lots of documentation on the service registry

doublethink/services.py

@@ -23,9 +23,53 @@ import os
 
 class ServiceRegistry(object):
     '''
-    status_info is dict, should have at least these fields
+    Simple service registry which stores service information in the rethinkdb
+    table 'services'.
+
+    Services are responsible for keeping their status information up to date
+    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
+    as in the example below. In addition, services may set arbitrary other
+    fields.
+
+    Some information about required fields:
+
+       '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".
+       '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
+           for the supplied role. Thus load values need to be comparable to
+           within the context of a single service, but comparing loads of
+           services of different roles does not necessarily make any sense.
+
+    About the 'id' field:
+
+        The only way that the service registry uniquely identifies a particular
+        instance of a service is using the 'id' field.
+
+        Services can supply their own 'id', or let rethinkdb generate a random
+        one.
+
+        If a service provides its own 'id', it should make it something
+        predictable and unique to each instance of the service. For example
+        `'%s:%s:%s' % (role, host, port)` might work for some services.
+
+        If, on the other hand, a server lets rethinkdb generate 'id', it will
+        need to remember the result returned by calls to `heartbeat()` and
+        supply the `id` value from there with every subsequent heartbeat.
+
+    Example service registry entry, with notes:
+
         {
-        'id': ...,   # generated by rethinkdb
+            'id': 'd0bed0be-d000-d000-f00d-abeefface0ff'     # generated by rethinkdb if not supplied
             'role': 'brozzler-worker',
             'load': 0.5, # load score
             'heartbeat_interval': 20.0,
@@ -33,36 +77,71 @@ class ServiceRegistry(object):
             'pid': 1234,                                     # set in svcreg.heartbeat() as a fallback
             'first_heartbeat': '2015-10-30T03:39:40.080814', # set in svcreg.heartbeat()
             'last_heartbeat': '2015-10-30T05:54:35.422866',  # set in svcreg.heartbeat()
-        ... plus anything else you want...
+            # ... plus anything else you want...
         }
     '''
-
     logger = logging.getLogger('doublethink.ServiceRegistry')
 
     def __init__(self, rr):
+        '''
+        Initialize the service registry.
+
+        Creates the database table if it does not exist.
+
+        Args:
+            rr (doublethink.Rethinker): a doublethink.Rethinker, which must
+                have `dbname` set
+        '''
         self.rr = rr
         self._ensure_table()
 
     def _ensure_table(self):
         dbs = self.rr.db_list().run()
+        assert self.rr.dbname
         if not self.rr.dbname in dbs:
-            self.logger.info('creating rethinkdb database %s', repr(self.rr.dbname))
+            self.logger.info(
+                    'creating rethinkdb database %s', repr(self.rr.dbname))
             self.rr.db_create(self.rr.dbname).run()
         tables = self.rr.table_list().run()
         if not 'services' in tables:
-            self.logger.info("creating rethinkdb table 'services' in database %s", repr(self.rr.dbname))
+            self.logger.info(
-            self.rr.table_create('services', shards=1, replicas=min(3, len(self.rr.servers))).run()
+                    "creating rethinkdb table 'services' in database %s",
-            # self.rr.table('sites').index_create...?
+                    repr(self.rr.dbname))
+            self.rr.table_create(
+                    'services', shards=1,
+                    replicas=min(3, len(self.rr.servers))).run()
+            # self.rr.table('services').index_create...?
 
     def heartbeat(self, status_info):
         '''
-        Returns updated status info on success, un-updated status info on
+        Update service status, indicating "up"-ness.
-        failure.
+
+        Args:
+            status_info (dict): a dictionary representing the status of the
+            service
+
+        `status_info` must have at least the fields 'role', 'load', and
+        'heartbeat_interval'. Some additional fields are populated
+        automatically by this method. If the field 'id' is absent, it will be
+        generated by rethinkdb.
+
+        See the ServiceRegistry class-level documentation for more information
+        about the various fields.
+
+        Returns:
+            On success, returns the modified status info dict. On failure
+            communicating with rethinkdb, returns `status_info` unmodified.
+
+        Raises:
+            Exception: if `status_info` is missing a required field, or a
+                `status_info['heartbeat_interval']` is not a number greater
+                than zero
         '''
         for field in 'role', 'heartbeat_interval', 'load':
             if not field in status_info:
                 raise Exception(
-                        'status_info is missing required field %s', field)
+                        'status_info is missing required field %s',
+                        repr(field))
         val = status_info['heartbeat_interval']
         if not (isinstance(val, float) or isinstance(val, int)) or val <= 0:
             raise Exception('heartbeat_interval must be a number > 0')
@@ -84,9 +163,16 @@ class ServiceRegistry(object):
             return status_info
 
     def unregister(self, id):
+        '''
+        Remove the service with id `id` from the 'services' table.
+        '''
         result = self.rr.table('services').get(id).delete().run()
-        if result != {'deleted':1,'errors':0,'inserted':0,'replaced':0,'skipped':0,'unchanged':0}:
+        if result != {
-            self.logger.warn('unexpected result attempting to delete id=%s from rethinkdb services table: %s', id, result)
+                'deleted':1, 'errors':0,'inserted':0,
+                'replaced':0,'skipped':0,'unchanged':0}:
+            self.logger.warn(
+                    'unexpected result attempting to delete id=%s from '
+                    'rethinkdb services table: %s', id, result)
 
     def leader(self, role_name, default=None):
         '''
@@ -108,16 +194,43 @@ class ServiceRegistry(object):
             self.rr.table('services', read_mode='majority').get(role_name).replace(lambda row: r.branch(r.branch(row, row['last_heartbeat'] > r.now() - row['heartbeat_interval'] * 3, False), row, default)).run()
         return self.rr.table('services', read_mode='majority').get(role_name).run()
 
-    def available_service(self, role):
+    def healthy_service(self, role):
+        '''
+        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.
+
+        Args:
+            role (str): role name
+
+        Returns:
+            the healthy service with the supplied `role` with the smallest
+            value of 'load'
+        '''
         try:
             result = self.rr.table('services').filter({"role":role}).filter(
-                lambda svc: r.now().sub(svc["last_heartbeat"]) < 3 * svc["heartbeat_interval"]   #.default(20.0)
+                lambda svc: r.now().sub(svc["last_heartbeat"]) < 3 * svc["heartbeat_interval"]
             ).order_by("load")[0].run()
             return result
         except r.ReqlNonExistenceError:
             return None
 
-    def available_services(self, role=None):
+    def healthy_services(self, role=None):
+        '''
+        Look up healthy services in the registry.
+
+        A service is considered healthy if its `last_heartbeat` is in the last
+        `3 * heartbeat_interval` seconds.
+
+        Args:
+            role (str, optional): role name
+
+        Returns:
+            If `role` is supplied, returns list of healthy services for the
+            given role, otherwise returns list of all healthy services. May
+            return an empty list.
+        '''
         try:
             query = self.rr.table('services')
             if role:
@@ -130,3 +243,6 @@ class ServiceRegistry(object):
         except r.ReqlNonExistenceError:
             return []
 
+    available_service = healthy_service
+    available_services = healthy_services
+

setup.py

@@ -3,7 +3,7 @@ import codecs
 
 setuptools.setup(
     name='doublethink',
-    version='0.2.0.dev74',
+    version='0.2.0.dev75',
     packages=['doublethink'],
     classifiers=[
         'Programming Language :: Python :: 2.7',