manager: basic scheduler w/ mock monitor data

circle/manager/scheduler.py

+from django.db.models import Sum
+
+
+class NotEnoughMemoryException(Exception):
+    def __init__(self, message=None):
+        if message is None:
+            message = "No node has enough memory to accomodate the guest."
+
+        Exception.__init__(self, message)
+
+
+class TraitsUnsatisfiableException(Exception):
+    def __init__(self, message=None):
+        if message is None:
+            message = "No node can satisfy all required traits of the guest."
+
+        Exception.__init__(self, message)
+
+
 def get_node(instance, nodes):
     ''' Select a node for hosting an instance based on its requirements.
     '''
-    # Return first Node or None
-    try:
-        req_traits = set(instance.req_traits.all())
-        nodes = [n for n in nodes if req_traits.issubset(n.traits.all())]
+    # check required traits
+    nodes = [n for n in nodes if has_traits(instance.req_traits.all(), n)]
+    if not nodes:
+        raise TraitsUnsatisfiableException()
+
+    # check required RAM
+    nodes = [n for n in nodes if has_enough_ram(instance.ram_size, n)]
+    if not nodes:
+        raise NotEnoughMemoryException()
+
+    # sort nodes first by processor usage, then priority
+    nodes.sort(key=lambda n: n.priority, reverse=True)
+    nodes.sort(key=processor_usage)
+
     return nodes[0]
-    except:
-        return None
+
+
+def has_traits(traits, node):
+    """True, if the node has all specified traits; otherwise, false.
+    """
+    traits = set(traits)
+    return traits.issubset(node.traits.all())
+
+
+def has_enough_ram(ram_size, node):
+    """True, if the node has enough memory to accomodate a guest requiring
+       ram_size mebibytes of memory; otherwise, false.
+    """
+    total = node.ram_size
+    used = 512  # TODO replace mock value with data from monitor
+    unused = total - used
+
+    overcommit = node.ram_size_with_overcommit
+    reserved = node.instance_set.aggregate(r=Sum('ram_size'))['r'] or 0
+    free = overcommit - reserved
+
+    return ram_size < unused and ram_size < free
+
+
+def processor_usage(node):
+    """Get a number indicating processor usage on the node.
+
+    Lower values indicate lower usage.
+    """
+    return 0  # TODO replace mock value with data from monitor