Do proper dependency calculations

Change-Id: Ic8a9bd2105ebfff3604a45b45eac39da3b1bbcf5
Signed-off-by: Zane Bitter <zbitter@redhat.com>

heat/engine/cloud_watch.py

@@ -52,6 +52,8 @@ class CloudWatchAlarm(Resource):
                   'Megabits/Second', 'Gigabits/Second', 'Terabits/Second',
                   'Count/Second', None]}}
 
+    strict_dependency = False
+
     def __init__(self, name, json_snippet, stack):
         super(CloudWatchAlarm, self).__init__(name, json_snippet, stack)
         self.instance_id = ''
@@ -96,6 +98,3 @@ class CloudWatchAlarm(Resource):
 
     def FnGetRefId(self):
         return unicode(self.name)
-
-    def strict_dependency(self):
-        return False

heat/engine/dependencies.py

@@ -0,0 +1,201 @@
+# vim: tabstop=4 shiftwidth=4 softtabstop=4
+
+#
+#    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 collections
+import itertools
+
+from heat.common import exception
+
+
+class CircularDependencyException(exception.OpenstackException):
+    message = _("Circular Dependency Found: %(cycle)s")
+
+
+class Dependencies(object):
+    '''Helper class for calculating a dependency graph'''
+
+    class Node(object):
+        def __init__(self, requires=None, required_by=None):
+            '''
+            Initialise the node, optionally with a set of keys this node
+            requires and/or a set of keys that this node is required by.
+            '''
+            self.require = requires and requires.copy() or set()
+            self.satisfy = required_by and required_by.copy() or set()
+
+        def copy(self):
+            '''Make a copy of the node'''
+            return Dependencies.Node(self.require, self.satisfy)
+
+        def reverse_copy(self):
+            '''Make a copy of the node with the edge directions reversed'''
+            return Dependencies.Node(self.satisfy, self.require)
+
+        def required_by(self, source=None):
+            '''
+            List the keys that require this node, and optionally add a
+            new one
+            '''
+            if source is not None:
+                self.satisfy.add(source)
+            return iter(self.satisfy)
+
+        def requires(self, target):
+            '''Add a key that this node requires'''
+            self.require.add(target)
+
+        def __isub__(self, target):
+            '''Remove a key that this node requires'''
+            self.require.remove(target)
+            return self
+
+        def __nonzero__(self):
+            '''Test if this node is a leaf (requires nothing)'''
+            return bool(self.require)
+
+        def stem(self):
+            '''Test if this node is a stem (required by nothing)'''
+            return not bool(self.satisfy)
+
+        def disjoint(self):
+            '''Test if this node is both a lead and a stem'''
+            return self and self.stem()
+
+        def __len__(self):
+            '''Count the number of keys required by this node'''
+            return len(self.require)
+
+        def __iter__(self):
+            '''Iterate over the keys required by this node'''
+            return iter(self.require)
+
+        def __str__(self):
+            '''Return a human-readable string representation of the node'''
+            return '{%s}' % ', '.join(str(n) for n in self)
+
+        def __repr__(self):
+            '''Return a string representation of the node'''
+            return repr(self.require)
+
+    def __init__(self, edges=[]):
+        '''
+        Initialise, optionally with a list of edges, in the form of
+        (requirer, required) tuples.
+        '''
+        self.deps = collections.defaultdict(self.Node)
+        for e in edges:
+            self += e
+
+    def __iadd__(self, edge):
+        '''Add another edge, in the form of a (requirer, required) tuple'''
+        requirer, required = edge
+
+        if required is None:
+            # Just ensure the node is created by accessing the defaultdict
+            self.deps[requirer]
+        else:
+            self.deps[required].required_by(requirer)
+            self.deps[requirer].requires(required)
+
+        return self
+
+    def __getitem__(self, last):
+        '''
+        Return a partial dependency graph consisting of the specified node and
+        all those that require it only.
+        '''
+        if last not in self.deps:
+            raise KeyError
+
+        def get_edges(key):
+            def requirer_edges(rqr):
+                # Concatenate the dependency on the current node with the
+                # recursive generated list
+                return itertools.chain([(rqr, key)], get_edges(rqr))
+
+            # Get the edge list for each node that requires the current node
+            edge_lists = itertools.imap(requirer_edges,
+                                        self.deps[key].required_by())
+            # Combine the lists into one long list
+            return itertools.chain.from_iterable(edge_lists)
+
+        if self.deps[last].stem():
+            # Nothing requires this, so just add the node itself
+            edges = [(last, None)]
+        else:
+            edges = get_edges(last)
+
+        return Dependencies(edges)
+
+    @staticmethod
+    def _deps_to_str(deps):
+        '''Convert the given dependency graph to a human-readable string'''
+        pairs = ('%s: %s' % (str(k), str(v)) for k, v in deps.items())
+        return '{%s}' % ', '.join(pairs)
+
+    def __str__(self):
+        '''
+        Return a human-readable string representation of the dependency graph
+        '''
+        return self._deps_to_str(self.deps)
+
+    def _edges(self):
+        '''Return an iterator over all of the edges in the graph'''
+        def outgoing_edges(rqr, node):
+            if node.disjoint():
+                yield (rqr, None)
+            else:
+                for rqd in node:
+                    yield (rqr, rqd)
+        return (outgoing_edges(*item) for item in self.deps.iteritems())
+
+    def __repr__(self):
+        '''Return a string representation of the object'''
+        return 'Dependencies([%s])' % ', '.join(repr(e) for e in edges)
+
+    def _toposort(self, deps):
+        '''Generate a topological sort of a dependency graph'''
+        def next_leaf():
+            for leaf, node in deps.items():
+                if not node:
+                    return leaf, node
+
+            # There are nodes remaining, but no more leaves: a cycle
+            cycle = self._deps_to_str(deps)
+            raise CircularDependencyException(cycle=cycle)
+
+        for iteration in xrange(len(deps)):
+            leaf, node = next_leaf()
+            yield leaf
+
+            # Remove the node and all edges connected to it before continuing
+            # to look for more leaves
+            for src in node.required_by():
+                deps[src] -= leaf
+            del deps[leaf]
+
+    def _mapgraph(self, func):
+        '''Map the supplied function onto every node in the graph.'''
+        return dict((k, func(n)) for k, n in self.deps.items())
+
+    def __iter__(self):
+        '''Return a topologically sorted iterator'''
+        deps = self._mapgraph(lambda n: n.copy())
+        return self._toposort(deps)
+
+    def __reversed__(self):
+        '''Return a reverse topologically sorted iterator'''
+        rev_deps = self._mapgraph(lambda n: n.reverse_copy())
+        return self._toposort(rev_deps)

heat/engine/manager.py

@@ -381,7 +381,7 @@ class EngineManager(manager.Manager):
         """
         stack = db_api.stack_get(context, stack_name)
         if stack:
-            return [r.name for r in stack.resources]
+            return [res.name for res in stack]
         else:
             return None
 
@@ -464,7 +464,7 @@ class EngineManager(manager.Manager):
                                       s.raw_template.parsed_template.template,
                                       s.id)
                     for a in wr.rule[action_map[new_state]]:
-                        ps.resources[a].alarm()
+                        ps[a].alarm()
 
         wr.last_evaluated = now
 

heat/engine/parser.py

@@ -17,8 +17,10 @@ import eventlet
 import json
 import itertools
 import logging
+
 from heat.common import exception
 from heat.engine import checkeddict
+from heat.engine import dependencies
 from heat.engine.resources import Resource
 from heat.db import api as db_api
 
@@ -69,41 +71,64 @@ class Stack(object):
         if parms is not None:
             self._apply_user_parameters(parms)
 
-        self.resources = {}
+        self.resources = dict((name,
-        for rname, rdesc in self.t['Resources'].items():
+                               Resource(name, data, self))
-            res = Resource(rname, rdesc, self)
+                              for (name, data) in self.t['Resources'].items())
-            self.resources[rname] = res
 
-            self.calulate_dependencies(res.t, res)
+        self.dependencies = dependencies.Dependencies()
+        for resource in self.resources.values():
+            resource.add_dependencies(self.dependencies)
+
+    def __iter__(self):
+        '''
+        Return an iterator over this template's resources in the order that
+        they should be started.
+        '''
+        return iter(self.dependencies)
+
+    def __reversed__(self):
+        '''
+        Return an iterator over this template's resources in the order that
+        they should be stopped.
+        '''
+        return reversed(self.dependencies)
+
+    def __len__(self):
+        '''Return the number of resources'''
+        return len(self.resources)
+
+    def __getitem__(self, key):
+        '''Get the resource with the specified name.'''
+        return self.resources[key]
+
+    def __contains__(self, key):
+        '''Determine whether the stack contains the specified resource'''
+        return key in self.resources
+
+    def keys(self):
+        return self.resources.keys()
+
+    def __str__(self):
+        return 'Stack "%s"' % self.name
 
     def validate(self):
         '''
-        http://docs.amazonwebservices.com/AWSCloudFormation/latest/ \
+        http://docs.amazonwebservices.com/AWSCloudFormation/latest/\
         APIReference/API_ValidateTemplate.html
         '''
         # TODO(sdake) Should return line number of invalid reference
 
         response = None
-        try:
-            order = self.get_create_order()
-        except KeyError as ex:
-            res = 'A Ref operation referenced a non-existent key '\
-                  '[%s]' % str(ex)
 
-            response = {'ValidateTemplateResult': {
+        for res in self:
-                        'Description': 'Malformed Query Response [%s]' % (res),
-                        'Parameters': []}}
-            return response
-
-        for r in order:
             try:
-                res = self.resources[r].validate()
+                result = res.validate()
             except Exception as ex:
                 logger.exception('validate')
-                res = str(ex)
+                result = str(ex)
-            finally:
+
-                if res:
+            if result:
-                    err_str = 'Malformed Query Response %s' % (res)
+                err_str = 'Malformed Query Response %s' % result
                 response = {'ValidateTemplateResult': {
                                 'Description': err_str,
                                 'Parameters': []}}
@@ -123,33 +148,6 @@ class Stack(object):
             response['ValidateTemplateResult']['Parameters'].append(res)
         return response
 
-    def resource_append_deps(self, resource, order_list):
-        '''
-        For the given resource first append it's dependancies then
-        it's self to order_list.
-        '''
-        for r in resource.depends_on:
-            self.resource_append_deps(self.resources[r], order_list)
-        if not resource.name in order_list:
-            order_list.append(resource.name)
-
-    def get_create_order(self):
-        '''
-        return a list of Resource names in the correct order
-        for startup.
-        '''
-        order = []
-        for r in self.t['Resources']:
-            if self.t['Resources'][r]['Type'] == 'AWS::EC2::Volume' or \
-               self.t['Resources'][r]['Type'] == 'AWS::EC2::EIP':
-                if len(self.resources[r].depends_on) == 0:
-                    order.append(r)
-
-        for r in self.t['Resources']:
-            self.resource_append_deps(self.resources[r], order)
-
-        return order
-
     def update_parsed_template(self):
         '''
         Update the parsed template after each resource has been
@@ -180,9 +178,8 @@ class Stack(object):
 
     def create_blocking(self):
         '''
-        create all the resources in the order specified by get_create_order
+        Create the stack and all of the resources.
         '''
-        order = self.get_create_order()
         self.status_set(self.IN_PROGRESS, 'Stack creation started')
 
         stack_status = self.CREATE_COMPLETE
@@ -197,8 +194,7 @@ class Stack(object):
                 logger.exception('create timeout conversion')
         tmo = eventlet.Timeout(secs_tmo)
         try:
-            for r in order:
+            for res in self:
-                res = self.resources[r]
                 if stack_status != self.CREATE_FAILED:
                     try:
                         res.create()
@@ -236,24 +232,21 @@ class Stack(object):
 
     def delete_blocking(self):
         '''
-        delete all the resources in the reverse order specified by
+        Delete all of the resources, and then the stack itself.
-        get_create_order().
         '''
-        order = self.get_create_order()
         failed = False
         self.status_set(self.DELETE_IN_PROGRESS)
 
-        for r in reversed(order):
+        for res in reversed(self):
-            res = self.resources[r]
             try:
                 res.delete()
             except Exception as ex:
                 failed = True
                 res.state_set(res.DELETE_FAILED)
                 logger.error('delete: %s' % str(ex))
+            else:
                 try:
-                re = db_api.resource_get(self.context, self.resources[r].id)
+                    db_api.resource_get(self.context, res.id).delete()
-                re.delete()
                 except Exception as ex:
                     # don't fail the delete if the db entry has
                     # not been created yet.
@@ -292,23 +285,20 @@ class Stack(object):
             if stack:
                 self.parsed_template_id = stack.raw_template.parsed_template.id
 
-        order = []
+        deps = self.dependencies[self[resource_name]]
-        self.resource_append_deps(self.resources[resource_name], order)
         failed = False
 
-        for r in reversed(order):
+        for res in reversed(deps):
-            res = self.resources[r]
             try:
                 res.delete()
-                re = db_api.resource_get(self.context, self.resources[r].id)
+                re = db_api.resource_get(self.context, res.id)
                 re.delete()
             except Exception as ex:
                 failed = True
                 res.state_set(res.DELETE_FAILED)
                 logger.error('delete: %s' % str(ex))
 
-        for r in order:
+        for res in deps:
-            res = self.resources[r]
             if not failed:
                 try:
                     res.create()
@@ -331,26 +321,6 @@ class Stack(object):
         pool = eventlet.GreenPool()
         pool.spawn_n(self.restart_resource_blocking, resource_name)
 
-    def calulate_dependencies(self, s, r):
-        if isinstance(s, dict):
-            for i in s:
-                if i == 'Fn::GetAtt':
-                    #print '%s seems to depend on %s' % (r.name, s[i][0])
-                    #r.depends_on.append(s[i][0])
-                    pass
-                elif i == 'Ref':
-                    #print '%s Refences %s' % (r.name, s[i])
-                    if r.strict_dependency():
-                        r.depends_on.append(s[i])
-                elif i == 'DependsOn':
-                    #print '%s DependsOn on %s' % (r.name, s[i])
-                    r.depends_on.append(s[i])
-                else:
-                    self.calulate_dependencies(s[i], r)
-        elif isinstance(s, list):
-            for index, item in enumerate(s):
-                self.calulate_dependencies(item, r)
-
     def _apply_user_parameters(self, parms):
         for p in parms:
             if 'Parameters.member.' in p and 'ParameterKey' in p:
@@ -358,7 +328,7 @@ class Stack(object):
                 try:
                     key_name = 'Parameters.member.%s.ParameterKey' % s[2]
                     value_name = 'Parameters.member.%s.ParameterValue' % s[2]
-                    logger.debug('appling user parameter %s=%s' %
+                    logger.debug('applying user parameter %s=%s' %
                                  (key_name, value_name))
                     self.parms[parms[key_name]] = parms[value_name]
                 except Exception:
@@ -402,15 +372,15 @@ class Stack(object):
             { "Fn::GetAtt" : [ "DBInstance", "Endpoint.Address" ] }
         '''
         def match_ref(key, value):
-            return key == 'Ref' and value in self.resources
+            return key == 'Ref' and value in self
 
         def handle_ref(arg):
-            return self.resources[arg].FnGetRefId()
+            return self[arg].FnGetRefId()
 
         def handle_getatt(args):
             resource, att = args
             try:
-                return self.resources[resource].FnGetAtt(att)
+                return self[resource].FnGetAtt(att)
             except KeyError:
                 raise exception.InvalidTemplateAttribute(resource=resource,
                                                          key=att)

heat/engine/resources.py

@@ -42,6 +42,9 @@ class Resource(object):
     UPDATE_FAILED = 'UPDATE_FAILED'
     UPDATE_COMPLETE = 'UPDATE_COMPLETE'
 
+    # If True, this resource must be created before it can be referenced.
+    strict_dependency = True
+
     def __new__(cls, name, json, stack):
         '''Create a new Resource of the appropriate class for its type.'''
 
@@ -55,7 +58,6 @@ class Resource(object):
         return ResourceClass(name, json, stack)
 
     def __init__(self, name, json_snippet, stack):
-        self.depends_on = []
         self.references = []
         self.stack = stack
         self.name = name
@@ -82,6 +84,26 @@ class Resource(object):
     def parsed_template(self):
         return self.stack.resolve_runtime_data(self.t)
 
+    def __str__(self):
+        return '%s "%s"' % (self.__class__.__name__, self.name)
+
+    def _add_dependencies(self, deps, fragment):
+        if isinstance(fragment, dict):
+            for key, value in fragment.items():
+                if key in ('DependsOn', 'Ref'):
+                    target = self.stack.resources[value]
+                    if key == 'DependsOn' or target.strict_dependency:
+                        deps += (self, target)
+                elif key != 'Fn::GetAtt':
+                    self._add_dependencies(deps, value)
+        elif isinstance(fragment, list):
+            for item in fragment:
+                self._add_dependencies(deps, item)
+
+    def add_dependencies(self, deps):
+        self._add_dependencies(deps, self.t)
+        deps += (self, None)
+
     def keystone(self):
         if self._keystone:
             return self._keystone
@@ -187,7 +209,7 @@ class Resource(object):
 
     def FnGetRefId(self):
         '''
-        http://docs.amazonwebservices.com/AWSCloudFormation/latest/UserGuide/ \
+        http://docs.amazonwebservices.com/AWSCloudFormation/latest/UserGuide/\
         intrinsic-function-reference-ref.html
         '''
         if self.instance_id is not None:
@@ -197,24 +219,18 @@ class Resource(object):
 
     def FnGetAtt(self, key):
         '''
-        http://docs.amazonwebservices.com/AWSCloudFormation/latest/UserGuide/ \
+        http://docs.amazonwebservices.com/AWSCloudFormation/latest/UserGuide/\
         intrinsic-function-reference-getatt.html
         '''
         return unicode(self.name)
 
     def FnBase64(self, data):
         '''
-        http://docs.amazonwebservices.com/AWSCloudFormation/latest/UserGuide/ \
+        http://docs.amazonwebservices.com/AWSCloudFormation/latest/UserGuide/\
             intrinsic-function-reference-base64.html
         '''
         return base64.b64encode(data)
 
-    def strict_dependency(self):
-        '''
-        If True, this resource must be created before it can be referenced.
-        '''
-        return True
-
 
 class GenericResource(Resource):
     properties_schema = {}

heat/tests/test_dependencies.py

@@ -0,0 +1,186 @@
+import nose
+import unittest
+from nose.plugins.attrib import attr
+
+from heat.engine.dependencies import Dependencies
+from heat.engine.dependencies import CircularDependencyException
+
+
+@attr(tag=['unit', 'dependencies'])
+@attr(speed='fast')
+class dependenciesTest(unittest.TestCase):
+
+    def _dep_test(self, func, checkorder, deps):
+        nodes = set.union(*[set(e) for e in deps])
+
+        d = Dependencies(deps)
+        order = list(func(d))
+
+        for n in nodes:
+            self.assertTrue(n in order, '"%s" is not in the sequence' % n)
+            self.assertEqual(order.count(n), 1)
+
+        self.assertEqual(len(order), len(nodes))
+
+        for l, f in deps:
+            checkorder(order.index(f), order.index(l))
+
+    def _dep_test_fwd(self, *deps):
+        def assertLess(a, b):
+            self.assertTrue(a < b,
+                            '"%s" is not less than "%s"' % (str(a), str(b)))
+        self._dep_test(iter, assertLess, deps)
+
+    def _dep_test_rev(self, *deps):
+        def assertGreater(a, b):
+            self.assertTrue(a > b,
+                            '"%s" is not greater than "%s"' % (str(a), str(b)))
+        self._dep_test(reversed, assertGreater, deps)
+
+    def test_single_node(self):
+        d = Dependencies([('only', None)])
+        l = list(iter(d))
+        self.assertEqual(len(l), 1)
+        self.assertEqual(l[0], 'only')
+
+    def test_disjoint(self):
+        d = Dependencies([('1', None), ('2', None)])
+        l = list(iter(d))
+        self.assertEqual(len(l), 2)
+        self.assertTrue('1' in l)
+        self.assertTrue('2' in l)
+
+    def test_single_fwd(self):
+        self._dep_test_fwd(('second', 'first'))
+
+    def test_single_rev(self):
+        self._dep_test_rev(('second', 'first'))
+
+    def test_chain_fwd(self):
+        self._dep_test_fwd(('third', 'second'), ('second', 'first'))
+
+    def test_chain_rev(self):
+        self._dep_test_rev(('third', 'second'), ('second', 'first'))
+
+    def test_diamond_fwd(self):
+        self._dep_test_fwd(('last', 'mid1'), ('last', 'mid2'),
+                           ('mid1', 'first'), ('mid2', 'first'))
+
+    def test_diamond_rev(self):
+        self._dep_test_rev(('last', 'mid1'), ('last', 'mid2'),
+                           ('mid1', 'first'), ('mid2', 'first'))
+
+    def test_complex_fwd(self):
+        self._dep_test_fwd(('last', 'mid1'), ('last', 'mid2'),
+                           ('mid1', 'mid3'), ('mid1', 'first'),
+                           ('mid3', 'first'), ('mid2', 'first'))
+
+    def test_complex_rev(self):
+        self._dep_test_rev(('last', 'mid1'), ('last', 'mid2'),
+                           ('mid1', 'mid3'), ('mid1', 'first'),
+                           ('mid3', 'first'), ('mid2', 'first'))
+
+    def test_many_edges_fwd(self):
+        self._dep_test_fwd(('last', 'e1'), ('last', 'mid1'), ('last', 'mid2'),
+                           ('mid1', 'e2'), ('mid1', 'mid3'),
+                           ('mid2', 'mid3'),
+                           ('mid3', 'e3'))
+
+    def test_many_edges_rev(self):
+        self._dep_test_rev(('last', 'e1'), ('last', 'mid1'), ('last', 'mid2'),
+                           ('mid1', 'e2'), ('mid1', 'mid3'),
+                           ('mid2', 'mid3'),
+                           ('mid3', 'e3'))
+
+    def test_dbldiamond_fwd(self):
+        self._dep_test_fwd(('last', 'a1'), ('last', 'a2'),
+                           ('a1', 'b1'), ('a2', 'b1'), ('a2', 'b2'),
+                           ('b1', 'first'), ('b2', 'first'))
+
+    def test_dbldiamond_rev(self):
+        self._dep_test_rev(('last', 'a1'), ('last', 'a2'),
+                           ('a1', 'b1'), ('a2', 'b1'), ('a2', 'b2'),
+                           ('b1', 'first'), ('b2', 'first'))
+
+    def test_circular_fwd(self):
+        d = Dependencies([('first', 'second'),
+                          ('second', 'third'),
+                          ('third', 'first')])
+        self.assertRaises(CircularDependencyException, list, iter(d))
+
+    def test_circular_rev(self):
+        d = Dependencies([('first', 'second'),
+                          ('second', 'third'),
+                          ('third', 'first')])
+        self.assertRaises(CircularDependencyException, list, reversed(d))
+
+    def test_self_ref(self):
+        d = Dependencies([('node', 'node')])
+        self.assertRaises(CircularDependencyException, list, iter(d))
+
+    def test_complex_circular_fwd(self):
+        d = Dependencies([('last', 'e1'), ('last', 'mid1'), ('last', 'mid2'),
+                          ('mid1', 'e2'), ('mid1', 'mid3'),
+                          ('mid2', 'mid3'),
+                          ('mid3', 'e3'),
+                          ('e3', 'mid1')])
+        self.assertRaises(CircularDependencyException, list, iter(d))
+
+    def test_complex_circular_rev(self):
+        d = Dependencies([('last', 'e1'), ('last', 'mid1'), ('last', 'mid2'),
+                          ('mid1', 'e2'), ('mid1', 'mid3'),
+                          ('mid2', 'mid3'),
+                          ('mid3', 'e3'),
+                          ('e3', 'mid1')])
+        self.assertRaises(CircularDependencyException, list, reversed(d))
+
+    def test_noexist_partial(self):
+        d = Dependencies([('foo', 'bar')])
+        get = lambda i: d[i]
+        self.assertRaises(KeyError, get, 'baz')
+
+    def test_single_partial(self):
+        d = Dependencies([('last', 'first')])
+        p = d['last']
+        l = list(iter(p))
+        self.assertEqual(len(l), 1)
+        self.assertEqual(l[0], 'last')
+
+    def test_simple_partial(self):
+        d = Dependencies([('last', 'middle'), ('middle', 'first')])
+        p = d['middle']
+        order = list(iter(p))
+        self.assertEqual(len(order), 2)
+        for n in ('last', 'middle'):
+            self.assertTrue(n in order,
+                            "'%s' not found in dependency order" % n)
+        self.assertTrue(order.index('last') > order.index('middle'))
+
+    def test_simple_multilevel_partial(self):
+        d = Dependencies([('last', 'middle'),
+                          ('middle', 'target'),
+                          ('target', 'first')])
+        p = d['target']
+        order = list(iter(p))
+        self.assertEqual(len(order), 3)
+        for n in ('last', 'middle', 'target'):
+            self.assertTrue(n in order,
+                            "'%s' not found in dependency order" % n)
+
+    def test_complex_partial(self):
+        d = Dependencies([('last', 'e1'), ('last', 'mid1'), ('last', 'mid2'),
+                          ('mid1', 'e2'), ('mid1', 'mid3'),
+                          ('mid2', 'mid3'),
+                          ('mid3', 'e3')])
+        p = d['mid3']
+        order = list(iter(p))
+        self.assertEqual(len(order), 4)
+        for n in ('last', 'mid1', 'mid2', 'mid3'):
+            self.assertTrue(n in order,
+                            "'%s' not found in dependency order" % n)
+
+
+# allows testing of the test directly, shown below
+if __name__ == '__main__':
+    sys.argv.append(__file__)
+    nose.main()