book.uuid):
+        for fd in self.flow_query_iter(conn, book.uuid):
             book.add(fd)
             self.populate_flow_detail(conn, fd)
 
     def populate_flow_detail(self, conn, fd):
-        for ad in self._atom_query_iter(conn, fd.uuid):
+        for ad in self.atom_query_iter(conn, fd.uuid):
             fd.add(ad)
 
 
@@ -558,6 +558,19 @@ class Connection(base.Connection):
         for book in gathered:
             yield book
 
+    def get_flows_for_book(self, book_uuid):
+        gathered = []
+        try:
+            with contextlib.closing(self._engine.connect()) as conn:
+                for row in self._converter.flow_query_iter(conn, book_uuid):
+                    flow_details = self._converter.populate_flow_detail(conn,
+                                                                        row)
+                    gathered.append(flow_details)
+        except sa_exc.DBAPIError as e:
+            raise exc.StorageFailure("Failed getting flow details", e)
+        for flow_details in gathered:
+            yield flow_details
+
     def get_flow_details(self, fd_uuid):
         try:
             flowdetails = self._tables.flowdetails
diff --git a/taskflow/persistence/backends/impl_zookeeper.py b/taskflow/persistence/backends/impl_zookeeper.py
index ae8096f..b60e93b 100644
--- a/taskflow/persistence/backends/impl_zookeeper.py
+++ b/taskflow/persistence/backends/impl_zookeeper.py
@@ -1,6 +1,7 @@
 # -*- coding: utf-8 -*-
 
 #    Copyright (C) 2014 AT&T Labs All Rights Reserved.
+#    Copyright (C) 2015 Rackspace Hosting All Rights Reserved.
 #
 #    Licensed under the Apache License, Version 2.0 (the "License"); you may
 #    not use this file except in compliance with the License. You may obtain
@@ -21,30 +22,18 @@ from kazoo.protocol import paths
 from oslo_serialization import jsonutils
 
 from taskflow import exceptions as exc
-from taskflow import logging
-from taskflow.persistence import base
-from taskflow.persistence import logbook
+from taskflow.persistence import path_based
 from taskflow.utils import kazoo_utils as k_utils
 from taskflow.utils import misc
 
-LOG = logging.getLogger(__name__)
 
-# Transaction support was added in 3.4.0
 MIN_ZK_VERSION = (3, 4, 0)
 
 
-class ZkBackend(base.Backend):
-    """A zookeeper backend.
-
-    This backend writes logbooks, flow details, and atom details to a provided
-    base path in zookeeper. It will create and store those objects in three
-    key directories (one for logbooks, one for flow details and one for atom
-    details). It creates those associated directories and then creates files
-    inside those directories that represent the contents of those objects for
-    later reading and writing.
+class ZkBackend(path_based.PathBasedBackend):
+    """A zookeeper-backed backend.
 
     Example configuration::
-
         conf = {
             "hosts": "192.168.0.1:2181,192.168.0.2:2181,192.168.0.3:2181",
             "path": "/taskflow",
@@ -52,24 +41,18 @@ class ZkBackend(base.Backend):
     """
     def __init__(self, conf, client=None):
         super(ZkBackend, self).__init__(conf)
-        path = str(conf.get("path", "/taskflow"))
-        if not path:
-            raise ValueError("Empty zookeeper path is disallowed")
-        if not paths.isabs(path):
+        if not self._path:
+            self._path = '/taskflow'
+        if not paths.isabs(self._path):
             raise ValueError("Zookeeper path must be absolute")
-        self._path = path
         if client is not None:
             self._client = client
             self._owned = False
         else:
-            self._client = k_utils.make_client(conf)
+            self._client = k_utils.make_client(self._conf)
             self._owned = True
         self._validated = False
 
-    @property
-    def path(self):
-        return self._path
-
     def get_connection(self):
         conn = ZkConnection(self, self._client, self._conf)
         if not self._validated:
@@ -87,52 +70,15 @@ class ZkBackend(base.Backend):
             raise exc.StorageFailure("Unable to finalize client", e)
 
 
-class ZkConnection(base.Connection):
+class ZkConnection(path_based.PathBasedConnection):
     def __init__(self, backend, client, conf):
-        self._backend = backend
-        self._client = client
+        super(ZkConnection, self).__init__(backend)
         self._conf = conf
-        self._book_path = paths.join(self._backend.path, "books")
-        self._flow_path = paths.join(self._backend.path, "flow_details")
-        self._atom_path = paths.join(self._backend.path, "atom_details")
+        self._client = client
         with self._exc_wrapper():
             # NOOP if already started.
             self._client.start()
 
-    def validate(self):
-        with self._exc_wrapper():
-            try:
-                if self._conf.get('check_compatible', True):
-                    k_utils.check_compatible(self._client, MIN_ZK_VERSION)
-            except exc.IncompatibleVersion as e:
-                raise exc.StorageFailure("Backend storage is not a"
-                                         " compatible version", e)
-
-    @property
-    def backend(self):
-        return self._backend
-
-    @property
-    def book_path(self):
-        return self._book_path
-
-    @property
-    def flow_path(self):
-        return self._flow_path
-
-    @property
-    def atom_path(self):
-        return self._atom_path
-
-    def close(self):
-        pass
-
-    def upgrade(self):
-        """Creates the initial paths (if they already don't exist)."""
-        with self._exc_wrapper():
-            for path in (self.book_path, self.flow_path, self.atom_path):
-                self._client.ensure_path(path)
-
     @contextlib.contextmanager
     def _exc_wrapper(self):
         """Exception context-manager which wraps kazoo exceptions.
@@ -146,8 +92,7 @@ class ZkConnection(base.Connection):
         except self._client.handler.timeout_exception as e:
             raise exc.StorageFailure("Storage backend timeout", e)
         except k_exc.SessionExpiredError as e:
-            raise exc.StorageFailure("Storage backend session"
-                                     " has expired", e)
+            raise exc.StorageFailure("Storage backend session has expired", e)
         except k_exc.NoNodeError as e:
             raise exc.NotFound("Storage backend node not found: %s" % e)
         except k_exc.NodeExistsError as e:
@@ -155,273 +100,50 @@ class ZkConnection(base.Connection):
         except (k_exc.KazooException, k_exc.ZookeeperError) as e:
             raise exc.StorageFailure("Storage backend internal error", e)
 
-    def update_atom_details(self, ad):
-        """Update a atom detail transactionally."""
-        with self._exc_wrapper():
-            txn = self._client.transaction()
-            ad = self._update_atom_details(ad, txn)
-            k_utils.checked_commit(txn)
-            return ad
-
-    def _update_atom_details(self, ad, txn, create_missing=False):
-        # Determine whether the desired data exists or not.
-        ad_path = paths.join(self.atom_path, ad.uuid)
-        e_ad = None
-        try:
-            ad_data, _zstat = self._client.get(ad_path)
-        except k_exc.NoNodeError:
-            # Not-existent: create or raise exception.
-            if not create_missing:
-                raise exc.NotFound("No atom details found with"
-                                   " id: %s" % ad.uuid)
-            else:
-                txn.create(ad_path)
-        else:
-            # Existent: read it out.
-            try:
-                ad_data = misc.decode_json(ad_data)
-                ad_cls = logbook.atom_detail_class(ad_data['type'])
-                e_ad = ad_cls.from_dict(ad_data['atom'])
-            except KeyError:
-                pass
-
-        # Update and write it back
-        if e_ad:
-            e_ad = e_ad.merge(ad)
-        else:
-            e_ad = ad
-        ad_data = base._format_atom(e_ad)
-        txn.set_data(ad_path,
-                     misc.binary_encode(jsonutils.dumps(ad_data)))
-        return e_ad
-
-    def get_atom_details(self, ad_uuid):
-        """Read a atom detail.
+    def _join_path(self, *parts):
+        return paths.join(*parts)
 
-        *Read-only*, so no need of zk transaction.
-        """
+    def _get_item(self, path):
         with self._exc_wrapper():
-            return self._get_atom_details(ad_uuid)
+            data, _ = self._client.get(path)
+        return misc.decode_json(data)
 
-    def _get_atom_details(self, ad_uuid):
-        ad_path = paths.join(self.atom_path, ad_uuid)
-        try:
-            ad_data, _zstat = self._client.get(ad_path)
-        except k_exc.NoNodeError:
-            raise exc.NotFound("No atom details found with id: %s" % ad_uuid)
-        else:
-            ad_data = misc.decode_json(ad_data)
-            ad_cls = logbook.atom_detail_class(ad_data['type'])
-            return ad_cls.from_dict(ad_data['atom'])
-
-    def update_flow_details(self, fd):
-        """Update a flow detail transactionally."""
-        with self._exc_wrapper():
-            txn = self._client.transaction()
-            fd = self._update_flow_details(fd, txn)
-            k_utils.checked_commit(txn)
-            return fd
-
-    def _update_flow_details(self, fd, txn, create_missing=False):
-        # Determine whether the desired data exists or not
-        fd_path = paths.join(self.flow_path, fd.uuid)
-        try:
-            fd_data, _zstat = self._client.get(fd_path)
-        except k_exc.NoNodeError:
-            # Not-existent: create or raise exception
-            if create_missing:
-                txn.create(fd_path)
-                e_fd = logbook.FlowDetail(name=fd.name, uuid=fd.uuid)
-            else:
-                raise exc.NotFound("No flow details found with id: %s"
-                                   % fd.uuid)
+    def _set_item(self, path, value, transaction):
+        data = misc.binary_encode(jsonutils.dumps(value))
+        if not self._client.exists(path):
+            transaction.create(path, data)
         else:
-            # Existent: read it out
-            e_fd = logbook.FlowDetail.from_dict(misc.decode_json(fd_data))
+            transaction.set_data(path, data)
 
-        # Update and write it back
-        e_fd = e_fd.merge(fd)
-        fd_data = e_fd.to_dict()
-        txn.set_data(fd_path, misc.binary_encode(jsonutils.dumps(fd_data)))
-        for ad in fd:
-            ad_path = paths.join(fd_path, ad.uuid)
-            # NOTE(harlowja): create an entry in the flow detail path
-            # for the provided atom detail so that a reference exists
-            # from the flow detail to its atom details.
-            if not self._client.exists(ad_path):
-                txn.create(ad_path)
-            e_fd.add(self._update_atom_details(ad, txn, create_missing=True))
-        return e_fd
+    def _del_tree(self, path, transaction):
+        for child in self._get_children(path):
+            self._del_tree(self._join_path(path, child), transaction)
+        transaction.delete(path)
 
-    def get_flow_details(self, fd_uuid):
-        """Read a flow detail.
-
-        *Read-only*, so no need of zk transaction.
-        """
-        with self._exc_wrapper():
-            return self._get_flow_details(fd_uuid)
-
-    def _get_flow_details(self, fd_uuid):
-        fd_path = paths.join(self.flow_path, fd_uuid)
-        try:
-            fd_data, _zstat = self._client.get(fd_path)
-        except k_exc.NoNodeError:
-            raise exc.NotFound("No flow details found with id: %s" % fd_uuid)
-
-        fd = logbook.FlowDetail.from_dict(misc.decode_json(fd_data))
-        for ad_uuid in self._client.get_children(fd_path):
-            fd.add(self._get_atom_details(ad_uuid))
-        return fd
-
-    def save_logbook(self, lb):
-        """Save (update) a log_book transactionally."""
-
-        def _create_logbook(lb_path, txn):
-            lb_data = lb.to_dict(marshal_time=True)
-            txn.create(lb_path, misc.binary_encode(jsonutils.dumps(lb_data)))
-            for fd in lb:
-                # NOTE(harlowja): create an entry in the logbook path
-                # for the provided flow detail so that a reference exists
-                # from the logbook to its flow details.
-                txn.create(paths.join(lb_path, fd.uuid))
-                fd_path = paths.join(self.flow_path, fd.uuid)
-                fd_data = jsonutils.dumps(fd.to_dict())
-                txn.create(fd_path, misc.binary_encode(fd_data))
-                for ad in fd:
-                    # NOTE(harlowja): create an entry in the flow detail path
-                    # for the provided atom detail so that a reference exists
-                    # from the flow detail to its atom details.
-                    txn.create(paths.join(fd_path, ad.uuid))
-                    ad_path = paths.join(self.atom_path, ad.uuid)
-                    ad_data = base._format_atom(ad)
-                    txn.create(ad_path,
-                               misc.binary_encode(jsonutils.dumps(ad_data)))
-            return lb
-
-        def _update_logbook(lb_path, lb_data, txn):
-            e_lb = logbook.LogBook.from_dict(misc.decode_json(lb_data),
-                                             unmarshal_time=True)
-            e_lb = e_lb.merge(lb)
-            lb_data = e_lb.to_dict(marshal_time=True)
-            txn.set_data(lb_path, misc.binary_encode(jsonutils.dumps(lb_data)))
-            for fd in lb:
-                fd_path = paths.join(lb_path, fd.uuid)
-                if not self._client.exists(fd_path):
-                    # NOTE(harlowja): create an entry in the logbook path
-                    # for the provided flow detail so that a reference exists
-                    # from the logbook to its flow details.
-                    txn.create(fd_path)
-                e_fd = self._update_flow_details(fd, txn, create_missing=True)
-                e_lb.add(e_fd)
-            return e_lb
-
-        with self._exc_wrapper():
-            txn = self._client.transaction()
-            # Determine whether the desired data exists or not.
-            lb_path = paths.join(self.book_path, lb.uuid)
-            try:
-                lb_data, _zstat = self._client.get(lb_path)
-            except k_exc.NoNodeError:
-                # Create a new logbook since it doesn't exist.
-                e_lb = _create_logbook(lb_path, txn)
-            else:
-                # Otherwise update the existing logbook instead.
-                e_lb = _update_logbook(lb_path, lb_data, txn)
-            k_utils.checked_commit(txn)
-            return e_lb
-
-    def _get_logbook(self, lb_uuid):
-        lb_path = paths.join(self.book_path, lb_uuid)
-        try:
-            lb_data, _zstat = self._client.get(lb_path)
-        except k_exc.NoNodeError:
-            raise exc.NotFound("No logbook found with id: %s" % lb_uuid)
-        else:
-            lb = logbook.LogBook.from_dict(misc.decode_json(lb_data),
-                                           unmarshal_time=True)
-            for fd_uuid in self._client.get_children(lb_path):
-                lb.add(self._get_flow_details(fd_uuid))
-            return lb
-
-    def get_logbook(self, lb_uuid):
-        """Read a logbook.
-
-        *Read-only*, so no need of zk transaction.
-        """
+    def _get_children(self, path):
         with self._exc_wrapper():
-            return self._get_logbook(lb_uuid)
+            return self._client.get_children(path)
 
-    def get_logbooks(self):
-        """Read all logbooks.
-
-        *Read-only*, so no need of zk transaction.
-        """
+    def _ensure_path(self, path):
         with self._exc_wrapper():
-            for lb_uuid in self._client.get_children(self.book_path):
-                yield self._get_logbook(lb_uuid)
-
-    def destroy_logbook(self, lb_uuid):
-        """Destroy (delete) a log_book transactionally."""
-
-        def _destroy_atom_details(ad_uuid, txn):
-            ad_path = paths.join(self.atom_path, ad_uuid)
-            if not self._client.exists(ad_path):
-                raise exc.NotFound("No atom details found with id: %s"
-                                   % ad_uuid)
-            txn.delete(ad_path)
-
-        def _destroy_flow_details(fd_uuid, txn):
-            fd_path = paths.join(self.flow_path, fd_uuid)
-            if not self._client.exists(fd_path):
-                raise exc.NotFound("No flow details found with id: %s"
-                                   % fd_uuid)
-            for ad_uuid in self._client.get_children(fd_path):
-                _destroy_atom_details(ad_uuid, txn)
-                txn.delete(paths.join(fd_path, ad_uuid))
-            txn.delete(fd_path)
+            self._client.ensure_path(path)
 
-        def _destroy_logbook(lb_uuid, txn):
-            lb_path = paths.join(self.book_path, lb_uuid)
-            if not self._client.exists(lb_path):
-                raise exc.NotFound("No logbook found with id: %s" % lb_uuid)
-            for fd_uuid in self._client.get_children(lb_path):
-                _destroy_flow_details(fd_uuid, txn)
-                txn.delete(paths.join(lb_path, fd_uuid))
-            txn.delete(lb_path)
+    def _create_link(self, src_path, dest_path, transaction):
+        if not self._client.exists(dest_path):
+            transaction.create(dest_path)
 
+    @contextlib.contextmanager
+    def _transaction(self):
+        transaction = self._client.transaction()
         with self._exc_wrapper():
-            txn = self._client.transaction()
-            _destroy_logbook(lb_uuid, txn)
-            k_utils.checked_commit(txn)
+            yield transaction
+            k_utils.checked_commit(transaction)
 
-    def clear_all(self, delete_dirs=True):
-        """Delete all data transactionally."""
+    def validate(self):
         with self._exc_wrapper():
-            txn = self._client.transaction()
-
-            # Delete all data under logbook path.
-            for lb_uuid in self._client.get_children(self.book_path):
-                lb_path = paths.join(self.book_path, lb_uuid)
-                for fd_uuid in self._client.get_children(lb_path):
-                    txn.delete(paths.join(lb_path, fd_uuid))
-                txn.delete(lb_path)
-
-            # Delete all data under flow detail path.
-            for fd_uuid in self._client.get_children(self.flow_path):
-                fd_path = paths.join(self.flow_path, fd_uuid)
-                for ad_uuid in self._client.get_children(fd_path):
-                    txn.delete(paths.join(fd_path, ad_uuid))
-                txn.delete(fd_path)
-
-            # Delete all data under atom detail path.
-            for ad_uuid in self._client.get_children(self.atom_path):
-                ad_path = paths.join(self.atom_path, ad_uuid)
-                txn.delete(ad_path)
-
-            # Delete containing directories.
-            if delete_dirs:
-                txn.delete(self.book_path)
-                txn.delete(self.atom_path)
-                txn.delete(self.flow_path)
-
-            k_utils.checked_commit(txn)
+            try:
+                if self._conf.get('check_compatible', True):
+                    k_utils.check_compatible(self._client, MIN_ZK_VERSION)
+            except exc.IncompatibleVersion as e:
+                raise exc.StorageFailure("Backend storage is not a"
+                                         " compatible version", e)
diff --git a/taskflow/persistence/base.py b/taskflow/persistence/base.py
index 0ce0925..a1f120d 100644
--- a/taskflow/persistence/base.py
+++ b/taskflow/persistence/base.py
@@ -119,6 +119,11 @@ class Connection(object):
         pass
 
     @abc.abstractmethod
+    def get_flows_for_book(self, book_uuid):
+        """Return an iterable of flowdetails for a given logbook uuid."""
+        pass
+
+    @abc.abstractmethod
     def get_flow_details(self, fd_uuid):
         """Fetches a flowdetails object matching the given uuid."""
         pass
diff --git a/taskflow/persistence/path_based.py b/taskflow/persistence/path_based.py
new file mode 100644
index 0000000..ea08025
--- /dev/null
+++ b/taskflow/persistence/path_based.py
@@ -0,0 +1,244 @@
+# -*- coding: utf-8 -*-
+
+#    Copyright (C) 2015 Rackspace Hosting All Rights Reserved.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+import abc
+import six
+
+from taskflow import exceptions as exc
+from taskflow.persistence import base
+from taskflow.persistence import logbook
+
+
+@six.add_metaclass(abc.ABCMeta)
+class PathBasedBackend(base.Backend):
+    """Base class for persistence backends that address data by path
+
+    Subclasses of this backend write logbooks, flow details, and atom details
+    to a provided base path in some filesystem-like storage. They will create
+    and store those objects in three key directories (one for logbooks, one
+    for flow details and one for atom details). They create those associated
+    directories and then create files inside those directories that represent
+    the contents of those objects for later reading and writing.
+    """
+
+    def __init__(self, conf):
+        super(PathBasedBackend, self).__init__(conf)
+        if conf is None:
+                conf = {}
+        self._path = conf.get('path', None)
+
+    @property
+    def path(self):
+        return self._path
+
+
+@six.add_metaclass(abc.ABCMeta)
+class PathBasedConnection(base.Connection):
+    def __init__(self, backend):
+        self._backend = backend
+        self._book_path = self._join_path(backend.path, "books")
+        self._flow_path = self._join_path(backend.path, "flow_details")
+        self._atom_path = self._join_path(backend.path, "atom_details")
+
+    @staticmethod
+    def _serialize(obj):
+        if isinstance(obj, logbook.LogBook):
+            return obj.to_dict(marshal_time=True)
+        elif isinstance(obj, logbook.FlowDetail):
+            return obj.to_dict()
+        elif isinstance(obj, logbook.AtomDetail):
+            return base._format_atom(obj)
+        else:
+            raise exc.StorageFailure("Invalid storage class %s" % type(obj))
+
+    @staticmethod
+    def _deserialize(cls, data):
+        if issubclass(cls, logbook.LogBook):
+            return cls.from_dict(data, unmarshal_time=True)
+        elif issubclass(cls, logbook.FlowDetail):
+            return cls.from_dict(data)
+        elif issubclass(cls, logbook.AtomDetail):
+            atom_class = logbook.atom_detail_class(data['type'])
+            return atom_class.from_dict(data['atom'])
+        else:
+            raise exc.StorageFailure("Invalid storage class %s" % cls)
+
+    @property
+    def backend(self):
+        return self._backend
+
+    @property
+    def book_path(self):
+        return self._book_path
+
+    @property
+    def flow_path(self):
+        return self._flow_path
+
+    @property
+    def atom_path(self):
+        return self._atom_path
+
+    @abc.abstractmethod
+    def _join_path(self, *parts):
+        """Accept path parts, and return a joined path"""
+
+    @abc.abstractmethod
+    def _get_item(self, path):
+        """Fetch a single item from the backend"""
+
+    @abc.abstractmethod
+    def _set_item(self, path, value, transaction):
+        """Write a single item to the backend"""
+
+    @abc.abstractmethod
+    def _del_tree(self, path, transaction):
+        """Recursively deletes a folder from the backend."""
+
+    @abc.abstractmethod
+    def _get_children(self, path):
+        """Get a list of child items of a path"""
+
+    @abc.abstractmethod
+    def _ensure_path(self, path):
+        """Recursively ensure that a path (folder) in the backend exists"""
+
+    @abc.abstractmethod
+    def _create_link(self, src_path, dest_path, transaction):
+        """Create a symlink-like link between two paths"""
+
+    @abc.abstractmethod
+    def _transaction(self):
+        """Context manager that yields a transaction"""
+
+    def _get_obj_path(self, obj):
+        if isinstance(obj, logbook.LogBook):
+            path = self.book_path
+        elif isinstance(obj, logbook.FlowDetail):
+            path = self.flow_path
+        elif isinstance(obj, logbook.AtomDetail):
+            path = self.atom_path
+        else:
+            raise exc.StorageFailure("Invalid storage class %s" % type(obj))
+        return self._join_path(path, obj.uuid)
+
+    def _update_object(self, obj, transaction, ignore_missing=False):
+        path = self._get_obj_path(obj)
+        try:
+            item_data = self._get_item(path)
+            existing_obj = self._deserialize(type(obj), item_data)
+            obj = existing_obj.merge(obj)
+        except exc.NotFound:
+            if not ignore_missing:
+                raise
+        self._set_item(path, self._serialize(obj), transaction)
+        return obj
+
+    def get_logbooks(self, lazy=False):
+        for book_uuid in self._get_children(self.book_path):
+            yield self.get_logbook(book_uuid, lazy)
+
+    def get_logbook(self, book_uuid, lazy=False):
+        book_path = self._join_path(self.book_path, book_uuid)
+        book_data = self._get_item(book_path)
+        book = self._deserialize(logbook.LogBook, book_data)
+        if not lazy:
+            for flow_details in self.get_flows_for_book(book_uuid):
+                book.add(flow_details)
+        return book
+
+    def save_logbook(self, book):
+        book_path = self._get_obj_path(book)
+        with self._transaction() as transaction:
+            self._update_object(book, transaction, ignore_missing=True)
+            for flow_details in book:
+                flow_path = self._get_obj_path(flow_details)
+                link_path = self._join_path(book_path, flow_details.uuid)
+                self._do_update_flow_details(flow_details, transaction,
+                                             ignore_missing=True)
+                self._create_link(flow_path, link_path, transaction)
+        return book
+
+    def get_flows_for_book(self, book_uuid, lazy=False):
+        book_path = self._join_path(self.book_path, book_uuid)
+        for flow_uuid in self._get_children(book_path):
+            yield self.get_flow_details(flow_uuid, lazy)
+
+    def get_flow_details(self, flow_uuid, lazy=False):
+        flow_path = self._join_path(self.flow_path, flow_uuid)
+        flow_data = self._get_item(flow_path)
+        flow_details = self._deserialize(logbook.FlowDetail, flow_data)
+        if not lazy:
+            for atom_details in self.get_atoms_for_flow(flow_uuid):
+                flow_details.add(atom_details)
+        return flow_details
+
+    def _do_update_flow_details(self, flow_detail, transaction,
+                                ignore_missing=False):
+        flow_path = self._get_obj_path(flow_detail)
+        self._update_object(flow_detail, transaction, ignore_missing)
+        for atom_details in flow_detail:
+            atom_path = self._get_obj_path(atom_details)
+            link_path = self._join_path(flow_path, atom_details.uuid)
+            self._create_link(atom_path, link_path, transaction)
+            self._update_object(atom_details, transaction, ignore_missing=True)
+        return flow_detail
+
+    def update_flow_details(self, flow_detail, ignore_missing=False):
+        with self._transaction() as transaction:
+            return self._do_update_flow_details(flow_detail, transaction,
+                                                ignore_missing)
+
+    def get_atoms_for_flow(self, flow_uuid):
+        flow_path = self._join_path(self.flow_path, flow_uuid)
+        for atom_uuid in self._get_children(flow_path):
+            yield self.get_atom_details(atom_uuid)
+
+    def get_atom_details(self, atom_uuid):
+        atom_path = self._join_path(self.atom_path, atom_uuid)
+        atom_data = self._get_item(atom_path)
+        return self._deserialize(logbook.AtomDetail, atom_data)
+
+    def update_atom_details(self, atom_detail, ignore_missing=False):
+        with self._transaction() as transaction:
+            return self._update_object(atom_detail, transaction,
+                                       ignore_missing)
+
+    def _do_destroy_logbook(self, book_uuid, transaction):
+        book_path = self._join_path(self.book_path, book_uuid)
+        for flow_uuid in self._get_children(book_path):
+            flow_path = self._join_path(self.flow_path, flow_uuid)
+            for atom_uuid in self._get_children(flow_path):
+                atom_path = self._join_path(self.atom_path, atom_uuid)
+                self._del_tree(atom_path, transaction)
+            self._del_tree(flow_path, transaction)
+        self._del_tree(book_path, transaction)
+
+    def destroy_logbook(self, book_uuid):
+        with self._transaction() as transaction:
+            return self._do_destroy_logbook(book_uuid, transaction)
+
+    def clear_all(self):
+        with self._transaction() as transaction:
+            for path in (self.book_path, self.flow_path, self.atom_path):
+                self._del_tree(path, transaction)
+
+    def upgrade(self):
+        for path in (self.book_path, self.flow_path, self.atom_path):
+            self._ensure_path(path)
+
+    def close(self):
+        pass