Status
Current state: Draft
Discussion thread: here
JIRA: KAFKA-3853
Released: <Kafka Version>
Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).
Motivation
KAFKA-3853 asks for an improvement to the new-consumer
option of the consumer group command. This command, when passed a consumer group that has no consumer (i.e., when the group state is Empty
), currently reports an error indicating that there is no active member:
$ bin/kafka-consumer-groups.sh --new-consumer --bootstrap-server localhost:9092 --describe --group group1 Note: This will only show information about consumers that use the Java consumer API (non-ZooKeeper-based consumers). Error: Consumer group 'group1' has no active members.
The requested improvement is returning offsets within the group (and leaving the consumer column empty) instead of returning the error message above. The error message can still be printed to to stderr
as a warning.
When the group is Stable
(i.e. when there are active consumers in the group), {{DescribeGroups Response}} returns the associated topic partition assignment for each member of the group, and that assignment can be used to extract the corresponding committed offset(s). However, if the group state is Empty
(i.e. when there are no active consumers in the group) there is no associated topic partition info in {{DescribeGroups Response}}. Therefore, a change in the protocol seems to be the only way to extract associated topic partitions of a group and the corresponding committed offsets.
Public Interfaces
This is the current schema for DescribeGroups (version 0).
DescribeGroups Request (Version: 0) => [group_ids] group_ids => STRING DescribeGroups Response (Version: 0) => [groups] groups => error_code group_id state protocol_type protocol [members] error_code => INT16 group_id => STRING state => STRING protocol_type => STRING protocol => STRING members => member_id client_id client_host member_metadata member_assignment member_id => STRING client_id => STRING client_host => STRING member_metadata => BYTES member_assignment => BYTES
As shown above, and as explained in the previous section, corresponding topic partitions can be extracted only through the individual (and active) members
of the group. The list members
is empty if the group is in Empty
state, so with this protocol there is no way to extract offsets of an Empty
group (problem 1).
Also, offset information for a Stable
group is not stored in the above Response
schema. Offsets associated with a Stable
consumer group are currently extracted using the committed
method of KafkaConsumer
and by creating a new and dummy member in the group for the sole purpose of making committed
calls on every topic partition of each of the other consumers in the group to extract the corresponding offsets. A cleaner approach would be extracting these offsets without having to create this dummy member (problem 2).
Proposed below is version 1 of the DescribeGroups
schema that solves both problems above by including the group offset information directly into the Response
schema (lines 17-21 below). Note that topic partitions reported under the offsets
field are going to be a superset of topic partitions reported under members:member_assignment
.
DescribeGroups Request (Version: 1) => [group_ids] group_ids => STRING DescribeGroups Response (Version: 1) => [groups] groups => error_code group_id state protocol_type protocol [members] [offsets] error_code => INT16 group_id => STRING state => STRING protocol_type => STRING protocol => STRING members => member_id client_id client_host member_metadata member_assignment member_id => STRING client_id => STRING client_host => STRING member_metadata => BYTES member_assignment => BYTES offsets => topic partition offset metadata topic => STRING partition => INT32 offset => INT64 metadata => STRING
Proposed Changes
The proposal is to include a list offsets
of tuples in the response to DescribeGroups request. Each of these tuples contains the following fields:
- topic: topic from which a previous member of the group consumed.
- partition: partition of the above topic for which offset information associated with the group exists
- offset: associated committed offset of the above topic partition within the group
- metadata: metadata associated with the above offset
Compatibility, Deprecation, and Migration Plan
There should be no impact to existing users of this protocol. The Request
schema remains unchanged, and the Response
schema is only expanded. There is no change in the existing Response
structure, and, therefore, users who use version 0 of the schema should not be affected. The code behind kafka-consumer-groups.sh
will also be updated (as requested in the JIRA) to return additional information (rows of topic partitions with no assigned consumer). In case it is necessary to provide the old output as well, a new parameter can be defined for the tool to report the new information; while the old way of running the tool still prints out the old output.
Rejected Alternatives
If there are alternative ways of accomplishing the same thing, what were they? The purpose of this section is to motivate why the design is the way it is and not some other way.