diff --git a/doublethink/services.py b/doublethink/services.py
index 1a22af5..aeca2d2 100644
--- a/doublethink/services.py
+++ b/doublethink/services.py
@@ -30,7 +30,7 @@ class ServiceRegistry(object):
     by calling `heartbeat(status_info)` periodically.
 
     `status_info` is a dict and must have at least the fields 'role', 'load',
-    and 'heartbeat_interval'. Certain other fields are populated automatically
+    and 'ttl'. Certain other fields are populated automatically
     as in the example below. In addition, services may set arbitrary other
     fields.
 
@@ -38,11 +38,10 @@ class ServiceRegistry(object):
 
        'role': The role of the service. `healthy_service()` and
            `healthy_services()` look up services using this field.
-       'heartbeat_interval': Specifies the expected time between heartbeats. If
-           a service's last heartbeat was more than `3 * heartbeat_interval`
-           seconds ago, it is considered to be "down". `healthy_services()`
-           and `healthy_service()` never return entries for services that are
-           considered "down".
+       'ttl': Specifies the expected time between heartbeats. If a service's 
+           last heartbeat was more than `ttl` seconds ago, it is considered 
+           to be "down". `healthy_services()` and `healthy_service()` never 
+           return entries for services that are considered "down".
        'load': An arbitrary numeric value. It is up to each service to populate
            this field in a way that makes sense to the particular service.
            `healthy_service(role)` returns the service with the lowest load
@@ -72,7 +71,7 @@ class ServiceRegistry(object):
             'id': 'd0bed0be-d000-d000-f00d-abeefface0ff'     # generated by rethinkdb if not supplied
             'role': 'brozzler-worker',
             'load': 0.5, # load score
-            'heartbeat_interval': 20.0,
+            'ttl': 20.0,
             'host': 'wbgrp-svc999.us.archive.org',           # set in svcreg.heartbeat() as a fallback
             'pid': 1234,                                     # set in svcreg.heartbeat() as a fallback
             'first_heartbeat': '2015-10-30T03:39:40.080814', # set in svcreg.heartbeat()
@@ -121,7 +120,7 @@ class ServiceRegistry(object):
             service
 
         `status_info` must have at least the fields 'role', 'load', and
-        'heartbeat_interval'. Some additional fields are populated
+        'ttl'. Some additional fields are populated
         automatically by this method. If the field 'id' is absent, it will be
         generated by rethinkdb.
 
@@ -134,17 +133,17 @@ class ServiceRegistry(object):
 
         Raises:
             Exception: if `status_info` is missing a required field, or a
-                `status_info['heartbeat_interval']` is not a number greater
+                `status_info['ttl']` is not a number greater
                 than zero
         '''
-        for field in 'role', 'heartbeat_interval', 'load':
+        for field in 'role', 'ttl', 'load':
             if not field in status_info:
                 raise Exception(
                         'status_info is missing required field %s',
                         repr(field))
-        val = status_info['heartbeat_interval']
+        val = status_info['ttl']
         if not (isinstance(val, float) or isinstance(val, int)) or val <= 0:
-            raise Exception('heartbeat_interval must be a number > 0')
+            raise Exception('ttl must be a number > 0')
         updated_status_info = dict(status_info)
         updated_status_info['last_heartbeat'] = r.now()
         if not 'first_heartbeat' in updated_status_info:
@@ -223,13 +222,13 @@ class ServiceRegistry(object):
         if candidate is not None:
             candidate['id'] = role
 
-            if not 'heartbeat_interval' in candidate:
+            if not 'ttl' in candidate:
                 raise Exception(
                         "candidate is missing required field "
-                        "'heartbeat_interval'")
-            val = candidate['heartbeat_interval']
+                        "'ttl'")
+            val = candidate['ttl']
             if not (isinstance(val, float) or isinstance(val, int)) or val <= 0:
-                raise Exception('heartbeat_interval must be a number > 0')
+                raise Exception('"ttl" must be a number > 0')
 
             candidate['first_heartbeat'] = r.now()
             candidate['last_heartbeat'] = r.now()
@@ -243,7 +242,7 @@ class ServiceRegistry(object):
                             lambda row: r.branch(
                                 r.branch(
                                     row,
-                                    row['last_heartbeat'] > r.now() - row['heartbeat_interval'] * 3,
+                                    row['last_heartbeat'] > r.now() - row['ttl'],
                                     False),
                                 row, candidate),
                             return_changes='always').run()
@@ -256,7 +255,7 @@ class ServiceRegistry(object):
 
         results = list(self.rr.table(
             'services', read_mode='majority').get_all(role).filter(
-                lambda row: row['last_heartbeat'] > r.now() - row['heartbeat_interval'] * 3).run())
+                lambda row: row['last_heartbeat'] > r.now() - row['ttl']).run())
         if results:
             return results[0]
         else:
@@ -267,7 +266,7 @@ class ServiceRegistry(object):
         Find least loaded healthy service in the registry.
 
         A service is considered healthy if its 'last_heartbeat' is in the last
-        `3 * heartbeat_interval` seconds.
+        `ttl` seconds.
 
         Args:
             role (str): role name
@@ -278,7 +277,7 @@ class ServiceRegistry(object):
         '''
         try:
             result = self.rr.table('services').filter({"role":role}).filter(
-                lambda svc: r.now().sub(svc["last_heartbeat"]) < 3 * svc["heartbeat_interval"]
+                lambda svc: r.now().sub(svc["last_heartbeat"]) < svc["ttl"]
             ).order_by("load")[0].run()
             return result
         except r.ReqlNonExistenceError:
@@ -289,7 +288,7 @@ class ServiceRegistry(object):
         Look up healthy services in the registry.
 
         A service is considered healthy if its `last_heartbeat` is in the last
-        `3 * heartbeat_interval` seconds.
+        `ttl` seconds.
 
         Args:
             role (str, optional): role name
@@ -304,7 +303,7 @@ class ServiceRegistry(object):
             if role:
                 query = query.filter({"role":role})
             query = query.filter(
-                lambda svc: r.now().sub(svc["last_heartbeat"]) < 3 * svc["heartbeat_interval"]   #.default(20.0)
+                lambda svc: r.now().sub(svc["last_heartbeat"]) < svc["ttl"]   #.default(20.0)
             ).order_by("load")
             result = query.run()
             return result
diff --git a/tests/test_svcreg.py b/tests/test_svcreg.py
index bbe7ace..4f3364c 100644
--- a/tests/test_svcreg.py
+++ b/tests/test_svcreg.py
@@ -55,17 +55,17 @@ def rr():
 def test_unique_service(rr):
     svcreg = doublethink.ServiceRegistry(rr)
     assert svcreg.unique_service('example-role') == None
-    # this raises an exception: no heartbeat_interval.
+    # this raises an exception: no ttl.
     with pytest.raises(Exception) as excinfo:
         svcreg.unique_service('example-role', candidate={})
     svc01 = {
         "role": "example-role",
-        "heartbeat_interval": 0.4,
+        "ttl": 0.4,
         "node": "test01.example.com"
     }
     svc02 = {
         "role": "example-role",
-        "heartbeat_interval": 0.4,
+        "ttl": 0.4,
         "node": "test02.example.com"
     }
     # register svc01. output should be svc01.
@@ -93,23 +93,23 @@ def test_service_registry(rr):
     with pytest.raises(Exception) as excinfo:
         svcreg.heartbeat({"role":"foo","load":1})
     with pytest.raises(Exception) as excinfo:
-        svcreg.heartbeat({"role":"foo","heartbeat_interval":1.0})
+        svcreg.heartbeat({"role":"foo","ttl":1.0})
     with pytest.raises(Exception) as excinfo:
-        svcreg.heartbeat({"heartbeat_interval":1.0,"load":1})
+        svcreg.heartbeat({"ttl":1.0,"load":1})
 
-    # invalid heartbeat interval (we accept anything for load and role)
+    # invalid ttl (we accept anything for load and role)
     with pytest.raises(Exception) as excinfo:
-        svcreg.heartbeat({"heartbeat_interval":-1,"role":"foo","load":1})
+        svcreg.heartbeat({"ttl":-1,"role":"foo","load":1})
     with pytest.raises(Exception) as excinfo:
-        svcreg.heartbeat({"heartbeat_interval":"strang","role":"foo","load":1})
+        svcreg.heartbeat({"ttl":"strang","role":"foo","load":1})
     with pytest.raises(Exception) as excinfo:
-        svcreg.heartbeat({"heartbeat_interval":[],"role":"foo","load":1})
+        svcreg.heartbeat({"ttl":[],"role":"foo","load":1})
     with pytest.raises(Exception) as excinfo:
-        svcreg.heartbeat({"heartbeat_interval":[1],"role":"foo","load":1})
+        svcreg.heartbeat({"ttl":[1],"role":"foo","load":1})
     with pytest.raises(Exception) as excinfo:
-        svcreg.heartbeat({"heartbeat_interval":{},"role":"foo","load":1})
+        svcreg.heartbeat({"ttl":{},"role":"foo","load":1})
     with pytest.raises(Exception) as excinfo:
-        svcreg.heartbeat({"heartbeat_interval":{1:2},"role":"foo","load":1})
+        svcreg.heartbeat({"ttl":{1:2},"role":"foo","load":1})
 
     assert svcreg.available_service("yes-such-role") == None
     assert svcreg.available_services("yes-such-role") == []
@@ -117,12 +117,12 @@ def test_service_registry(rr):
     svc0 = {
         "role": "yes-such-role",
         "load": 100.0,
-        "heartbeat_interval": 0.4,
+        "ttl": 0.4,
     }
     svc1 = {
         "role": "yes-such-role",
         "load": 200.0,
-        "heartbeat_interval": 0.4,
+        "ttl": 0.4,
     }
     svc0 = svcreg.heartbeat(svc0)
     svc1 = svcreg.heartbeat(svc1)
@@ -188,12 +188,12 @@ def test_service_registry(rr):
     svc0 = {
         "role": "yes-such-role",
         "load": 100.0,
-        "heartbeat_interval": 0.4,
+        "ttl": 0.4,
     }
     svc1 = {
         "role": "yes-such-role",
         "load": 200.0,
-        "heartbeat_interval": 0.4,
+        "ttl": 0.4,
     }
     svc0 = svcreg.heartbeat(svc0)
     svc1 = svcreg.heartbeat(svc1)
@@ -205,22 +205,22 @@ def test_service_registry(rr):
     svc0 = {
         "role": "yes-such-role",
         "load": 100.0,
-        "heartbeat_interval": 0.4,
+        "ttl": 0.4,
     }
     svc1 = {
         "role": "yes-such-role",
         "load": 200.0,
-        "heartbeat_interval": 0.4,
+        "ttl": 0.4,
     }
     svc2 = {
         "role": "another-such-role",
         "load": 200.0,
-        "heartbeat_interval": 0.4,
+        "ttl": 0.4,
     }
     svc3 = {
         "role": "yet-another-such-role",
         "load": 200.0,
-        "heartbeat_interval": 0.4,
+        "ttl": 0.4,
     }
     svc0 = svcreg.heartbeat(svc0)
     svc1 = svcreg.heartbeat(svc1)
@@ -245,7 +245,7 @@ def test_svcreg_heartbeat_server_down(rr):
     svc0 = {
         "role": "role-foo",
         "load": 100.0,
-        "heartbeat_interval": 0.4,
+        "ttl": 0.4,
     }
     # no exception thrown
     svc0 = svcreg.heartbeat(svc0)