| Table of Contents |
|---|
Status
Current state:"Approved"
Discussion thread: here
...
Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).
Motivation
The DescribeQuorum API as defined in KIP-595 is intended to allow the admin client to query the status of the Kraft quorum, including information about voter lag.
...
This KIP proposes making DescribeQuorum API accessible via the admin client and augmenting the DescribeQuorum API response with more information to be able to ascertain the liveness and lag of the voters in the quorum more accurately.
Public Interfaces
Additional Classes to expose the DescribeMetadataQuorumResult API to the admin client:
| Code Block |
|---|
public class DescribeMetadataQuorumResult {
private final KafkaFuture<QuorumInfo> quorumInfo;
DescribeMetadataQuorumResult(KafkaFuture<QuorumInfo> quorumInfo) {
this.quorumInfo = quorumInfo;
}
/**
* Returns a future containing the QuorumInfo
*/
public KafkaFuture<QuorumInfo> quorumInfo() {
return quorumInfo;
}
} |
...
| Code Block |
|---|
public class DescribeMetadataQuorumOptions extends AbstractOptions<DescribeMetadataQuorumOptions> {
} |
DescribeMetadataQuorum Handler in the Admin Client
| Code Block |
|---|
/**
* Describes the state of the metadata quorum.
* <p>
* This is a convenience method for {@link #describeMetadataQuorum(DescribeMetadataQuorumOptions)} with default options.
* See the overload for more details.
*
* @return the {@link DescribeMetadataQuorumResult} containing the result
*/
default DescribeMetadataQuorumResult describeMetadataQuorum() {
return describeMetadataQuorum(new DescribeMetadataQuorumOptions());
}
/**
* Describes the state of the metadata quorum.
* <p>
* The following exceptions can be anticipated when calling {@code get()} on the futures obtained from
* the returned {@code DescribeMetadataQuorumResult}:
* <ul>
* <li>{@link org.apache.kafka.common.errors.ClusterAuthorizationException}
* If the authenticated user didn't have {@code DESCRIBE} access to the cluster.</li>
* <li>{@link org.apache.kafka.common.errors.TimeoutException}
* If the request timed out before the controller could list the cluster links.</li>
* </ul>
*
* @param options The {@link DescribeMetadataQuorumOptions} to use when describing the quorum.
* @return the {@link DescribeMetadataQuorumResult} containing the result
*/
DescribeMetadataQuorumResult describeMetadataQuorum(DescribeMetadataQuorumOptions options); |
Proposed change in the DescribeQuorum Response:
| Code Block |
|---|
"apiKey": 55,
"type": "response",
"name": "DescribeQuorumResponse",
"validVersions": "0-1",
"flexibleVersions": "0+",
"fields": [
{ "name": "ErrorCode", "type": "int16", "versions": "0+",
"about": "The top level error code."},
{ "name": "Topics", "type": "[]TopicData",
"versions": "0+", "fields": [
{ "name": "TopicName", "type": "string", "versions": "0+", "entityType": "topicName",
"about": "The topic name." },
{ "name": "Partitions", "type": "[]PartitionData",
"versions": "0+", "fields": [
{ "name": "PartitionIndex", "type": "int32", "versions": "0+",
"about": "The partition index." },
{ "name": "ErrorCode", "type": "int16", "versions": "0+"},
{ "name": "LeaderId", "type": "int32", "versions": "0+", "entityType": "brokerId",
"about": "The ID of the current leader or -1 if the leader is unknown."},
{ "name": "LeaderEpoch", "type": "int32", "versions": "0+",
"about": "The latest known leader epoch"},
{ "name": "HighWatermark", "type": "int64", "versions": "0+"},
{ "name": "CurrentVoters", "type": "[]ReplicaState", "versions": "0+" },
{ "name": "Observers", "type": "[]ReplicaState", "versions": "0+" }
]}
]}],
"commonStructs": [
{ "name": "ReplicaState", "versions": "0+", "fields": [
{ "name": "ReplicaId", "type": "int32", "versions": "0+", "entityType": "brokerId" },
{ "name": "LogEndOffset", "type": "int64", "versions": "0+",
"about": "The last known log end offset of the follower or -1 if it is unknown"},
{ "name": "LastFetchTimestamp", "type": "int64", "versions": "1+", "ignorable": true, "default": -1,
"about": "The last known leader wall clock time time when a follower fetched from the leader. This is reported as -1 both for the current leader or if it is unknown for a voter"},
{ "name": "LastCaughtUpTimestamp", "type": "int64", "versions": "1+", "ignorable": true, "default": -1,
"about": "The leader wall clock append time of the offset for which the follower made the most recent fetch request. This is reported as the current time for the leader and -1 if unknown for a voter"}
]}
] |
kafka-metadata-quorum.sh
The output from this API will also be captured in the kafka-metadata-quorum.sh. The output of the --describe replication command in the tool as defined in KIP-595 will change as follows:
| Code Block |
|---|
> bin/kafka-metadata-quorum.sh --describe replication ReplicaId LogEndOffset Lag LastFetchTimestamp LastCaughtUpTimestamp Status 0 234134 0 tnow tnow Leader 1 234130 4 t2 t6 Follower 2 234100 34 t3 t7 Follower 3 234124 10 t4 t8 Observer 4 234130 4 t5 t9 Observer |
Proposed Changes
This KIP proposes exposing the DescribeQuorum API to the admin client and adding two new fields per replica (including voters and observers) to the DescribeQuorum API response.
...
LastFetchTimestamp
This metric will be reported for each voter. This is a good approximation of the “liveness” of the voters and can be used to detect a network partition in the quorum.
This information is already known to the leader for all voters and only needs to be added to the responseLastCaughtUpTimestamp
This metric will be reported for each voter. This is akin to the metric used to track lag for replicas in ISR and it measures the approximate lag between the leader and the replica based on the offsets requested in the fetch requests and when they were made. The metric gives a measure of when was the last time a replica caught up to the leader. .
To compute this metric, the Replica state maintains a few bits of information about fetch requests as they are received. Some of this information is not tracked by the Raft layer but it can be easily added in. The cost to track this information is minimal and it only requires some additional bookkeeping during pre-existing processing for a fetch.
NOTE: Given the leader is always caught up to itself, the Last Caught Up Time for the leader will be the leader's wall clock time when it responded to theDescribeQuorumrequest.
Compatibility, Deprecation, and Migration Plan
The API response is versioned and the newly added fields will bump the message version on the response. This should handle any issues around compatibility.
Rejected Alternatives
Use Existing Fields
An alternative here was to not add in this information and use existing information available in the response to compute voter lag. As discussed in the motivation section, the only information available in the response at the moment is the offset, which does not allow for a very reliable mechanism to ascertain voter lag.
Track Some Other Information
Another possibility here is to add information which allows us to tell the voter lag more accurately. An argument can be made against the accuracy of the lag as measured by the additional fields proposed in this KIP. A more complete view of the voters would be to add in Replica Time/Offset Information
...