Add list_consumer_group_offsets()

Support fetching the offsets of a consumer group.

Note: As far as I can tell (the Java code is a little inscrutable), the
Java AdminClient doesn't allow specifying the `coordinator_id` or the
`partitions`.

But I decided to include them because they provide a lot of additional
flexibility:

1. allowing users to specify the partitions allows this method to be used even for
older brokers that don't support the OffsetFetchRequest_v2

2. allowing users to specify the coordinator ID gives them a way to
bypass a network round trip. This method will frequently be used for
monitoring, and if you've got 1,000 consumer groups that are being
monitored once a minute, that's ~1.5M requests a day that are
unnecessarily duplicated as the coordinator doesn't change unless
there's an error.

kafka/admin/kafka.py

@@ -1,8 +1,12 @@
 from __future__ import absolute_import
 
+from collections import defaultdict
 import copy
 import logging
 import socket
+
+from kafka.vendor import six
+
 from kafka.client_async import KafkaClient, selectors
 import kafka.errors as Errors
 from kafka.errors import (
@@ -12,8 +16,9 @@
 from kafka.protocol.admin import (
     CreateTopicsRequest, DeleteTopicsRequest, DescribeConfigsRequest, AlterConfigsRequest, CreatePartitionsRequest,
     ListGroupsRequest, DescribeGroupsRequest)
-from kafka.protocol.commit import GroupCoordinatorRequest
+from kafka.protocol.commit import GroupCoordinatorRequest, OffsetFetchRequest
 from kafka.protocol.metadata import MetadataRequest
+from kafka.structs import TopicPartition, OffsetAndMetadata
 from kafka.version import __version__
 
 log = logging.getLogger(__name__)
@@ -585,5 +590,75 @@ def list_consumer_groups(self):
         # TODO this is completely broken, as it needs to send to the group coordinator
         # return self._send(request)
 
+    def list_consumer_group_offsets(self, group_id, group_coordinator_id=None,
+                                    partitions=None):
+        """Fetch Consumer Group Offsets.
+
+        Note:
+        This does not verify that the group_id or partitions actually exist
+        in the cluster.
+
+        As soon as any error is encountered, it is immediately raised.
+
+        :param group_id: The consumer group id name for which to fetch offsets.
+        :param group_coordinator_id: The node_id of the group's coordinator
+            broker. If set to None, will query the cluster to find the group
+            coordinator. Explicitly specifying this can be useful to prevent
+            that extra network round trip if you already know the group
+            coordinator. Default: None.
+        :param partitions: A list of TopicPartitions for which to fetch
+            offsets. On brokers >= 0.10.2, this can be set to None to fetch all
+            known offsets for the consumer group. Default: None.
+        :return dictionary: A dictionary with TopicPartition keys and
+            OffsetAndMetada values. Partitions that are not specified and for
+            which the group_id does not have a recorded offset are omitted. An
+            offset value of `-1` indicates the group_id has no offset for that
+            TopicPartition. A `-1` can only happen for partitions that are
+            explicitly specified.
+        """
+        group_offsets_listing = {}
+        if group_coordinator_id is None:
+            group_coordinator_id = self._find_group_coordinator_id(group_id)
+        version = self._matching_api_version(OffsetFetchRequest)
+        if version <= 3:
+            if partitions is None:
+                if version <= 1:
+                    raise ValueError(
+                        """OffsetFetchRequest_v{} requires specifying the
+                        partitions for which to fetch offsets. Omitting the
+                        partitions is only supported on brokers >= 0.10.2.
+                        For details, see KIP-88.""".format(version))
+                topics_partitions = None
+            else:
+                # transform from [TopicPartition("t1", 1), TopicPartition("t1", 2)] to [("t1", [1, 2])]
+                topics_partitions_dict = defaultdict(set)
+                for topic, partition in partitions:
+                    topics_partitions_dict[topic].add(partition)
+                topics_partitions = list(six.iteritems(topics_partitions_dict))
+            request = OffsetFetchRequest[version](group_id, topics_partitions)
+            response = self._send_request_to_node(group_coordinator_id, request)
+            if version > 1:  # OffsetFetchResponse_v1 lacks a top-level error_code
+                error_type = Errors.for_code(response.error_code)
+                if error_type is not Errors.NoError:
+                    # optionally we could retry if error_type.retriable
+                    raise error_type(
+                        "Request '{}' failed with response '{}'."
+                        .format(request, response))
+            # transform response into a dictionary with TopicPartition keys and
+            # OffsetAndMetada values--this is what the Java AdminClient returns
+            for topic, partitions in response.topics:
+                for partition, offset, metadata, error_code in partitions:
+                    error_type = Errors.for_code(error_code)
+                    if error_type is not Errors.NoError:
+                        raise error_type(
+                            "Unable to fetch offsets for group_id {}, topic {}, partition {}"
+                            .format(group_id, topic, partition))
+                    group_offsets_listing[TopicPartition(topic, partition)] = OffsetAndMetadata(offset, metadata)
+        else:
+            raise NotImplementedError(
+                "Support for OffsetFetch v{} has not yet been added to KafkaAdmin."
+                .format(version))
+        return group_offsets_listing
+
     # delete groups protocol not yet implemented
     # Note: send the request to the group's coordinator.

kafka/structs.py

@@ -72,6 +72,7 @@
     ["topic", "partition", "leader", "replicas", "isr", "error"])
 
 OffsetAndMetadata = namedtuple("OffsetAndMetadata",
+    # TODO add leaderEpoch: OffsetAndMetadata(offset, leaderEpoch, metadata)
     ["offset", "metadata"])
 
 OffsetAndTimestamp = namedtuple("OffsetAndTimestamp",