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Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

Further Improvements and Proposals on Mirror Maker 2

Accepted

• KIP-545: support automated consumer offset sync across clusters in MM 2.0
• KIP-597: MirrorMaker2 internal topics Formatters
• KIP-690: Add additional configuration to control MirrorMaker 2 internal topics naming convention
• KIP-710: Full support for distributed mode in dedicated MirrorMaker 2.0 clusters
• KIP-716: Allow configuring the location of the offset-syncs topic with MirrorMak
• KIP-720: Deprecate MirrorMaker v1er2

Under Discussion

• KIP-781: Improve MirrorMaker2's client configuration

Abandoned

• KIP-656: MirrorMaker2 Exactly-once Semantics (superseded by KIP-618: Exactly-Once Support for Source Connectors and KIP-710: Full support for distributed mode in dedicated MirrorMaker 2.0 clusters)
• KIP-712: Shallow Mirroring

Motivation

MirrorMaker has been used for years in large-scale production environments, but not without several problems:

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• Leverages the Kafka Connect framework and ecosystem.

• Includes both source and sink connectors.

• Includes a high-level driver that manages connectors in a dedicated cluster.
• Detects new topics, partitions.

• Automatically syncs topic configuration between clusters.

• Manages downstream topic ACL.

• Supports "active/active" cluster pairs, as well as any number of active clusters.

• Supports cross-datacenter replication, aggregation, and other complex topologies.

• Provides new metrics including end-to-end replication latency across multiple data centers/clusters.

• Emits offsets required to migrate consumers between clusters.

• Tooling for offset translation.

• MirrorMaker-compatible legacy mode.

• No rebalancing.

Public Interfaces

Public Interfaces

This KIP subsumes the small interface change proposed in KIP-416: Notify SourceTask of ACK'd offsets, metadata

New classes and interfaces include:

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The following are the SourceConnector's additional configuration properties.

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property

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default value

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description

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consumer.*

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n/a

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passed to the consumer

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In addition, internal client properties can be overridden as follows:

And similarly for the SinkConnector:

These properties are specified in the MirrorConnectorConfig class, which is shared by all MirrorMaker connectors (MirrorSourceConnector, MirrorCheckpointConnector, etc). Some properties are relevant to specific Connectors, e.g. "emit.heartbeats" is only relevant to the MirrorHeartbeatConnector.

Example Configuration

A sample configuration file ./config/connect-mirror-source.properties is provided:

name = local-mirror-source
topics = .*
connector.class = org.apache.kafka.connect.mirror.MirrorSourceConnector
tasks.max = 1


# for demo, source and target clusters are the same
source.cluster.alias = upstream
source.cluster.bootstrap.servers = localhost:9092
target.cluster.bootstrap.servers = localhost:9092


# use ByteArrayConverter to ensure that records are not re-encoded
key.converter = org.apache.kafka.connect.converters.ByteArrayConverter
value.converter = org.apache.kafka.connect.converters.ByteArrayConverter

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--%<----


# emulate legacy mirror-maker
replication.policy.class = org.apache.kafka.mirror.LegacyReplicationPolicy


# disable all new features
renamerefresh.topics.enabled = false
refresh.topics = false
refresh.groupsgroups.enabled = false
emit.checkpoints.enables = false
emit.heartbeats.enabled = false
sync.topic.configs.enabled = false
sync.topic.acls.enabled = false


--%<----

MirrorMaker Clusters

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clusters = primary, backup
cluster.primary.brokerbootstrap.listservers = localhost:9091
cluster.backup.brokerbootstrap.listservers = localhost:9092 

This can be run as follows:

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name = MirrorSourceConnector
connector.class = org.apache.kafka.connect.mirror.MirrorSourceConnector
source.cluster.alias = primary
target.cluster.alias = backup
source.cluster.bootstrap.servers = localhost:9091
target.cluster.bootstrap.servers = localhost:9092
key.converter.class = org.apache.kafka.connect.converters.ByteArrayConverter
value.converter.class = org.apache.kafka.connect.converters.ByteArrayConverter 

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clusters = primary, backup
primary.bootstrap.servers = localhost:9091
backup.bootstrap.servers = localhost:9092 
primary->backup.topics = .*
primary->backup.emit.heartbeats.enabled = false

In this case, two MirrorSourceConnectors, two MirrorCheckpointConnectors, and two MirrorHeartbeatConnectors will be created by the driver and the primary→backup connectors will start replicating all source topics at launch, with heartbeats disabled. Sub-configurations like primary->backup.x.y.z will be applied to all connectors in that source→target flow (MirrorSourceConnector, MirrorCheckpointConnector), avoiding the need to configure each connector individually.

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us-west.offset.storage.topic = mm2-offsets
us-west->us-east.emit.heartbeats.enabled = false

Second, launch one or more MirrorMaker cluster nodes:

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# connector.properties
name = local-mirror-source
connector.class = org.apache.kafka.connect.mirror.MirrorSourceConnector
tasks.max = 1
topics=.*

source.cluster.alias = upstream
source.cluster.bootstrap.servers = host1:9092
target.cluster.bootstrap.servers = host2:9092

# use ByteArrayConverter to ensure that in and out records are exactly the same
key.converter = org.apache.kafka.connect.converters.ByteArrayConverter
value.converter = org.apache.kafka.connect.converters.ByteArrayConverter

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• MirrorSourceConnector
• MirrorSinkConnector (coming soon)
• MirrorCheckpointConnector
• MirrorHeartbeatConnector

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PUT /connectors/us-west-source/config HTTP/1.1

{
    "name": "us-west-source",
    "connector.class": "org.apache.kafka.connect.mirror.MirrorSourceConnector",
    "source.cluster.alias": "us-west",
    "target.cluster.alias": "us-east",
    "source.cluster.bootstrap.servers": "us-west-host1:9091",
    "topics": ".*"
}

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• Command-line tools wrapping RemoteClusterUtils. Document cross-cluster migration procedure.
• Broker and KafkaConsumer support for unambiguous remote topic names, e.g. with a reserved separator character.
• CheckpointSinkConnector/Task to support upstream checkpointing when a single Connect cluster is used.
• Additional metrics.
• Support reconfiguration of running Tasks without stop, start.
• Exactly-once semantics in Connect and MM2.
• Improve offset translation, e.g. with timestamps or RecordBatch headers, to get closer to exact offset of latest commit.
• Prevent duplicate streams from "diamond problem" (fan-out-fan-in).
• REST API for start, stop, status of underlying Connectors.
• Support multiple clusters/workers/herders in ConnectDistributed to obviate MM2-specific driver.KIP-416: Notify SourceTask of ACK'd offsets, metadata

Rejected Alternatives

• We could release this as an independent project, but we feel that cluster replication should be one of Kafka's fundamental features.

• We could deprecate MirrorMaker but keep it around. However, we see this as a natural evolution of MirrorMaker, not an alternative solution.

• We could update MirrorMaker rather than completely rewrite it. However, we'd end up recreating many features provided by the Connect framework, including the REST API, configuration, metrics, worker coordination, etc.

• We could build on Uber's uReplicator, which solves some of the problems with MirrorMaker. However, uReplicator uses Apache Helix to provide features that overlap with Connect, e.g. REST API, live configuration changes, cluster management, coordination etc. A native MirrorMaker should only use native parts of Apache Kafka. That said, uReplicator is a major inspiration for the MM2 design.

• We could provide a high-level REST API for remote-controlling a MirrorMaker cluster. However, this has overlapping concerns with Connect.

• Instead of separate connectors for heartbeats and checkpoints, we could do everything in a single connector. However, this restricts how Converters and Transformations can be used. For example, you might want a replication pipeline that converts to JSON, but that doesn't necessarily mean you also want heartbeats to be JSON-encoded. It's possible to create a separate KafkaProducer within MirrorSourceConnector to emit Heartbeats and Checkpoints without going through the Connect transformation pipeline, but I don't want to introduce the additional configuration properties for a whole new producer.

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