diff --git a/xos/api/utility/onboarding.py b/xos/api/utility/onboarding.py
index 1144387..c04be52 100644
--- a/xos/api/utility/onboarding.py
+++ b/xos/api/utility/onboarding.py
@@ -69,7 +69,7 @@
 
             result.append( ("XOS", self.is_ready(xos)) )
 
-            for sc in xos.service_controllers.all():
+            for sc in xos.loadable_modules.all():
                 result.append( (sc.name, self.is_ready(sc)) )
 
         result = "\n".join( ["%s: %s" % (x[0], x[1]) for x in result] )
@@ -86,7 +86,7 @@
         xos=xos[0]
 
         result = []
-        for sc in xos.service_controllers.all():
+        for sc in xos.loadable_modules.all():
             result.append(sc.name)
 
         return HttpResponse( json.dumps(result), content_type="application/javascript")
@@ -98,7 +98,7 @@
 
         xos=xos[0]
 
-        sc=xos.service_controllers.filter(name=service)
+        sc=xos.loadable_modules.filter(name=service)
         if not sc:
             return HttpResponse("Not Found", status_code=404)
 
diff --git a/xos/core/admin.py b/xos/core/admin.py
index fa28734..83492ab 100644
--- a/xos/core/admin.py
+++ b/xos/core/admin.py
@@ -1032,10 +1032,9 @@
 
 
 class ServiceAdmin(XOSBaseAdmin):
-    list_display = ("backend_status_icon", "name", "kind",
-                    "versionNumber", "enabled", "published")
+    list_display = ("backend_status_icon", "name", "kind", "enabled", "published")
     list_display_links = ('backend_status_icon', 'name', )
-    fieldList = ["backend_status_text", "name", "kind", "description", "controller", "versionNumber", "enabled", "published",
+    fieldList = ["backend_status_text", "name", "kind", "description", "controller", "enabled", "published",
                  "view_url", "icon_url", "public_key", "private_key_fn", "service_specific_attribute", "service_specific_id"]
     fieldsets = [
         (None, {'fields': fieldList, 'classes': ['suit-tab suit-tab-general']})]
@@ -1061,7 +1060,7 @@
 class ServiceControllerAdmin(XOSBaseAdmin):
     list_display = ("backend_status_icon", "name",)
     list_display_links = ('backend_status_icon', 'name',)
-    fieldList = ["backend_status_text", "name", "xos", "base_url", "synchronizer_run", "synchronizer_config", "no_start"]
+    fieldList = ["backend_status_text", "name", "xos", "version", "provides", "requires", "base_url", "synchronizer_run", "synchronizer_config", "no_start"]
     fieldsets = [
         (None, {'fields': fieldList, 'classes': ['suit-tab suit-tab-general']})]
     inlines = [ServiceControllerResourceInline]
diff --git a/xos/core/models/__init__.py b/xos/core/models/__init__.py
index 3630d9d..abf8da2 100644
--- a/xos/core/models/__init__.py
+++ b/xos/core/models/__init__.py
@@ -4,7 +4,7 @@
 from .xosmodel import XOS, XOSVolume
 from .service import Service, Tenant, TenantWithContainer, CoarseTenant, ServicePrivilege, TenantRoot, TenantRootPrivilege, TenantRootRole, TenantPrivilege, TenantRole, Subscriber, Provider
 from .service import ServiceAttribute, TenantAttribute, ServiceRole
-from .service import ServiceController, ServiceControllerResource
+from .service import ServiceController, ServiceControllerResource, LoadableModule, LoadableModuleResource
 from .tag import Tag
 from .role import Role
 from .site import Site, Deployment, DeploymentRole, DeploymentPrivilege, Controller, ControllerRole, ControllerSite, SiteDeployment,Diag
diff --git a/xos/core/models/service.py b/xos/core/models/service.py
index a9b3aea..2685fe4 100644
--- a/xos/core/models/service.py
+++ b/xos/core/models/service.py
@@ -1,5 +1,8 @@
 import json
+import operator
+
 from operator import attrgetter
+from distutils.version import LooseVersion
 
 from core.models import PlCoreBase, PlCoreBaseManager, SingletonModel, XOS
 from core.models.plcorebase import StrippedCharField
@@ -16,7 +19,6 @@
        return xos[0]
     else:
        return None
-
 
 class AttributeMixin(object):
     # helper for extracting things from a json-encoded
@@ -65,27 +67,90 @@
                                             None,
                                             attrname))
 
-class ServiceController(PlCoreBase):
-    xos = models.ForeignKey(XOS, related_name='service_controllers', help_text="Pointer to XOS", default=get_xos)
+class LoadableModule(PlCoreBase):
+    xos = models.ForeignKey(XOS, related_name='loadable_modules', help_text="Pointer to XOS", default=get_xos)
     name = StrippedCharField(max_length=30, help_text="Service Name")
     base_url = StrippedCharField(max_length=1024, help_text="Base URL, allows use of relative URLs for resources", null=True, blank=True)
 
-    synchronizer_run = StrippedCharField(max_length=1024, help_text="synchronizer run command", null=True, blank=True)
-    synchronizer_config = StrippedCharField(max_length=1024, help_text="synchronizer config file", null=True, blank=True)
-
-    no_start = models.BooleanField(help_text="Do not start the XOS UI inside of the UI docker container", default=False)
+    version = StrippedCharField(blank=True, null=True,
+        max_length=30, help_text="Version of Service Controller", default = "1.0.0")
+    provides = StrippedCharField(blank=True, null=True,
+        max_length=254, help_text="Comma-separated list of things provided")
+    requires = StrippedCharField(blank=True, null=True,
+        max_length=254, help_text="Comma-separated list of required Service Controllers")
 
     def __unicode__(self): return u'%s' % (self.name)
 
     def save(self, *args, **kwargs):
-       super(ServiceController, self).save(*args, **kwargs)
+       super(LoadableModule, self).save(*args, **kwargs)
 
        if self.xos:
            # force XOS to rebuild
            # XXX somewhat hackish XXX
            self.xos.save(update_fields=["updated"])
 
-class ServiceControllerResource(PlCoreBase):
+    def get_provides_list(self):
+        prov_list = []
+        if self.provides and self.provides.strip():
+            for prov in self.provides.split(","):
+                prov=prov.strip()
+                if "=" in prov:
+                    (name, version) = prov.split("=",1)
+                    name = name.strip()
+                    version = version.strip()
+                else:
+                    name = prov
+                    version = "1.0.0"
+                prov_list.append( {"name": name, "version": version} )
+
+        # every controller provides itself
+        prov_list.append( {"name": self.name, "version": self.version} )
+
+        return prov_list
+
+
+    @classmethod
+    def dependency_check(cls, dep_list):
+        missing = []
+        satisfied = []
+        operators = {">=": operator.ge,
+                     "<=": operator.le,
+                     ">": operator.gt,
+                     "<": operator.lt,
+                     "!=": operator.ne,
+                     "=": operator.eq}
+        for dep in dep_list:
+            dep = dep.strip()
+            name = dep
+            version = None
+            this_op = None
+            for op in operators.keys():
+                if op in dep:
+                    (name, version) = dep.split(op,1)
+                    name = name.strip()
+                    version = version.strip()
+                    this_op = operators[op]
+                    break
+            found=False
+            scs = ServiceController.objects.all()
+            for sc in scs:
+                for provide in sc.get_provides_list():
+                    if (provide["name"] != name):
+                        continue
+                    if not this_op:
+                        satisfied.append(sc)
+                        found=True
+                        break
+                    elif this_op(LooseVersion(provide["version"]), LooseVersion(version)):
+                        satisfied.append(sc)
+                        found=True
+                        break
+            if not found:
+                missing.append(dep)
+
+        return (satisfied, missing)
+
+class LoadableModuleResource(PlCoreBase):
     KIND_CHOICES = (('models', 'Models'),
                     ('admin', 'Admin'),
                     ('admin_template', 'Admin Template'),
@@ -104,8 +169,8 @@
                       ('yaml', 'YAML'),
                       ('raw', 'raw'))
 
-    service_controller = models.ForeignKey(ServiceController, related_name='service_controller_resources',
-                                help_text="The Service Controller this resource is associated with")
+    loadable_module = models.ForeignKey(LoadableModule, related_name='loadable_module_resources',
+                                help_text="The Loadable Module this resource is associated with")
 
     name = StrippedCharField(max_length=30, help_text="Object Name")
     subdirectory = StrippedCharField(max_length=1024, help_text="optional subdirectory", null=True, blank=True)
@@ -117,11 +182,21 @@
 
     @property
     def full_url(self):
-        if self.service_controller and self.service_controller.base_url:
-            return urlparse.urljoin(self.service_controller.base_url, self.url)
+        if self.loadable_module and self.loadable_module.base_url:
+            return urlparse.urljoin(self.loadable_module.base_url, self.url)
         else:
             return self.url
 
+class ServiceController(LoadableModule):
+    synchronizer_run = StrippedCharField(max_length=1024, help_text="synchronizer run command", null=True, blank=True)
+    synchronizer_config = StrippedCharField(max_length=1024, help_text="synchronizer config file", null=True, blank=True)
+
+    no_start = models.BooleanField(help_text="Do not start the XOS UI inside of the UI docker container", default=False)
+
+class ServiceControllerResource(LoadableModuleResource):
+    class Meta:
+        proxy = True
+
 class Service(PlCoreBase, AttributeMixin):
     # when subclassing a service, redefine KIND to describe the new service
     KIND = "generic"
@@ -133,7 +208,7 @@
         max_length=30, help_text="Kind of service", default=KIND)
     name = StrippedCharField(max_length=30, help_text="Service Name")
     versionNumber = StrippedCharField(blank=True, null=True,
-        max_length=30, help_text="Version of Service Definition")
+        max_length=30, help_text="Version of Service Definition")   # deprecated
     published = models.BooleanField(default=True)
     view_url = StrippedCharField(blank=True, null=True, max_length=1024)
     icon_url = StrippedCharField(blank=True, null=True, max_length=1024)
diff --git a/xos/core/models/xosmodel.py b/xos/core/models/xosmodel.py
index 361dd63..ab7edd6 100644
--- a/xos/core/models/xosmodel.py
+++ b/xos/core/models/xosmodel.py
@@ -35,12 +35,12 @@
         # If `services` is empty, then only rebuild the UI
         # Otherwise, only rebuild the services listed in `services`
         with transaction.atomic():
-            for service_controller in self.service_controllers.all():
-                if (services) and (service_controller.name not in services):
+            for loadable_module in self.loadable_modules.all():
+                if (services) and (loadable_module.name not in services):
                     continue
-                for scr in service_controller.service_controller_resources.all():
-                   scr.save()
-                service_controller.save()
+                for lmr in loadable_module.loadable_module_resources.all():
+                   lmr.save()
+                loadable_module.save()
             self.save()
 
 class XOSVolume(PlCoreBase):
diff --git a/xos/synchronizers/onboarding/steps/sync_xos.py b/xos/synchronizers/onboarding/steps/sync_xos.py
index 80753e4..a9d2089 100644
--- a/xos/synchronizers/onboarding/steps/sync_xos.py
+++ b/xos/synchronizers/onboarding/steps/sync_xos.py
@@ -46,9 +46,9 @@
         # is in error state, but it is important that a single broken service does
         # not takedown the entirety of XOS.
 
-        for scr in xos.service_controllers.all():
+        for scr in xos.loadable_modules.all():
             if (scr.backend_status is not None) and (scr.backend_status.startswith("0")):
-                raise DeferredException("Detected unsynced service controller. Deferring.")
+                raise DeferredException("Detected unsynced loadable module. Deferring.")
 
         self.create_docker_compose()
 
diff --git a/xos/synchronizers/onboarding/xosbuilder.py b/xos/synchronizers/onboarding/xosbuilder.py
index fb71a28..5289fc7 100644
--- a/xos/synchronizers/onboarding/xosbuilder.py
+++ b/xos/synchronizers/onboarding/xosbuilder.py
@@ -8,7 +8,7 @@
 import xmlrpclib
 
 from xos.config import Config
-from core.models import Service, ServiceController, ServiceControllerResource, XOS
+from core.models import Service, ServiceController, ServiceControllerResource, LoadableModule, LoadableModuleResource, XOS
 from xos.logger import Logger, logging
 
 from django.utils import timezone
@@ -33,7 +33,7 @@
 
     def get_base_dest_dir(self, scr):
         xos_base = "opt/xos"
-        service_name = scr.service_controller.name
+        service_name = scr.loadable_module.name
         base_dirs = {"models": "%s/services/%s/" % (xos_base, service_name),
                      "admin": "%s/services/%s/" % (xos_base, service_name),
                      "admin_template": "%s/services/%s/templates/" % (xos_base, service_name),
@@ -125,7 +125,7 @@
 #    def get_controller_docker_lines(self, controller, kinds):
 #        need_service_init_py = False
 #        dockerfile=[]
-#        for scr in controller.service_controller_resources.all():
+#        for scr in controller.loadable_module_resources.all():
 #            if scr.kind in kinds:
 #                lines = self.get_docker_lines(scr)
 #                dockerfile = dockerfile + lines
@@ -156,7 +156,7 @@
         need_service_init_py = False
         script=[]
         inits=[]
-        for scr in list(controller.service_controller_resources.all()):
+        for scr in list(controller.loadable_module_resources.all()):
             if not (scr.kind in kinds):
                 continue
 
@@ -196,7 +196,7 @@
 
     def check_controller_unready(self, controller):
         unready_resources=[]
-        for scr in controller.service_controller_resources.all():
+        for scr in controller.loadable_module_resources.all():
             if (not scr.backend_status) or (not scr.backend_status.startswith("1")):
                 unready_resources.append(scr)
 
@@ -230,14 +230,14 @@
 
         dockerfile = ["FROM %s" % xos.source_ui_image]
         script = []
-        for controller in ServiceController.objects.all():
+        for controller in LoadableModule.objects.all():
             if self.check_controller_unready(controller):
-                 logger.warning("Controller %s has unready resources" % str(controller))
+                 logger.warning("Loadable Module %s has unready resources" % str(controller))
                  continue
 
             #dockerfile = dockerfile + self.get_controller_docker_lines(controller, self.UI_KINDS)
             script = script + self.get_controller_script_lines(controller, self.UI_KINDS)
-            if controller.service_controller_resources.filter(kind="models").exists():
+            if controller.loadable_module_resources.filter(kind="models").exists():
                 app_list.append("services." + controller.name)
                 migration_list.append(controller.name)
 
@@ -258,6 +258,10 @@
     def create_synchronizer_dockerfile(self, controller):
         self.build_tainted = False
 
+        if not controller.loadable_module_resources.filter(kind="synchronizer").exists():
+            # it doesn't have a synchronizer, therefore it doesn't need a dockerfile
+            return None
+
         # bake in the synchronizer from this controller
         sync_lines = self.get_controller_script_lines(controller, self.SYNC_CONTROLLER_KINDS)
 
@@ -277,10 +281,9 @@
         # It's important to bake all services in, because some services'
         # synchronizers may depend on models from another service.
         app_list = []
-        for c in ServiceController.objects.all():
-            #dockerfile = dockerfile + self.get_controller_docker_lines(c, self.SYNC_ALLCONTROLLER_KINDS)
+        for c in LoadableModule.objects.all():
             script = script + self.get_controller_script_lines(c, self.SYNC_ALLCONTROLLER_KINDS)
-            if c.service_controller_resources.filter(kind="models").exists():
+            if c.loadable_module_resources.filter(kind="models").exists():
                 app_list.append("services." + c.name)
 
         self.create_xos_app_data(controller.name, script, app_list, None)
@@ -344,7 +347,7 @@
                      logger.warning("Controller %s has unready resources" % str(c))
                      continue
 
-                 if c.service_controller_resources.filter(kind="synchronizer").exists():
+                 if c.loadable_module_resources.filter(kind="synchronizer").exists():
                      if c.synchronizer_run and c.synchronizer_config:
                          command = 'bash -c "sleep 120; cd /opt/xos/synchronizers/%s; python ./%s -C %s"' % (c.name, c.synchronizer_run, c.synchronizer_config)
                      else:
diff --git a/xos/tosca/custom_types/xos.m4 b/xos/tosca/custom_types/xos.m4
index c935eef..6372f9c 100644
--- a/xos/tosca/custom_types/xos.m4
+++ b/xos/tosca/custom_types/xos.m4
@@ -80,6 +80,18 @@
                 type: string
                 required: false
                 description: Base url, to allow resources to use relative URLs
+            version:
+                type: string
+                required: false
+                description: Version number of this Service Controller
+            provides:
+                type: string
+                required: false
+                description: Comma-separated list of things provided
+            requires:
+                type: string
+                required: false
+                description: Comma-separated list of requirements
             models:
                 type: string
                 required: false
diff --git a/xos/tosca/custom_types/xos.yaml b/xos/tosca/custom_types/xos.yaml
index a7ab219..44ade1f 100644
--- a/xos/tosca/custom_types/xos.yaml
+++ b/xos/tosca/custom_types/xos.yaml
@@ -188,6 +188,18 @@
                 type: string
                 required: false
                 description: Base url, to allow resources to use relative URLs
+            version:
+                type: string
+                required: false
+                description: Version number of this Service Controller
+            provides:
+                type: string
+                required: false
+                description: Comma-separated list of things provided
+            requires:
+                type: string
+                required: false
+                description: Comma-separated list of requirements
             models:
                 type: string
                 required: false
diff --git a/xos/tosca/resources/servicecontroller.py b/xos/tosca/resources/servicecontroller.py
index 1ad4a17..e14c604 100644
--- a/xos/tosca/resources/servicecontroller.py
+++ b/xos/tosca/resources/servicecontroller.py
@@ -5,14 +5,14 @@
 sys.path.append("/opt/tosca")
 from translator.toscalib.tosca_template import ToscaTemplate
 
-from core.models import ServiceController, ServiceControllerResource
+from core.models import ServiceController, ServiceControllerResource, LoadableModule, LoadableModuleResource
 
 from xosresource import XOSResource
 
 class XOSServiceController(XOSResource):
     provides = "tosca.nodes.ServiceController"
     xos_model = ServiceController
-    copyin_props = ["base_url", "synchronizer_run", "synchronizer_config"]
+    copyin_props = ["version", "provides", "requires", "base_url", "synchronizer_run", "synchronizer_config"]
 
     def postprocess_resource_prop(self, obj, kind, format):
         values = self.get_property(kind)
@@ -39,7 +39,7 @@
                     value = parts[-1]
 
 
-                scr = ServiceControllerResource.objects.filter(service_controller=obj, name=name, kind=kind, format=format)
+                scr = LoadableModuleResource.objects.filter(loadable_module=obj, name=name, kind=kind, format=format)
                 if scr:
                     scr=scr[0]
                     if (scr.url != value) or (scr.subdirectory!=subdirectory):
@@ -49,7 +49,7 @@
                         scr.save()
                 else:
                     self.info("adding resource %s" % kind)
-                    scr = ServiceControllerResource(service_controller=obj, name=name, kind=kind, format=format, url=value, subdirectory=subdirectory)
+                    scr = LoadableModuleResource(loadable_module=obj, name=name, kind=kind, format=format, url=value, subdirectory=subdirectory)
                     scr.save()
 
     def postprocess(self, obj):
@@ -66,3 +66,12 @@
         self.postprocess_resource_prop(obj, "rest_service", "python")
         self.postprocess_resource_prop(obj, "rest_tenant", "python")
 
+    def save_created_obj(self, xos_obj):
+        if xos_obj.requires and xos_obj.requires.strip():
+            (satisfied, missing) = ServiceController.dependency_check([x.strip() for x in xos_obj.requires.split(",")])
+            if missing:
+                raise Exception("missing dependencies for ServiceController %s: %s" % (xos_obj.name, ", ".join(missing)))
+
+        super(XOSServiceController, self).save_created_obj(xos_obj)
+
+
diff --git a/xos/tosca/resources/servicecontrollerresource.py b/xos/tosca/resources/servicecontrollerresource.py
index 96ea83d..91e61dd 100644
--- a/xos/tosca/resources/servicecontrollerresource.py
+++ b/xos/tosca/resources/servicecontrollerresource.py
@@ -5,7 +5,7 @@
 sys.path.append("/opt/tosca")
 from translator.toscalib.tosca_template import ToscaTemplate
 
-from core.models import ServiceControllerResource, ServiceController
+from core.models import ServiceControllerResource, ServiceController, LoadableModuleResource, LoadableModule
 
 from xosresource import XOSResource
 
@@ -19,7 +19,7 @@
 
         controller_name = self.get_requirement("tosca.relationships.UsedByController", throw_exception=throw_exception)
         if controller_name:
-            args["service_controller"] = self.get_xos_object(ServiceController, throw_exception=throw_exception, name=controller_name)
+            args["loadable_module"] = self.get_xos_object(ServiceController, throw_exception=throw_exception, name=controller_name)
 
         return args
 
diff --git a/xos/tosca/resources/xosresource.py b/xos/tosca/resources/xosresource.py
index f65a231..427b5eb 100644
--- a/xos/tosca/resources/xosresource.py
+++ b/xos/tosca/resources/xosresource.py
@@ -238,12 +238,16 @@
 
         return args
 
+    def save_created_obj(self, xos_obj):
+        xos_obj.save()
+
     def create(self):
         xos_args = self.get_xos_args()
         xos_obj = self.xos_model(**xos_args)
         if self.user:
             xos_obj.caller = self.user
-        xos_obj.save()
+
+        self.save_created_obj(xos_obj)
 
         self.info("Created %s '%s'" % (self.xos_model.__name__,str(xos_obj)))