diff --git a/compose/parallel.py b/compose/parallel.py
index b3ca01530..f400b2235 100644
--- a/compose/parallel.py
+++ b/compose/parallel.py
@@ -64,18 +64,30 @@ def _no_deps(x):
     return []
 
 
+class State(object):
+    def __init__(self, objects):
+        self.objects = objects
+
+        self.started = set()   # objects being processed
+        self.finished = set()  # objects which have been processed
+        self.failed = set()    # objects which either failed or whose dependencies failed
+
+    def is_done(self):
+        return len(self.finished) + len(self.failed) >= len(self.objects)
+
+    def pending(self):
+        return set(self.objects) - self.started - self.finished - self.failed
+
+
 def parallel_execute_stream(objects, func, get_deps):
     if get_deps is None:
         get_deps = _no_deps
 
     results = Queue()
+    state = State(objects)
 
-    started = set()   # objects being processed
-    finished = set()  # objects which have been processed
-    failed = set()    # objects which either failed or whose dependencies failed
-
-    while len(finished) + len(failed) < len(objects):
-        for event in feed_queue(objects, func, get_deps, results, started, finished, failed):
+    while not state.is_done():
+        for event in feed_queue(objects, func, get_deps, results, state):
             yield event
 
         try:
@@ -89,10 +101,10 @@ def parallel_execute_stream(objects, func, get_deps):
         obj, _, exception = event
         if exception is None:
             log.debug('Finished processing: {}'.format(obj))
-            finished.add(obj)
+            state.finished.add(obj)
         else:
             log.debug('Failed: {}'.format(obj))
-            failed.add(obj)
+            state.failed.add(obj)
 
         yield event
 
@@ -105,26 +117,26 @@ def queue_producer(obj, func, results):
         results.put((obj, None, e))
 
 
-def feed_queue(objects, func, get_deps, results, started, finished, failed):
-    pending = set(objects) - started - finished - failed
+def feed_queue(objects, func, get_deps, results, state):
+    pending = state.pending()
     log.debug('Pending: {}'.format(pending))
 
     for obj in pending:
         deps = get_deps(obj)
 
-        if any(dep in failed for dep in deps):
+        if any(dep in state.failed for dep in deps):
             log.debug('{} has upstream errors - not processing'.format(obj))
             yield (obj, None, UpstreamError())
-            failed.add(obj)
+            state.failed.add(obj)
         elif all(
-            dep not in objects or dep in finished
+            dep not in objects or dep in state.finished
             for dep in deps
         ):
             log.debug('Starting producer thread for {}'.format(obj))
             t = Thread(target=queue_producer, args=(obj, func, results))
             t.daemon = True
             t.start()
-            started.add(obj)
+            state.started.add(obj)
 
 
 class UpstreamError(Exception):