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Status

Current state: Drafting

Discussion thread: Not ready yet

JIRA: KAFKA-7904

Motivation

Today, a topic partition may be categorized as:

(1) Fully in sync (inSyncReplicas = allReplicasMap)

(2) UnderReplicated

(3) UnderMinIsr

(4) Offline

(3) and (4) are failure scenarios in which clients will face unavailability (producers with acks=ALL will fail to produce if ISR count is under the configured "min.insync.replicas" count).

(2) Under-replicated partitions occur whenever the inSyncReplicas set is not equal to the allReplicasMap, which can happen when:

• Repartitioning
• Broker restarts
• Transient network issues
• Broker failure

The current categorization of topic partitions has a gap as an UnderReplicatedPartition does not tell operators if the reduced ISR set is intentional (repartitioning/restarts) or if there may be something wrong such as a broker has completely failed. This makes it hard for operators as setting an alert for UnderReplicatedPartitions may not be effective as it may be too noisy, and increasing the # of samples needed to trigger the alert increases the time to detect failures.

This KIP aims to fill this gap by proposing a new categorization of partitions: AtMinIsr, which consists of partitions that only have the minimum number of insync replicas remaining (as configured by "min.insync.replicas").

If a partition is "AtMinIsr", then it suggests something severe has happened, but more importantly that one more failure can result in unavailability so some sort of action should be taken (ex. repartitioning).

Example 1:

1 partition

minIsrCount=2

ISR=[0,1,2]

1. Broker 0 fails

• ISR=[1,2]
• partition is UnderReplicatedPartition and AtMinIsr

2. Broker 1 fails

• ISR=[2]
• partition is UnderReplicatedPartition and UnderMinIsr

In this example, AtMinIsr triggers when UnderReplicatedPartition triggers and tells us that 1 more failure will cause producers with ack=ALL to be unavailable.

Example 2: 

1 partition

minIsrCount=1

ISR=[0,1,2]

1. Broker 0 fails

• ISR=[1,2]
• partition is UnderReplicatedPartition

2. Broker 1 fails

• ISR=[2]
• partition is UnderReplicatedPartition and AtMinIsr

3. Broker 2 fails

• ISR=[]
• partition is OfflinePartition

In this example, AtMinIsr triggers when there is only 1 insync replica remaining, and tells us that 1 more failure will cause the partition to go completely offline!

Public Interfaces

We will introduce a new metric and a new TopicCommand option to identify AtMinIsr partitions.

ReplicaManager.AtMinIsrPartitions


--at-min-isr-partitions

Proposed Changes

We will add the gauge to Partition.scala:

newGauge("AtMinIsr",
  new Gauge[Int] {
    def value = {
      if (isAtMinIsr) 1 else 0
    }
  },
  tags
)


...


def isAtMinIsr: Boolean = {
  leaderReplicaIfLocal match {
    case Some(leaderReplica) =>
      inSyncReplicas.size == leaderReplica.log.get.config.minInSyncReplicas
    case None =>
      false
  }
}

And TopicCommand.scala:

private val reportAtMinIsrPartitionsOpt = parser.accepts("at-min-isr-partitions",
  "if set when describing topics, only show partitions whose isr count is equal to the configured minimum. Not supported with the --zookeeper option.")


private def hasAtMinIsrPartitions(partitionDescription: PartitionDescription) = {
  partitionDescription.isr.size == partitionDescription.minIsrCount
}


private def shouldPrintAtMinIsrPartitions(partitionDescription: PartitionDescription) = {
  opts.reportAtMinIsrPartitions && hasAtMinIsrPartitions(partitionDescription)
}

Compatibility, Deprecation, and Migration Plan

The new TopicCommand option requires use of AdminClient so it will not be available with the --zookeeper option.

Rejected Alternatives

None so far.